Faculty of Social and Life SciencesBiology

DISSERTATIONKarlstad University Studies

2011:3

Pär Gustafsson

Forest – stream linkages

Brown trout (Salmo trutta) responses to woody debris, terrestrial invertebrates and light

Pär Gustafsson

Forest – stream linkages

Brown trout (Salmo trutta) responses to woody debris, terrestrial invertebrates and light

Karlstad University Studies2011:3

Pär Gustafsson. Forest – stream linkages: Brown trout (Salmo trutta) responses to woody debris, terrestrial invertebrates and light

DISSERTATION

Karlstad University Studies 2011:3ISSN 1403-8099 ISBN 978-91-7063-335-5

© The author

Distribution:Karlstad UniversityFaculty of Social and Life SciencesBiologySE-651 88 Karlstad, Sweden+46 54 700 10 00

www.kau.se

Print: Universitetstryckeriet, Karlstad 2011

Abstract

The five papers in this thesis focus on woody debris, terrestrial invertebrates and light,

three factors influenced by riparian zone structure, potentially affecting streams

and brown trout (Salmo trutta). The individual strength of these stressors and

their interactions with each other are not well studied, and their effects may

differ both spatially and temporally as well as with the size-structure of specific

fish populations.

Using a combination of laboratory and field experiments, I examined the effects

of woody debris, terrestrial invertebrates and light on prey availability and on

the growth rates, diets and behavior of different size-classes of trout. My field

experiments showed that addition of high densities of large wood affected trout

growth in a positive way. This positive effect of large wood on trout growth

may be related to prey abundance, as indicated by the high standing crop of

aquatic macroinvertebrates on the wood. The effect on trout may also be

related to decreased energy expenditures in wood habitats, as trout increased the

ratio between numbers of prey captured and time spent active and that

swimming activity and level of aggression decreased as wood densities were

increased in a stream tank experiment. Terrestrial invertebrates are generally

assumed to be a high quality prey resource for fish and my field experiments

showed that reduction of terrestrial invertebrate inputs had a negative effect on

trout growth. The availability of terrestrial prey in the stream was also coupled

to trout diet and linked to growth, as fish with high growth rates had high

proportions of terrestrial prey in their diets. Light (PAR) did not have an effect

on chlorophyll biomass, nor was there an effect on aquatic macroinvertebrates

or trout. Other factors, such as low nutrient content may have limited the

effects. Many of my results were dependent on fish-size. I observed for example

that large trout had higher capture rates on surface-drifting terrestrial prey than

small trout when prey densities were high, but at low prey densities, the

consumption of terrestrial prey by large and small trout were similar. Moreover,

although large wood and terrestrial invertebrates affected growth of both small

and large trout, the effects were generally more consistent for large trout.

Changes in riparian forests typically induce an array of interacting effects that

call for further research. However, this thesis show that some factors may have

profound effects on stream biota also as single stressors and that positive

effects from wood, by adding trees to streams, may partly compensate for

negative effects associated with riparian deforestation.

Contents

Publications 5

Introduction 6

Woody debris 7

Terrestrial invertebrates 9

Light 11

Objectives 12

Materials and methods 13

Study area 13

Sampling and experimental design 14

Summary of results 19

Discussion 23

Concluding remarks and future research 29

Acknowledgements 31

References 32

Appendices: Papers I-V

5

Publications

This thesis is based on the following manuscripts and published papers, referred

to in the text by their Roman numerals (I-V). Paper I is reprinted with the

permission from John Wiley and Sons.

I. Gustafsson, P., Bergman, E. and Greenberg, L. A. (2010). Functional

response and size-dependent foraging on aquatic and terrestrial prey

by brown trout (Salmo trutta L.). Ecology of Freshwater Fish, 19: 170-177.

II. Erős, T., Gustafsson, P., Greenberg L. A. and Bergman, E. (2011).

Aquatic-terrestrial linkages: effects of terrestrial invertebrate input

and light on a boreal stream community. Manuscript.

III. Gustafsson, P., Greenberg, L. A. and Bergman E. (2011). Influence

of woody debris and terrestrial invertebrates on prey resources for

stream living fish. Manuscript.

IV. Gustafsson, P., Greenberg, L. A. and Bergman E. (2011). Effects of

woody debris and terrestrial invertebrates on the diet and growth of

brown trout (Salmo trutta) in a boreal stream. Manuscript.

V. Gustafsson, P., Greenberg, L. A. and Bergman E. (2011). Effects of

large wood density on brown trout (Salmo trutta) behavior in artificial

streams. Manuscript.

6

Introduction

Subsidies of energy and material from donor systems may have large impacts on

the distribution and density of animal populations in recipient systems.

Consequently, any change in the donor system may potentially influence the

conditions for species living in the recipient system (Polis et al. 1997, Kawaguchi

et al. 2003). Freshwater streams and their adjacent riparian zones are ecosystems

closely linked by the reciprocal flux of energy and the movement of various

organisms over habitat boundaries. The terrestrial habitat may influence its

aquatic counterpart by regulating the input of light and allochthonous material,

whereas the aquatic habitat may contribute to terrestrial food webs via the

emergence of aquatic insects (Baxter et al. 2005). Although forests have been

harvested for centuries it was not until the 1950s that the mechanization of

forestry started and led, over a period of about 25 years, to large clear-cut areas

and an eight-fold increase in the quantity of timber removed in, for example,

Sweden (Crisp et al. in Northcote and Hartman 2004). As a consequence, the

connectivity between terrestrial and aquatic habitats has been interrupted,

several pathways for energy and material flux have been disturbed and

conditions have thus changed for many species, including trout (Naiman and

Decamps 1997, Northcote and Hartman 2004).

According to the river continuum concept low order headwater streams are

more influenced by the composition and structure of the biota along the stream

bank than high order streams (Vannote et al. 1980). A large water-surface-to-

volume ratio and a relatively high proportion of stream surface covered by

overhanging riparian vegetation may constrain the amount of sunlight reaching

the water and thus influence autochthonous primary production, especially in

small streams (Hill et al. 2001). Conversely, a large allochtonous input of riparian

foliage, needles and other plant debris may balance the lack of solar energy by

powering basic heterotrophic processes, which may in turn have effects on

macroinvertebrates and fish. Acting as a top consumer in many of these smaller

streams and important also for recreational purposes (i.e. game fishing), brown

trout (Salmo trutta) are often of particular interest when studying the effects of

forestry and riparian management on stream ecosystems. The links between fish

and riparian zones are several but generally involve 1) geomorphic processes, 2)

amount of shade and cover, 3) input of organic material and 4) water quality

(Growns et al. 2003 and references therein).

7

To extrapolate and predict the consequences of effects linked to forest

management is challenging. As forestry operations include multiple stressors,

the evaluation of combined and cumulative effects becomes even more difficult,

but, as pointed out by Hartman (in Northcote and Hartman 2004), nevertheless

important. Thus, the fundamental constituent in this thesis has been to combine

factors connected to forestry and study both their single and joint effects on

aquatic organisms in general and brown trout in particular. The thesis focuses

on three factors linked to riparian zones, woody debris, terrestrial invertebrates and

light, and how they influence size-dependent foraging behavior, diet, growth and

prey resources for brown trout (S. trutta).

Woody debris

In-stream wood, and especially large wood, is recognized as one of the key

structural components in shaping stream communities. Submerged wood

influences geomorphic processes (Keller and Swanson 1979) as well as the

transport and retention of sediment and organic material (Bilby and Likens

1980, Speaker et al. 1984). The main effects of large wood on fish habitat

typically include reduced stream velocity (Keim et al. 2002), increased pool

frequency, depth and structural complexity (Lisle 1986, Bilby and Ward 1991,

Fausch and Northcote 1992, Urabe and Nakano 1998, Roni and Quinn 2001,

Dahlström 2005). By increasing the amount of cover and complexity, large

wood may provide fish with refuge from both aquatic and terrestrial predators

(Werner et al. 1983, Shirvell 1990, Reinhardt and Healey 1997) and result in

decreased competition between individuals by increasing visual isolation

between conspecifics and decrease territory size (Kalleberg 1958, Basquill and

Grant 1998, Sundbaum and Näslund 1998, Imre et al. 2002). However, high

structural complexity from large wood may not only reduce the visual isolation

between conspecifics but also between predators and their prey (Savino and

Stein 1982, Wilzbach et al. 1986) and thus be a negative stressor for drift-feeding

fish (Wilzbach et al. 1986, O´Brian and Showalter 1993, Kemp et al. 2005).

Complex cover and habitat, like that created by large wood, may be especially

important during times of stress, such as when stream-flows are low or high

(Dolloff and Warren 2003). In small, drought-prone streams, pools made from

large wood may provide the only refuge during low flows.

8

Woody debris may also influence the food resources for fish by offering a high

quality surface for aquatic macroinvertebrates (Hilderbrand et al. 1997, Drury

and Kelso 2000, Spänhoff et al. 2000, Johnson et al. 2003). This may be

especially important in high-order streams with relatively little hard substrates,

where submerged wood may contribute substantially to the total surface area of

hard substrates available for colonization by algae and macroinvertebrates

(Wallace and Benke 1984, O´Connor 1992, Hax and Golladay 1993). However,

even in headwater streams, large wood increases the available substrate area and

in contrast to a mineral substrate, large wood slowly decays, leaving interstitial

spaces that offer invertebrates both feeding areas and refuges from predators,

droughts and strong currents (Lisle 1986, Drury and Kelso 2000, Spänhoff et al.

2000). Additionally, large wood may support both higher densities and higher

biomass of macroinvertebrates than a mineral streambed (Hernandez et al.

2005), presumably because of wood’s large size, high structural complexity and

nutrient content. The presence of large wood also increases taxonomic diversity

since wood-associated taxa are not always found on the streambed, whereas

most benthic taxa are also found on wood (Johnson et al. 2003, Hernandez et al.

2005, Bond et al. 2006).

Overall, the effects on aquatic habitats from woody debris have led to a general

assumption that habitats with wood are positive for stream fish populations.

Many studies also report increased biomass and/or density of salmonids in

habitats after wood addition or in areas with abundant wood (Saunders and

Smith 1962, Fausch and Northcote 1992, Flebbe et al. 1995, Roni and Quinn

2001, Lehane et al. 2002, Sweka and Hartman 2006). Knowledge about the

underlying mechanisms of how wood affects salmonid populations is however

fairly limited. For instance, very few experimental field studies have been

designed to examine the direct effects of large wood on specific growth rates of

brown trout. The effects of woody debris on lotic systems may also vary with

scale, the characteristics of the stream, stream size, wood size, density and

arrangement of the wood itself, species and fish size. For example, some studies

suggest that large wood mainly concentrates mobile fish, with minor

consequences for the size of the whole population (Riley and Fausch 1995). On

the other hand, Lehane et al. (2002) observed increased biomass in wood-

treatments without redistribution from other areas and Degerman et al. (2004)

observed that both fish-size and abundance of brown trout was positively

related to large wood in Swedish streams. As fish mature, their use of large

wood in streams can also shift in relation to their size and food preferences.

9

Moreover, falling trees will more likely end up perpendicular to stream margins

in small streams than in large streams, and thus more often create pools and

wood clusters in small streams (Bilby and Ward 1989, Richmond and Fausch

1995). As wood decomposes and decomposition rates vary with tree species,

the effects of large wood in streams may also be both cumulative and vary over

time, with time scales ranging from months to centuries (Hyatt and Naiman

2001, Roberts et al. 2004).

Despite its recognized importance, large wood is often in short supply in many

streams. The low availability of large wood has been attributed to an intense

forestry management that during the 19th-century not only increased the

fragmentation and loss of forest habitats, but also reduced the recruitment of

large wood to the streams running through these forests (Northcote and

Hartman 2004). While input of leaves and other small organic debris of

terrestrial origin may reappear soon after a clear-cut, input of large wood often

takes several decades (Dahlström 2005). A reasonable assumption is that forest

harvesting that includes the removal of whole trees will have an impact on

aquatic habitats many years after logging has ceased. Murphy and Koski (1989)

concluded that large wood loadings were reduced by 70% some 90 years after

logging, and that a return to pre-logging levels would take approximately 250

years. Moreover, studies have shown that large in-stream wood is strongly

connected with basin area logged (Bisson et al. 1987, Bilby and Ward 1991) and

that wood densities are considerably higher in Swedish old-growth forest

streams than in streams flowing through managed forests (Dahlström 2005 and

references therein).

Terrestrial invertebrates

It has long been recognized that a substantial amount of energy and material

that enters headwater streams is derived from the surrounding riparian forest

(Hynes 1970, Vannote et al. 1980). Yet, not until recently have researchers

started to appreciate and assess the importance of terrestrial invertebrates as a

prey subsidy for fish (Baxter et al. 2005). The use of this resource by fish, which

represents a direct linkage between trout and trees, is however both logical and

well supported. First, several studies have shown that a reduction of riparian

vegetation may negatively influence the input of terrestrial invertebrate biomass

to streams. In an early study by Mason and MacDonald (1982), they found that

10

terrestrial invertebrate biomass input was more than 400% greater in forested

than in afforested reaches. Similar patterns have also been reported by

Kawaguchi and Nakano (2001). Correlations have also been established

between type of riparian vegetation and biomass input of terrestrial

invertebrates (Edwards and Huryn 1996, Kawaguchi and Nakano 2001, Allan et

al. 2003, Baxter et al. 2005), with invertebrate input from deciduous forests

generally being greater than from coniferous forests. Second, this allochtonous

food resource may become integrated into stream food webs through its use by

stream-dwelling fish, especially salmonids. In ground-breaking work by Allen

(1951), he suggested that trout populations in streams could not be supported

by production of aquatic macroinvertebrates alone; instead they required a

contribution from terrestrial invertebrates as well. Later studies have also

established relationships between invertebrates derived from riparian vegetation

and diet of salmonids and some have documented that during certain periods of

the year terrestrial invertebrates may contribute over 50% of the total diet

biomass (Zadorina 1988, Bridcut and Giller 1995, Wipfli 1997, Nakano et al.

1999a, Bridcut 2000, Kawaguchi et al. 2003, Dineen et al. 2007a). Third, albeit

debated, terrestrial invertebrates have been suggested to be an energetically

favorable prey due to their higher energy density (cal g-1 dry weight) (Cummins

and Wuycheck 1971), generally larger size and greater mass than aquatic

macroinvertebrates (Nakano et al. 1999a, Kawaguchi et al. 2003). Finally, the

peak in the input of terrestrial invertebrates typically occurs in summer and may

correspond with a period of the year when the abundance of aquatic benthic

macroinvertebrates is low and metabolic requirements of fish are high (Cloe and

Garman 1996, Nakano and Murakami 2001, Elliot 1994). Some studies have

suggested that the input of terrestrial invertebrates to headwater streams may

actually equal the in situ production of aquatic benthic macroinvertebrates

(Mason and MacDonald 1982, Cloe and Garman 1996, Wipfli 1997). Thus,

given that the input of terrestrial invertebrates is an important food resource for

trout, changes in this resource may have an effect on both trout individuals and

populations.

The relationship between terrestrial invertebrate inputs and the population

response by brown trout is however not well studied. Only a few studies have

experimentally studied the effects of a reduction of terrestrial invertebrates on

salmonid species. Nakano et al. (1999b) showed that reducing terrestrial inputs

in a Japanese deciduous forest resulted in an increase in use of aquatic

macroinvertebrates by Dolly Varden char (Salvelinus malma), with subsequent

11

indirect top-down effects that cascaded through the food web. In a review,

Baxter et al. (2005) stressed that few studies have quantified the direct effects of

reduced terrestrial invertebrate input on fish growth. However later, Baxter et al.

(2007) showed that a reduction of terrestrial prey reduced the growth of Dolly

Varden in a Japanese deciduous stream, and Sweka and Hartman (2008)

modeled that a depletion of terrestrial invertebrates could reduce the growth of

brook trout. Field experiments examining the effects of terrestrial inputs are still

few, and as most field studies have been conducted in deciduous forests; the

question arises if similar responses would be seen in salmonids living in

coniferous forest streams, where terrestrial invertebrate inputs are assumed to

be lower. Furthermore, research must also consider the temporal and

taxonomical variations of this subsidy and, as trout might have ontogenetically

different foraging strategies and thus differ in their ability to use terrestrial prey

resources, also consider the effects of size-related foraging differences.

Light

Sunlight directly influences stream food webs via bottom-up processes. The

general consensus is that increased light is positively correlated with primary

production (Feminella et al. 1989, Hill et al. 1995, Hill and Dimick 2002, Kiffney

et al. 2004), and thus increased light is expected, for example, to favor increased

abundance of grazers. This might in turn increase the food base for trout.

Nevertheless it may be difficult to predict the effects of increased light as

several other factors such as nutrients, temperature, presence of herbivores and

type of substrate also affect production of periphyton (Rosemond 1993, Hill et

al. 1995, Kiffney et al. 2003, 2004). In a study by Ambrose and Wilzbach (2004),

where light input to streams was increased via a controlled reduction of riparian

vegetation, no clear evidence of increased periphyton biomass by light was

observed in streams with low nutrient levels. However, in streams with higher

nutrient levels the increased light resulted in higher primary production. Similar

results were observed by Rosemond et al. (2000) and Stockner and Shortreed

(1978), thus suggesting that nutrients, and not light may act as a limiting factor

for primary production in many small streams.

Increased incident light may have positive effects on trout through increased

foraging efficiency (Schutz and Northcote 1972). Wilzbach et al. (1986) found a

relationship between surface light and the amount of prey captured by

12

salmonids. Hence, increased light, as might be obtained by removal of riparian

vegetation, may be beneficial for trout either through indirect trophic effects or

by direct effects. On the other hand, several studies have shown that since light

input is closely linked to water temperature (Barton et al. 1985, Stott and Marks

2000, Kiffney et al. 2003, 2004), an increase in light may become negative for

salmonids. For example, Stott and Marks (2000) reported that clear-cutting

increased the daily mean maximum temperature by 5.3-7 °C. Such a dramatic

increase could be harmful as the difference between optimal foraging

temperatures and temperatures where energy intake drops rapidly is fairly small

for brown trout (Elliot 1994). Thus, high temperature stress in trout is more

likely to occur in afforested streams than in forested streams.

Objectives

The main objective in this thesis was to study the effects of woody debris, terrestrial

invertebrate prey subsidies and light on prey availability, and on growth rates, diets

and behavior of different size-classes of brown trout (Salmo trutta). This was

examined by using a combination of laboratory and field experiments. In the

first laboratory experiment (paper I) my interest was to study wild brown trout

and observe size-dependent foraging behavior when provided surface-drifting

terrestrial invertebrates and/or benthic invertebrates at different densities and

relative abundances. Then, during two subsequent field experiments, I

continued to examine the effects of terrestrial invertebrates on trout by

manipulating the terrestrial prey subsidy and combine this with manipulations

of either light input (paper II) or woody debris (paper III and IV). The

objective for these studies was to test the individual and combined effects of

reduced or ambient terrestrial invertebrate prey input, increased or ambient light

levels and depleted or enhanced densities of large wood on prey availability and

on the growth and diet of two size-classes of brown trout in a coniferous

headwater stream. The initial aim with manipulating light conditions in paper II

was to study if light (PAR), as a single factor, could affect primary production

and ultimately study the consequences for benthic macroinvertebrates and trout.

As inputs of terrestrial invertebrates typically vary over season with highest

inputs during summer, a central approach in paper II was to study the temporal

variation and biotic effects of this resource. Potential food resources for fish in

streams may be affected both directly and indirectly by the surrounding forest

trough which they flow. Therefore, my focus in paper III was to study how

13

manipulations of terrestrial invertebrate subsidies and large in-stream wood

affected total prey resources for trout. As differences in food-base may have

effect on trout, a central interest for me in paper IV was to study the effects of

terrestrial prey subsidies and large wood on the diet and growth of trout. To

further examine the effects from woody debris on trout I conducted another

laboratory experiment (paper V) where my focus was to study the effects of

large wood on brown trout foraging and agonistic interactions. Specifically, I

explored if the response to different densities of large wood was size-dependent

and if the response differed between single fish of different sizes and same-aged

and mixed-aged pairs of trout when foraging on surface-drifting terrestrial prey.

Materials and methods

Study area

The studies in paper I and V were conducted at the stream aquarium facility at

Karlstad University during winter 2005/2006 and 2008/2009, respectively. The

facility was built in 2004 and provides a wide range of experimental tanks

suitable for research studies (Figure 1).

Figure 1. One of the stream tanks used in paper V (W++ treatment).

The field studies in paper II

in Sundtjärnsbäcken (X-

coniferous-forested second order stream situated in the county of Värmland in

western Sweden (Figure 2

13.1 km2).

Figure 2. The geographic location of and IV, situated in Glaskogen nature reserve, county

Sampling and experimental design

In paper I, wild brown trout were caught by electrofishing in the River

Barlindshultsälven (X-Y: 6605725

Karlstad University. After an acclimatization period of approximately three

weeks the functional response curves and se

classes of trout (0+, 1+, ≥2+) were studied using surface (terrestrial

fly) and/or benthic (aquatic

and relative abundances. During experiments fish were kept isolated in tanks

(200 L) at 12 °C, and a new set of fish was used for each prey type experiment.

A total of 42 fish were used for the

arrangements). Foraging behavior was measured as the number of prey

captured per second, and the data were fitted with a modified Hollings (1959)

equation. Selectivity was measured as the proportion of each prey type c

in relation to the availability of that prey in the environment. Additionally, I also

recorded age-specific differences in vertical distribution between attacks.

Paper II, III and IV is

factorial design with the following treatments:

14

paper II (2006), III (2007) and IV (2007) were carried out

-Y: 6612710-1303106), a 1.5-2.5 m wide,

forested second order stream situated in the county of Värmland in

2). The stream empties into the Lake Övre Gla (area:

eographic location of Sundtjärnsbäcken, the study stream in papersGlaskogen nature reserve, county of Värmland, Sweden.

ampling and experimental design

, wild brown trout were caught by electrofishing in the River

Y: 6605725-1302471) and immediately transported to

Karlstad University. After an acclimatization period of approximately three

weeks the functional response curves and selectivity patterns of three age/

≥2+) were studied using surface (terrestrial, i.e. house

ic (aquatic, i.e. Gammarus) prey at a broad range of densities

and relative abundances. During experiments fish were kept isolated in tanks

, and a new set of fish was used for each prey type experiment.

A total of 42 fish were used for the whole experiment (4-6 replicates x 3

arrangements). Foraging behavior was measured as the number of prey

captured per second, and the data were fitted with a modified Hollings (1959)

equation. Selectivity was measured as the proportion of each prey type c

in relation to the availability of that prey in the environment. Additionally, I also

specific differences in vertical distribution between attacks.

is based on two field experiments, both using

factorial design with the following treatments: Paper II (May – Oct 2006):

carried out

, mainly

forested second order stream situated in the county of Värmland in

The stream empties into the Lake Övre Gla (area:

papers II, III

, wild brown trout were caught by electrofishing in the River

1302471) and immediately transported to

Karlstad University. After an acclimatization period of approximately three

lectivity patterns of three age/size

, i.e. house

) prey at a broad range of densities

and relative abundances. During experiments fish were kept isolated in tanks

, and a new set of fish was used for each prey type experiment.

6 replicates x 3

arrangements). Foraging behavior was measured as the number of prey

captured per second, and the data were fitted with a modified Hollings (1959)

equation. Selectivity was measured as the proportion of each prey type captured

in relation to the availability of that prey in the environment. Additionally, I also

specific differences in vertical distribution between attacks.

using a 2x2

Oct 2006): 1)

15

reduced input of terrestrial invertebrates (T), 2) light enhancement (L), 3) a

combination of “T” and “L” - TL and 4) unmanipulated reaches (C) (Figure 3a).

Paper III and IV (June – Aug 2007): 1) large wood enhancement (W), 2)

reduced input of terrestrial invertebrates (TR), 3) a combination of “W” and

“TR” (WTR) and 4) unmanipulated reaches (U) (Figure 4a). By using this design

and these manipulations I could mimic some effects from forestry and study

how light and terrestrial invertebrates influenced benthic macroinvertebrates,

trout diet and growth (paper II), and how woody debris and terrestrial

invertebrates affected total prey biomass (paper III) and the diet and growth of

brown trout (paper IV). All treatments were replicated three times and

randomly assigned to twelve 20 m long study reaches. To prevent fish from

escaping, each reach was blocked at the down- and upstream end by metal nets.

To minimize cumulative longitudinal treatment effects, all enclosures were

separated by a 40 m long unmanipulated buffer zone.

Reduction of terrestrial invertebrates in both paper II (T and TL enclosures),

III and IV (TR and WTR enclosures) was done by constructing 20 m long, 2.5 m

high wooden frames that were covered with UV-resistant transparent

greenhouse plastic (Figure 3b). An additional 20 meters of “tent” was built

upstream of each reduction enclosure to further decrease terrestrial invertebrate

input. In paper II, light, measured as PAR (Photosynthetic Active Radiation,

µm s-1 m-2), was artificially increased by hanging eight lamps (2x58 W

fluorescent light tubes per lamp) approximately 40 cm over the stream surface.

Light devices were suspended from the same type of wooden frames used in

enclosures where terrestrial input was reduced. To control for the presence of

light fixtures in the illuminated enclosures (L and TL), eight lamp models of the

same size as the lamp fixtures were placed in both unmanipulated (C) and

terrestrial reduction (T) enclosures in the same spatial arrangement as in the

illuminated enclosures.

To increase the volume of large wood in paper III and IV I added fresh birch

(Betula pendula) logs to six of the enclosures. The total volume of wood in each

wood-enclosure was standardized to 98 m-3 ha-1, thus mimicking conditions

found in old-growth forests in Scandinavia, i.e. before modern forestry

(Dahlström 2005 and references therein). Logs in small streams more often than in

large streams are distributed in a random fashion and oriented perpendicular to

the stream bank (Bilby and Ward 1989, Richmond and Fausch 1995), often

forming pool-like habitats. Therefore I concentrated the logs at four different

16

locations within each wood-enclosure, creating pool-like areas with high

structural complexity upstream of each wood aggregation (Figure 4b).

Figure 3. a) Illustration of a stream segment containing unmanipulated enclosures and enclosures with reduced or normal input of terrestrial invertebrates (T.I.) combined with enhanced or ambient light conditions. b) One of the enclosures covered by a transparent plastic “tent” to reduce input of terrestrial invertebrates.

Extra

light &

ambient

T.I.

Ambient

light &

ambient

T.I.

Extra

light &

reduced

T.I.

Ambient

Light &

reduced

T.I.

L C TL T

40 m buffer zones

b)

a)

17

Figure 4. a) Illustration of a stream segment containing unmanipulated enclosures and enclosures with reduced or normal input of terrestrial invertebrates (T.I.) combined with wood or no-wood added. b) One of the enclosures with added large wood.

For all three field papers, fish were electrofished in a nearby stream,

translocated, anesthetized with tricaine methane-sulfonate (MS-222), measured,

weighed (length to nearest mm and weight to nearest 0.1 g) and individually

tagged with a PIT-tag (Passive Integrated Transponder, ID-100, Trovan

system). The tags were injected into the body cavity with a syringe. Tagged fish

were kept for observation for 24 h followed by a randomized release of 10-

11small (mean length p. II: 79mm, p. III & IV: 103mm) and 6-7 large (mean

length p. II: 137mm, p. III & IV 142mm) trout into each enclosure. Densities

Large

wood &

ambient

T.I.

No wood &

ambient

T.I.

Large

wood &

reduced

T.I.

No wood &

reduced

T.I.

W U WTR TR

40 m buffer zones

a)

b)

18

used were well within the range found in similar streams in the surrounding

area.

For paper II, sampling started in the middle of May 2006 and ended in mid-

October with sampling every 1-5 weeks, for paper III and IV sampling started

in middle of June and ended in mid-August with sampling every 2-5 weeks.

During fish sampling, every 4-5 weeks, trout were caught by electrofishing

(maximum of 2 passes), anesthetized with MS-222, length measured, weighed

and stomach flushed (Twomey and Giller 1990) for gut contents. Fish that

could not be recaptured or accidentally died during the experiments were

replaced with new fish to keep equal densities.

Terrestrial invertebrates were sampled every second (III, IV) or third (II) week

by suspending six pan traps over the stream in each enclosure for 72 h. For

both paper II and III, benthic macroinvertebrates were sampled by taking six

random Surber samples per enclosure and both aquatic and terrestrial drift was

sampled every 4-5 weeks by placing drift nets in each enclosure around dusk.

Abiotic parameters (width, depth and velocity) were measured along transects in

each enclosure once a month. In paper II, stream surface irradiance (PAR) was

sampled at each transect on a weekly basis during the whole season. Primary

production (measured as g chl.a cm-2) was sampled by removing ceramic tiles

from each enclosure once a month. All tiles had been incubated in the stream

three weeks prior to the start of the experiment. Water samples were taken

throughout the study periods and analyzed for basic parameters and nutrients.

Air and stream temperatures (both inside and outside “tents”) were

continuously measured via digital loggers placed at lower, middle and upper

sections of the experimental area.

Colonization of benthic macroinvertebrates on wood in paper III was studied

by adding sample logs to the stream two weeks prior to experimental start and

collect logs every third week during the whole study period. Wood associated

prey in each enclosure was then quantified by multiplying the total amount of

wood area by the density of macroinvertebrates based on the colonization data

(g m-2).

In paper V wild brown trout (Salmo trutta) of two different size classes, “small”:

[84.3 ±1.81 mm] (mean ± SE) and “large” [127.9 ±2.28 mm], were collected by

electrofishing in the River Barlindshultsälven (X-Y: 6605725-1302471) and

19

directly transported to the laboratory at Karlstad University. After an

acclimatization period of about 6 weeks, the swimming activity, surface drift

foraging and agonistic interactions with conspecifics was studied in three

artificial stream tanks, each with one of three wood densities, ranging from 0 to

0.034 m3/m2. The wood densities used represented conditions in a clear-cut

forest where recruitment of wood has been eliminated, a managed forest and an

old-growth forest (Figure 1). A three-factor experimental design, testing the

effects of wood (W++, W+ and W-); fish (small and large fish); and fish size

combinations (1 small fish, 1 large fish, 2 small fish, 2 large fish, 1 small + 1

large fish) was used. All treatments were replicated six times. All trials were

recorded on video and tapes were later used for quantifying: 1) activity (% time

spent swimming), 2) agonistic interactions between conspecifics (# min-1), 3)

foraging efficiency (% prey items caught), 4) for calculations of captured prey

per time unit active (PTA) and 5) vertical position of the fish during trials.

Summary of results

Paper I

The functional response of 0+ and older trout differed significantly, both when

the fish were fed benthic and surface prey. Capture rates were higher for large

trout than for small trout, but there were no significant differences between

prey types. Capture rates when both prey items were present simultaneously

also differed significantly between size-classes, again with large trout having

higher capture rates than small trout. Capture rates within each size class did not

vary significantly with prey density. Furthermore, the two–prey experiment also

revealed that 1+ trout ate significantly more surface-drifting prey than 0+ but

also more than ≥2+ trout, which also 0+ trout did. There were size-specific

differences in vertical positioning where 1+ and 2+ trout remained in the upper

part of the water column more often than 0+ trout between attacks of surface

prey. Instead, 0+ trout sought refuge at the bottom between attacks

significantly more often than 1+ and ≥2+.

20

Paper II

The input of terrestrial invertebrates to the uncovered stream sections showed a

seasonal trend. An initially low input during spring was followed by an increase

as temperatures increased and foliation was completed. Input to uncovered

enclosures peaked in July at about 100 mg m-2 d-1 and was followed by

progressively decreasing levels from August to October. The difference in

biomass input between tent covered enclosures and uncovered enclosures was

significant throughout the season, but with the biggest differences from June to

August. The large reduction in terrestrial invertebrate subsidies did not induce a

top-down effect on lower trophic levels (i.e. macroinvertebrates and algae)

through an increased foraging on aquatic macroinvertebrates by trout. Instead,

the reduced resource input had a direct negative and significant impact on trout

growth rates. This effect was seasonal and partly size-dependent, with large

trout during mid- to late summer showing the greatest reduction in growth but

where small trout also were negatively affected, mainly during and thus

corresponding temporally with a period when differences in terrestrial inputs

between reduction enclosures and unmanipulated enclosures were greatest.

Moreover, the diet was affected; large trout ate fewer terrestrial invertebrates in

enclosures with reduced terrestrial inputs than in enclosures with ambient

terrestrial inputs.

No effect of light could be observed on chlorophyll a biomass or on the density

or species composition of benthic macroinvertebrates or drift. This is despite

the fact that light intensity (measured as PAR) was increased more than 2.5

times above ambient light levels. Moreover, light had no effect on trout diet or

trout growth.

Paper III

The flux of terrestrial invertebrates showed great temporal variation, with

terrestrial input peaking in mid-June when uncovered enclosures received

approximately 250 mg falling invertebrates m-2 d-1. This peak was followed by

decreasing inputs and from mid-July and onwards the inputs varied between 25-

50 mg m-2 d-1. Terrestrial invertebrate inputs were significantly affected by the

tents with uncovered enclosures, on average, receiving nearly 2.5 times more

terrestrial invertebrate biomass than covered enclosures. The largest input

21

differences occurred during the first half of the study period, thus reflecting the

seasonal input pattern.

The proportion of wood-associated aquatic macroinvertebrate biomass relative

to total aquatic biomass increased from 35% in June to over 50% in August.

Total aquatic biomass, including both benthic and wood-associated taxa was

46.7% and 86.9% greater in the two wood treatments (W, WTR) than in the two

no-wood treatments (U, TR), in June and August respectively, a difference that

was highly significant. These numbers are to be compared to a mean wood

surface area of approximately 7m2/enclosure and a mean streambed surface

area of about 23m2/enclosure. In terms of total potential prey biomass, the

treatment with added large wood and ambient levels of terrestrial invertebrate

inputs (W) had the highest standing crop of invertebrates (benthic, wood and

terrestrial combined), whereas the treatment with reduced terrestrial input and

no wood added (TR) had the lowest standing crop. The two remaining

treatments (U, WTR) had intermediate levels.

The colonization of aquatic macroinvertebrates on submerged wood was fast

and reached maximum biomass within 4 weeks. Chironomids and Simulium were

the most frequent colonists, comprising about 65% of total biomass, which

differed from the taxonomic composition on mineral substrates where these

taxa comprised approximately 15% of total biomass.

Biomass of terrestrial drift was significantly lower in covered enclosures than in

uncovered. The differences in biomass were also revealed in significant

differences in the proportions of terrestrial invertebrates in the drift. The

terrestrial component of the drift regularly exceeded 70% of total drift in

uncovered enclosures as compared to 15-40% in covered. Taxonomically there

were no significant differences in the drift between treatments, neither for

aquatic nor for terrestrial taxa. There was, however, a nearly significant effect of

higher biomass of Lepidoptera and Brachycera in uncovered enclosures. No

differences in total aquatic biomass or taxonomical differences in the drift were

revealed between treatments.

22

Paper IV

The addition of large wood to the stream had a significantly positive effect on

trout growth rates, whereas reduced terrestrial invertebrate subsidies had a

significantly negative effect. Growth rates were highest in the treatment with

added wood and ambient terrestrial invertebrate inputs, lowest in the treatment

with no added wood and reduced terrestrial invertebrate inputs and

intermediate in the other two treatments. The response to wood, but not to

terrestrial invertebrate input, was also size-dependent, with growth of large trout

being positively affected during the whole study period while smaller trout was

positively affected only during the first half.

Analyses of diet composition (biomass based proportions) showed no

differences between treatments with and without added wood. The reduction of

terrestrial invertebrate inputs, however, greatly affected the diet. Trout in

enclosures with reduced inputs had a significantly lower proportion of terrestrial

invertebrate biomass in the diet than trout experiencing ambient terrestrial

inputs. This was especially evident for Lepidoptera larvae, which constituted as

much as 30% of total diet biomass in enclosures with ambient terrestrial inputs

whereas of minor importance in enclosures with reduced inputs. Overall, 20-

40% of trout diet in uncovered enclosures consisted of terrestrial invertebrates.

No differences between size-classes regarding terrestrial invertebrate content in

the diet were observed.

As expected, the addition of wood significantly changed current velocity and

mean depth. In enclosures where wood was added, seasonal mean depth

increased from approximately 10 cm to 15 cm. The greatest difference however

occurred in June with mean depths in wood and no-wood enclosures at

approximately 14 cm and 8 cm respectively. The addition of wood also

decreased the seasonal mean velocity by approximately 250%, from 0.28 to 0.08

cm s-1.

Paper V

The presence of large wood significantly reduced swimming activity of the fish,

with swimming activity decreasing with increasing habitat complexity. Large fish

were less active than small fish and single fish swam less than paired fish,

23

apparently an effect of the observed higher frequency of agonistic interactions

in paired trials. In general, the rate of agonistic interactions decreased

significantly with increasing wood density, with the most substantial drop

occurring between the intermediate and the highest wood density. There was no

significant effect of size, indicating that both size-classes were similarly

aggressive although large fish, when paired with a small fish, initiated nearly all

aggressive interactions. An effect of size-class combination was also observed,

where individuals within same-sized pairs showed significantly more aggressive

nips than individuals in pairs of different size. Large and small trout captured,

on average, equal proportions of surface drifting prey and size-class

combination did not affect the number of successful captures. The proportion

of captured prey was, however, inversely related to wood density for large but

not for small trout.

In order to get a measurement of energy gain related to energy spent I

expressed capture rates as a ratio between the number of prey captured and the

time spent actively swimming. Doing that I found that trout captured

approximately 3.3 times more prey per minute active (PTA) at the highest wood

density as compared to the wood-free treatment. Size-class combination also

had a significant effect, showing a higher PTA for single fish treatments than

paired, regardless of the size of the other fish. This was probably due to that

single fish were less active than when in pairs. Despite generally higher PTA in

large fish, the difference did not result in a significant effect of fish size.

Analyses of fish position revealed that large fish significantly more often than

small sought shelter at the bottom under the logs during trials with abundant

wood.

Discussion

The reduction in growth by trout in response to the reduction in terrestrial

invertebrate inputs (paper II and IV) may represent a direct link between trees

and fish. In both studies the supply of terrestrial prey was, on average, several

hundred percent higher in unmanipulated treatments than in treatments with

reduced terrestrial inputs. Diet analyses also revealed that terrestrial

invertebrates were frequently utilized as prey and that the reduction in terrestrial

prey in both paper II and IV had effect on diet, as trout from uncovered

treatments with ambient terrestrial inputs consumed more terrestrial prey than

24

trout in covered enclosures. A high utilization of terrestrial prey by stream living

fish and that reduced growth may be an effect of terrestrial prey shortage

corroborates with previous findings (Bridcut and Giller 1995, Wipfli 1997,

Nakano et al. 1999a, Webster and Hartman 2005, Baxter et al. 2007). During

summer, the standing crop of suitable aquatic macroinvertebrate prey for fish is

often low due to prior emergence by adults, leaving only small size of instars in

streams (Hynes 1970, Cloe and Garman 1996). In addition, metabolic demands

of the fish are expected to be high in summer due to high water temperatures

(Elliott 1994). Thus, large differences in the availability of the terrestrial prey

resource in covered vs. uncovered enclosures, during a period of the year when

metabolic rates are expected to be high and the availability of the alternative

prey resource, aquatic macroinvertebrates, is in limited supply may thus have

lead to reduced fish growth.

The low supply of terrestrial food may also have been accentuated by nutritional

differences between the two prey resources. Terrestrial invertebrates are

generally believed to have higher energy content (cal/g dry weight) than aquatic

macroinvertebrates (Cummins and Wuycheck 1971), and terrestrial

invertebrates are also assumed to be generally larger and have higher mass

compared to aquatic macroinvertebrates (Nakano et al. 1999a, Kawaguchi et al.

2003). Thus, terrestrial invertebrates may be an energetically favorable prey.

Support for this explanation can also be found in a study by Sweka and

Hartman (2008), who used a bioenergetics model to conclude that a reduction

of terrestrial invertebrates decreased growth of brook trout and to maintain

growth trout had to increase its consumption of aquatic invertebrates by 130%.

Thus, the low growth rates in trout from treatments with reduced terrestrial

inputs in paper II and IV may be caused by an overall prey shortage in terms of

numbers of prey as well as by a difference in energetic quality of the prey.

Previous studies have shown a positive relationship between body size and the

contribution of surface-drifting terrestrial prey to the diet of salmonids

(Dunbrack and Dill 1983, Mäki-Petäys et al. 1997, Montori et al. 2006, Teixeira et

al. 2006, Dineen et al. 2007a). This pattern may be attributed to morphological

differences between small and large fish (i.e. gape limitation), which possibly

explains the size-specific differences that I observed in paper I. Due to much

shorter handling times, large trout were better than small trout in capturing both

aquatic and terrestrial prey. Larger reaction distances in large fish may also result

in that large fish more easily detect surface-drifting invertebrates than small

25

(Dunbrack and Dill 1983). This may also explain the higher selectivity for

surface prey in large than small trout in paper I. In natural streams, size specific

diet-differences may also be due to habitat partitioning in which large trout use

pools to a greater extent than small trout (Greenberg et al. 1996, Heggenes 1996,

Mäki-Petäys et al. 1997, Heggenes et al. 1999, Armstrong et al. 2003, Ayllón et al.

2010) and thus likely increase their encounter rates with the normally larger and

surface drifting terrestrial prey.

Despite the results in paper I, the reduction of terrestrial prey in the field

studies in paper II and paper IV resulted in negative effects on growth for

both size-classes. However, in paper II, large trout was more consistently

affected than small, while in paper IV, both size-classes were equally affected

by the prey reduction. This was also supported by diet analyses as size-

dependent dietary differences in the consumption of terrestrial invertebrates

was observed in paper II while no such differences were revealed in paper IV.

Although the exact reason for this discrepancy remains uncertain, it may be

explained both by differences in trout size, differences in relative trout size

(large vs. small) between studies and the observed differences in both

temporality and magnitude of terrestrial inputs between years. For example, the

relative difference in size between small and large fish was larger in paper II

than in paper IV and indicate that prey preferences may have been more similar

in paper IV. Additionally, in a study by Montori et al. (2006) where terrestrial

content in diet was size-dependent the size differences between small and large

fish was much greater than what I had. In my studies, the temporal pattern of

terrestrial input also differed between years with inputs in paper II and III (IV)

peaking in July and June respectively. This, in relation to fish size, may also have

affected both diet and growth.

Both the ambient and the reduced levels of terrestrial invertebrate inputs in

papers II and III (IV) resemble levels observed in studies comparing inputs in

forested and unforested areas (Mason and McDonald 1982, Edwards and

Huryn 1996, Kawaguchi and Nakano 2001, Allan et al. 2003), although the

values in especially paper III are greater than what previously has been

reported from coniferous dominated streams and actually comparable to many

deciduous streams (See Baxter et al. 2005 for a review). This indicates that

coniferous forests contain substantial prey resources for fish and that a

reduction of this food source to especially headwater streams, recognized as

comparatively unproductive (Utz and Hartman 2006), and may have major

26

consequences for fish. Moreover, from a taxonomical perspective, terrestrial

larvae were both an important element in terrestrial invertebrate input and in

the diet of trout, as shown in papers III and IV. As these larvae are highly

nutritious and overhanging vegetation is believed to be especially important for

the input of terrestrial larvae (Kawaguchi and Nakano 2001), a reduction of

riparian forest will likely reduce a very important prey subsidy for trout.

Terrestrial larvae have a phenology in which they often are produced in

immense quantities during a limited period, typically associated with specific

host tree species (Jones and Despland 2006). This temporal pattern was also

observed in paper III in which lepidoptera larvae dominated the input

particularly in June. Dietary and growth data from paper IV indicate that both

size-classes of fish in uncovered enclosures with unrestricted access to terrestrial

larvae not only utilized the resource but also experienced higher growth rates

than fish in covered enclosures with a limited access to this particular prey

source.

Large wood is a well documented link between the stream and its surrounding

riparian forest affecting both aquatic macroinvertebrates and fish. The rapid

colonization rates by aquatic macroinvertebrates on wood and the observed

biomass values in paper III corroborates with previous findings (e.g. Drury and

Kelso 2000, Bond et al. 2006) and indicate that wood is a preferred substrate.

However, the much greater total aquatic macroinvertebrate biomass in wood-

enclosures than in no-wood enclosures observed in paper III is largely an

effect of increasing the area available for colonizing by the addition of wood.

Nevertheless, wood acting as an advantageous substrate and thus attracting a

variety of macroinvertebrates may also lead to higher area specific biomass than

on mineral substrates (e.g. Hilderbrand et al. 1997, Hoffman and Hering 2000,

Spänhoff et al. 2000, Benke and Wallace 2003 and references therein, Hernandez et

al. 2005). Total macroinvertebrate biomass may have thus been enhanced also

by an increase in habitat quality.

The generally positive effects on trout growth rates from wood in paper IV

may have several explanations. First, as previously outlined, the total standing

crop of aquatic macroinvertebrates was much higher in the wood than in no-

wood treatments. Since availability of prey biomass has been shown to be

positively related to growth of salmonids (Bilby and Bisson 1987), habitats with

abundant wood containing approximately 50-80% higher biomass of aquatic

fauna than enclosures without wood (paper III), may have directly influenced

27

trout growth. In relation to the no-wood habitats, the higher prey biomass in

wood- habitats also indicates that abundant wood may be especially important

during summer when aquatic macroinvertebrates often are in short supply (Cloe

and Garman 1996, Dineen et al. 2007b). Second, large wood may also change

the physical habitat and affect foraging efficiency in fish. According to foraging

models, the most profitable holding position for a drift-feeding fish in terms of

maximizing net energy gain, has a low current velocity for high capture rates

(Hughes and Dill 1990) but situated near fast-flowing water with high rates of

drifting prey (Bachman 1984, Fausch 1984). A large amount of wood may thus

have increased the number of profitable holding stations for fish in paper IV.

Third, high wood densities increase habitat complexity and may also increase

the visual isolation between competitors and decrease the interactions between

conspecifics (Kalleberg 1958, Sundbaum and Näslund 1998, Imre et al. 2002).

This may reduce energy expenditure and enhance growth. In paper V, where

trout behavior was tested against a gradient of wood densities I also observed

that both swimming activity and the rate of agonistic interactions decreased

with increased wood densities when foraging on surface-drifting prey. The

aggressive behavior was, however, not completely relaxed until the highest

wood density. This wood density corresponds to the wood density used in

paper III and IV and to wood densities reported from old-growth forest

streams (Dahlström 2005). However, habitats with high complexity can also

obstruct foraging due to shading effects (Wilzbach 1986, Sundbaum and

Näslund 1998, Kemp 2005). This was observed in paper V where the

proportion of surface prey captured was reduced in the most complex habitat,

particularly for large fish. The size-dependent effect may be explained by that

large fish more than small utilize large in-stream structures (Fausch 1993,

Armstrong et al. 2003 and references therein) such as large wood. Support for this

may also be found in paper V where large fish during trials with abundant

wood more than small sought shelter at the bottom under logs. This behavior

may in turn be related to trade-offs between predation risks and foraging

(Power 1987) but also reflect the relatively poorer maneuvering skills by large

fish in structurally complex habitats (Videler 1993). However, the PTA-value

(prey per time unit active) in paper V was significantly higher for all trout at the

highest wood density, thus indicating potentially higher net energy gain. To sum

up, the addition of large wood to the stream with its concomitant structural

effects in combination with higher prey abundance may have caused the

observed positive effects on growth in wood habitats.

28

The effects on growth from large wood in paper IV were generally stronger for

large trout than for small trout thus indicating that large wood and wood

formed pools may be more important for large trout than for small trout. This

may be due to size-dependent habitat partitioning in which large trout by

inhabiting wood pools also benefit more from the advantages. This spatial

segregation may in turn be linked to dominance hierarchies in which large fish

outrival small fish and/or to trade-offs between predatory risks and the benefits

of foraging in pools (Bohlin 1977, Power 1985, Power 1987, Fausch and White

1986, Nakano 1995). As mentioned, large fish may utilize large in-stream

structures more than small fish, which also uses cobbles and gravel as refuge or

for feeding purposes (Fausch 1993, Armstrong et al. 2003 and references therein). A

similar result was also found in Paper V where large fish more than small

sought shelter at the bottom under logs during trials. However, the growth data

in paper IV also indicated a weak positive effect of wood for small trout,

especially during the first half of the study period. Theoretically, habitat

segregation by fish size in wood treatments may, relative to no-wood

treatments, have decreased the intraspecific niche overlap, relaxed the

competition and thus been positive for all trout (Schlosser 1987). To an

unknown extent, also small trout may have inhabited and directly benefited

from the wood habitats. The less consistent effect for small trout over season is

more difficult to explain but might be linked to the increasing water levels from

June to July-August making more riffles available. This may then have resulted

in reduced dependence on wood habitats for especially small fish and thus

reduced the growth differences between wood and no-wood habitats.

The dietary data from paper IV suggest that large wood did not increase the

proportion of aquatic prey despite an established higher aquatic prey biomass in

the wood-treatments (paper III). However, as diet sampling was conducted

around midday and only twice over the study period it is difficult to draw any

firm conclusions about foraging in the field. The observed positive effects on

growth in paper IV may still be related to prey intake but can also be related to

decreased energy expenditure in wood-habitats. Support for an increased net-

energy gain in wood habitats may actually be found in paper V where the PTA-

values for trout living in complex wood habitats were significantly higher than

for trout in simple wood-free habitats.

Despite the increase in photosynthetic active radiation (PAR) to the stream, no

effects on the stream biota could be observed in the study in paper II. The

29

hypothesis was that a high increase in solar radiation during the whole growing

season would positively influence the primary production and thus induce a

bottom-up effect on macroinvertebrates (i.e. grazers) and subsequently on fish.

The lack of such results either indicates that light was insufficient compared to

periphyton requirements or that light, as a single factor, was unable to stimulate

growth of periphyton. Given that the light-enhanced enclosures had light levels

of 55-60 µmol m-2 s-1 and that stream periphyton is light saturated at

approximately 150 µmol m-2 s-1 (Hill et al. 2001), light levels were probably far

from optimal (due to technical and logistical constraints further increasing of

light was not feasible). Nevertheless, moderate light increases from 20-25 to 60

µmol m-2 s-1 have been shown to increase photosynthesis in periphyton (Hill et

al. 1995), and thus the lack of differences in algal and macroinvertebrate

abundance between light-enhanced and non-enhanced enclosures was still

rather unexpected.

There may be interactions between light, nutrients and temperature, obfuscating

any measurable light effect (Rosemond 1993, Hill et al. 1995, Rosemond et al.

2000). Hill et al. (1995) argued that even though light probably functions as the

limiting factor in most shaded streams it may be substituted by nutrients when

light levels rise above a minimum level. Similarly, Stockner and Shortreed (1978,

see also Allan 1995) observed that increased solar radiation had very little effect

on primary production unless nutrients were added to the system. Ambrose et

al. (2004) also reported that increased light had no effect on growth of algae in

nutrient-poor streams, but did have an effect in streams with a naturally high

nutrient content. Hence, even if light levels were sufficient for increased

photosynthesis, the highly oligotrophic water in Sundtjärnsbäcken may have

limited the growth of algae.

Concluding remarks and future research

The papers included in this thesis describe some of the relationships between

riparian forests, brown trout and their associated prey in boreal streams. Based

on my results it is apparent that forests may have profound effects on stream

biota in general and brown trout in particular. The papers focusing on terrestrial

invertebrates demonstrate the importance of this subsidy as a food resource for

trout while the papers focusing on large wood show that woody debris may

significantly influence the aquatic macroinvertebrates community and have

30

effects on both behavior and growth of brown trout. When combining the

positive effects from wood addition on aquatic macroinvertebrate biomass in

the field studies with the likely negative effects of reduced terrestrial

invertebrate input (paper III), the four treatments may be ranked according to

their potential prey availability. The addition of wood combined with ambient

terrestrial inputs (W) offered the highest potential food base which resulted in

growth rates of trout in paper IV being highest here, whereas reduced

terrestrial input in the absence of large wood (TR) had the lowest food base and

growth rates of trout were also lowest here. The two remaining treatments (U

and WTR) had intermediate invertebrate biomasses and trout growth rates.

These results illustrate a relationship between food abundance and growth

(Bilby and Bisson 1987, Elliott 1994) and in this case the indirect effects of large

wood and direct effects of terrestrial invertebrate subsidies. Managers should

thus recognize both terrestrial invertebrate input and recruitment of large wood

when assessing the full potential of streams to support fish. The positive effects

from large wood also propose that adding trees to streams may be a way to

compensate for negative effects associated with riparian deforestation.

The field experiments in this thesis were relatively long-term and conducted in a

forest stream with enclosures mimicking natural conditions at a fairly large scale.

Nevertheless, one question is whether the effects of the manipulations would be

the same in an open system, constant through time and have long term effects

on trout fitness and life-history? For instance, it could be argued that the effects

of large wood may differ within and between years or that an unrestricted

movement of fish may dampen the effects. It is however also recognized that

the survival of juvenile salmonids during winter largely depend on their growth

during the previous summer (Shuler et al. 1994 and references therein, Quinn and

Peterson 1996) why better growth in my wood-treatments possibly would

enhance winter-survival. As suggested by Gowan and Fausch (1996), the

addition of large wood to a stream may also increase total population biomass if

fish, that otherwise might not have survived, fill up those areas abandoned by

other fish migrating to more profitable (wood) habitats. In addition, the impact

of terrestrial invertebrates, whose phenology is largely linked to weather

conditions may also vary both temporally and quantitively. However, paper II,

III and IV, with focus on terrestrial invertebrates during two subsequent years

showed similar effects on trout growth despite large differences in both timing

and magnitude between years. Thus, even with an array of potentially

obfuscating variables, some stressors may still be persistent through time. These

31

findings also corroborate the conclusions of Fausch et al. (2010), who report

that even when inputs of terrestrial invertebrates varied considerably between

years this did not change the actual result on stream biota.

Many of the fundamental physical effects of forestry on lotic environments may

be similar, regardless of the region studied. It is, however, also well known that

forest-stream relations involve a complex array of elements interacting together

with an outcome often depending on the particular system. For instance, the

ways forestry effects are reflected in a population of fish may depend on fish

species, population structure and fish body-size. These difficulties are stressed

by Northcote and Hartman (2004) who argue that a detailed and quantitative

understanding of the links between forests and stream ecosystems should be

considered as a key issue for future research. Moreover, Crisp et al. (in

Northcote and Hartman 2004) state that the historical lack of methodical

thoroughness in experimental designs, complemented with mostly short-term

studies may lead to risky generalizations. Research designed to reveal the relative

importance of different factors linked to forestry affecting stream biota has also

been recognized as important for stream managers (Fausch et al. 2010).

Although difficult in many ways, a major undertaking and a central approach for

future research should thus be to continue untangling the factors involved in

forest-stream relations in boreal systems and to study both their individual and

combined effects over a broad spectrum of conditions. I also suggest that

research continues to combine field experiments with natural-like laboratory

experiments and expand the work to other study periods in an attempt to reveal

the effects from different stressors on the whole life-cycle of trout. Finally,

besides providing data over direct effects on prey biomass, trout growth and

behavior, it is also essential that research examine the quantitative responses

from a range of realistic levels from various stressors.

Acknowledgments

In retrospect, I admit that when I started as a PhD-student back in 2005 I was

not fully aware of all the challenges and the sometimes enormous workload I

had in front of me. A naivety for which I´m quite thankful… At the same time,

I’m very pleased and satisfied that I got the opportunity to go through with this,

it has been really interesting and worth the effort! I have beyond doubt learned

a lot, visited places I never would have visited and met people from all over the

32

world. So, when things now are about to be wrapped up, the feeling of relief

and joy is naturally accompanied with a tinge of sadness.

I wish to thank all my colleagues at the Biology Department for offering a

relaxed, but yet a stimulating atmosphere for doing research. It has been some

really nice years and I will surely miss you. My hope is that in the future I will

have the opportunity to work with at least some of you again. Special

recognition and very much appreciation of course, go to my supervisors Dr.

Eva Bergman and Professor Larry Greenberg. Your highly professional advice

and comments have been a main factor in progressively improving my thinking,

writing and, hopefully, also my research – thank you! I hope we will stay in

contact; we can always discuss house renovation, antiques or child-rearing but

we also have some papers to publish☺! Much gratitude also to the

undergraduate students who have been involved in my different projects –

Tomas Axelsson, Oskar Calsson, Matilda Elgerud, Lizel Glenn and David

Zetterqvist – your efforts and assistance both in the field and the lab have been

fundamental and I am greatly indebted to you! I also would like to thank my

Hungarian friend and co-worker from paper II, Dr. Tibor Erős, for the year we

spent working together. Despite the insane mosquito clouds, flooding events

and “midnight-lunches” in Glaskogen during field work, it was unquestionably a

very happy time! Many thanks also go to Mats Gustafsson and Annika Nyholm

who, besides being good friends also provided invaluable support in many

practical and “nutritional” matters during my field work.

Finally, my beloved Mia, Kerstin and Erik; during periods of pressure and

frustration you have been my sanctuary, and your unconditional love and

support have been invaluable for me to complete this work - you are my everything

and my future.

References

Allan, J. D. (1995). Stream Ecology. Structure and function of running waters.

Kluwer Academic Publishers.

Allan, J. D., Wipfli, M. S., Caouette, J. P., Prussian, A. and Rodgers, J. (2003).

Influence of streamside vegetation on inputs of terrestrial invertebrates

to salmonid food webs. Canadian Journal of Fisheries and Aquatic

Sciences, 60: 309-320.

33

Allen, K. R. (1951). The Horowiki stream: a study of a trout population. N.Z.

Mar. Dep. Fish. Bull., 10: 1-231.

Ambrose, H. E. and Wilzbach, M. A. (2004). Periphyton response to increased

light and salmon carcass introduction in northern California streams. J.

N. Am. Benthol. Soc., 23: 701-712.

Armstrong, J. D., Kemp, P. S., Kennedy, G. J. A., Ladle, M. and Milner, N. J.

(2003). Habitat requirements of Atlantic salmon and brown trout in

rivers and streams. Fish. Res., 62: 143–170.

Ayllón, D., Almodovar, A., Nicola, G. G. and Elvira, B. (2010). Ontogenetic

and spatial variations in brown trout habitat selection. Ecology of

Freshwater Fish, 19: 420–432.

Bachman, R. A. (1984). Foraging behaviour of free ranging wild and hatchery

brown trout in a stream. Trans. Am. Fish. Soc. 119: 1-32.

Barton, D. R., Taylor, W. D. and Biette, R. M. (1985). Dimensions or riparian

buffer strips required to maintain trout habitat in southern Ontario

streams. North American Journal of fisheries Management, 5: 364-378.

Basquill, S. P. and Grant, J. W. A. (1998). An increase in habitat complexity

reduces aggression and monopolization of food by zebra fish (Danio

rerio). Canadian Journal of Zoology, 76: 770-772.

Baxter, C. V., Fausch, K. D. and Saunders, W. C. (2005). Tangled webs:

reciprocal flows of invertebrate prey link streams and riparian zones.

Freshwater Biology, 50: 201-220.

Baxter, C. V., Fausch, K. D., Murakami, M. and Chapman, P. L. (2007).

Invading rainbow trout usurp a terrestrial prey subsidy from native charr

and reduce their growth and abundance. Oecologia, 153: 461-470.

Benke, A. C. and Wallace, J. B. (2003). Influence of Wood on Invertebrate

Communities in Streams and Rivers. American Fisheries Society

Symposium, 37: 149-177.

Bilby, R. E., and Likens, G. E. (1980). Importance of organic debris dams in the

structure and function of stream eco-systems. Ecology, 61: 1107-1113.

Bilby, R. E. and Bisson, P. A. (1987). Emigration and production of hatchery

coho salmon (Oncorhynchus kisutch) stocked in streams draining an

old-growth and a clear-cut watershed. Canadian Journal of Fisheries and

Aquatic Sciences, 44: 1397-1407.

Bilby, R. E. and Ward, J. W. (1989). Changes in characteristics and function of

woody debris with increasing size of streams in western Washington.

Transactions of the American Fisheries Society, 118: 368-378.

34

Bilby, R. E. and Ward, J. W. (1991). Characteristics and function of large woody

debris in streams draining old-growth, clear-cut and second-growth

forests in southwestern Washington. Canadian Journal of Fisheries and

Aquatic Sciences, 48: 2499-2508.

Bisson, P. A., Bilby, R. E., Bryant, M. D., Dolloff, C. A., Grette, G. B., House,

R. A., Murphy, M., Koski, V. and Sedell, J. R. (1987). Large Woody

Debris in forested streams in the Pacific Northwest: past, present and

future. In Salo, E. O. and Cundy, T. W. (ed), Streamside Management:

Forestry and Fisheries interactions. No. 57, Institute of Forest

Resources, University of Washington: 143-190.

Bohlin, T. (1977). Habitat selection and intercohort competition of juvenile sea-

trout Salmo trutta. Oikos, 29: 112-117.

Bond, N. R., Sabater, S., Glaister, A., Roberts, S. and Vanderkruk, K. (2006).

Colonization of introduced timber by algae and invertebrates, and its

potential role in aquatic ecosystem restoration. Hydrobiologia, 556: 303-

316.

Bridcut, E. E. (2000). A study of terrestrial and aerial macroinvertebrates on

river banks and their contribution to drifting fauna and Salmonid diets in

a Scottish catchment. Hydrobiologia, 427: 83-100.

Bridcut, E. E. and Giller, P. S. (1995) Diet variability and foraging strategies in

brown trout (Salmo trutta): an analysis from subpopulations to

individuals. Canadian Journal of Fisheries and Aquatic Sciences, 52:

2543-2552.

Cloe, W. W. and Garman, G. C. (1996). The energetic importance of terrestrial

arthropod inputs to three warm-water streams. Freshwater Biology, 36:

105-115.

Cummins, K. W. and Wuycheck, J. C. (1971). Caloric equivalents for

investigations in ecological energetics. Int. Ver. Theor. Angew. Limno.

Verh., 18: 1-158.

Dahlström, N. (2005). Function and dynamics of woody debris in boreal forest

streams. Doctoral dissertation thesis, Department of Ecology and

Environmental Science, Umeå University, Sweden.

Degerman, E., Sers, B., Törnblom, J. and Angelstam, P. (2004). Large woody

debris and brown trout in small forest streams – towards targets for

assessment and management of riparian landscapes. Ecological Bulletins,

51: 233-239.

35

Dineen, G., Harrison S. S. C. and Giller P. S. (2007a). Diet partitioning in

sympatric Atlantic salmon and brown trout in streams with contrasting

riparian vegetation. Journal of Fish Biology, 71: 17-38.

Dineen, G., Harrison S. S. C. and Giller P. S. (2007b). Seasonal analysis of

aquatic and terrestrial invertebrate supply to streams with grassland and

deciduous riparian vegetation. Biology and Environment: proceedings of

the royal Irish academy, 107b: 167-182.

Dolloff, C. A. and Warren, M. L. Jr. (2003). Fish Relationships with Large

Wood in Small Streams. American Fisheries Society Symposium, 37:

179-193.

Drury, D. M. and Kelso, W. E. (2000). Invertebrate colonization of woody

debris in coastal plain streams. Hydrobiologia, 434: 63-72.

Dunbrack, R. L. and Dill, L. M. (1983). A model of size dependent surface

feeding in a stream dwelling Salmonid. Environmental Biology of Fishes,

8: 203-216.

Edwards, E. D. and Huryn, A. D. (1996). Effect of riparian land use on

contributions of terrestrial invertebrates to streams. Hydrobiologia, 337:

151-159.

Elliott, J. M. (1994). Quantitative ecology and the brown trout. Oxford

University press.

Fausch, K. D. (1984). Profitable stream positions for salmonids – relating

specific growth-rate to net energy gain. Canadian Journal of Zoology, 62:

441–451.

Fausch, K. D. and White, R. J. (1986). Competition among Juveniles of Coho

Salmon, Brook Trout, and Brown Trout in a Laboratory Stream, and

Implications for Great Lakes Tributaries. Transactions of the American

Fisheries Society, 115: 363-381

Fausch, K. D. and Northcote, T. G. (1992). Large Woody Debris and Salmonid

Habitat in a Small Coastal British-Columbia Stream. Canadian Journal of

Fisheries and Aquatic Sciences, 49: 682-693.

Fausch, K. D. (1993). Experimental Analysis of Microhabitat Selection by

Juvenile Steelhead (Oncorhynchus mykiss) and Coho Salmon (O.

kisutch) in a British Columbia Stream. Canadian Journal of Fisheries and

Aquatic Sciences, 50: 1198–1207.

Fausch. K. D., Baxter, C. V. and Murakami, M. (2010). Multiple stressors in

north temperate streasm: lessons from linked forest-stream ecosystems

in northern Japan. Freshwater Biology, 55: 120-134.

36

Feminella, J. W., Power, M. E. and Resh, V. H. (1989). Periphyton responses to

invertebrate grazing and riparian canopy in three northern California

coastal streams. Freshwater Biology, 22: 445-457.

Flebbe, P. A. and Dolloff, C A. (1995). Trout use of Woody Debris and Habitat

in Appalachian Wilderness Streams of North Carolina. North American

Journal of fisheries Management, 15: 579-590.

Gowan, C. and Fausch, K. D. (1996). Long-term demographic responses of

trout populations to habitat manipulation in six Colorado streams.

Ecological applications, 6: 931-946.

Greenberg, L., Svendsen, P. and Harby, A. (1996). Availability of microhabitats

and their use by brown trout (Salmo trutta) and grayling (Thymallus

thymallus) in the River Vojman, Sweden. Regulated rivers-research and

management, 12: 287-303.

Growns, I., Gehrke, P. C., Astles, K. L., and Pollard, D. A. (2003). A

comparison of fish assemblages associated with different riparian

vegetation types in the Hawkesbury-Nepean River system. Fish. Manage.

Ecol., 10: 209-220.

Hax, C. L. and Golladay, S. W. (1993). Macroinvertebrate colonization and

biofilm development on leaves and wood in a boreal river. Freshwater

Biology, 29: 79-87.

Heggenes, J. (1996). Habitat selection by brown trout (Salmo trutta) and young

Atlantic salmon (S. salar) in streams: static and dynamic hydraulic

modelling. Regulated Rivers: Research and Management, 12: 155–169

Heggenes, J., Bagliniére J. L. and Cunjak, R. A. (1999). Spatial niche variability

for young Atlantic salmon (Salmo salar) and Brown trout (S. trutta) in

heterogeneous streams. Ecology of Freshwater Fish, 8: 1-21

Hilderbrand, R. H., Lemly, A. D., Dolloff, C. A. and Harpster K. L. (1997).

Effects of large woody debris placement on stream channels and benthic

macroinvertebrates. Canadian Journal of Fisheries and Aquatic Sciences,

54: 931-939.

Hernandez, O., Merritt, R. W. and Wipfli, M. S. (2005). Benthic invertebrate

community structure is influenced by forest succession after clearcut

logging in southeastern Alaska. Hydrobiologia, 533: 45-59.

Hill, W. R., Ryon, M. G. and Schilling, E. M. (1995). Light limitation in a

stream ecosystem: responses by primary producers and consumers.

Ecology, 76: 1297-1309.

Hill, W. R., Mulholland P. J. and Marzolf, E. R. (2001). Stream ecosystem

responses to forest leaf emergence in spring. Ecology, 82: 2306-2319.

37

Hill, W. R. and Dimick, S. M. (2002). Effects of riparian leaf dynamics on

periphyton photosynthesis and light utilization efficiency. Freshwater

Biology, 47: 1245-1256.

Hoffman, A. and Hering, D. (2000). Wood-associated Macroinvertebrate Fauna

in Central European Streams. Internat. Rev. Hydrobiol., 85: 25-48.

Holling, C. S. (1959). Some characteristics of simple types of predation and

parasitism. Canadian entomologist, 91: 385-398.

Hughes, N. F. and Dill, L. M. (1990).Position Choice by Drift-Feeding

Salmonids: Model and Test for Arctic Grayling (Thymallus arcticus) in

Subarctic Mountain Streams, Interior Alaska. Canadian Journal of

Fisheries and Aquatic Sciences, 47: 2039-2048.

Hyatt, T. L. and Naiman, R. J. (2001). The residence time of large woody debris

in the Queets River, Washington, USA. Ecological Applications, 11: 191-

202.

Hynes, H. B. N. (1970). The ecology of running waters. Toronto: University of

Toronto Press.

Imre, I., Grant, J. W. A., and Keeley, E. R. (2002). The effects of visual isolation

on territory size and population density of juvenile rainbow trout

(Oncorhynchus mykiss). Canadian Journal of Fisheries and Aquatic

Sciences, 59: 303–309.

Johnson, L. B., Breneman, D. B. and Richards, C. (2003). Macroinvertebrate

community structure and function associated with large wood in low

gradient streams. River Research and Applications, 19: 199-218.

Jones, B. C. J. and Despland, E. (2006). Effects of synchronization with host

plant phenology occur early in the larval development of a spring

folivore. Can. J. Zool., 84: 628-633.

Kalleberg, H. (1958). Observations in a stream tank of territoriality and

competition in juvenile salmon and trout (Salmo salar L. and S. trutta L.).

Report of the Institute of Freshwater Research, Drottningholm, 39: 55–

98.

Kawaguchi, Y. and Nakano, S. (2001). Contribution of terrestrial invertebrates

to the annual resource budget for salmonids in forest and grassland

reaches of a headwater stream. Freshwater Biology, 46: 303-316.

Kawaguchi, Y., Taniguchi, Y. and Nakano, S. (2003). Terrestrial invertebrate

input determine the local abundance of stream fishes in a forested

stream. Ecology, 84: 701-708.

38

Keim, R. F., A. E. Skaugset, D. S. Bateman. (2002). Physical Aquatic Habitat II.

Pools and Cover Affected buy Large Woody Debris in Three Western

Oregon Streams. North American Journal of Fisheries Management, 22:

151-164.

Keller, E. A. and Swanson, F. J. (1979). Effects of large organic material on

channel form and fluvial processes. Earth Surf. Process., 4: 361–380

Kemp, P. S., Armstrong, J. D. and Gilvear, D. J. (2005). Behavioural responses

of juvenile Atlantic salmon (Salmo salar) to presence of boulders. River

Research and Implications, 21: 1053–1060.

Kiffney, P. M., Richardson, J. S. and Bull, J. P. (2003). Responses of periphyton

and insects to experimental manipulation of riparian buffer width along

forest streams. J. Appl. Ecol., 40: 1060-1076.

Kiffney, P. M., Richardson, J. S. and Bull, J. P. (2004). Establishing light as a

causal mechanism structuring stream communities in response to

experimental manipulation of riparian buffer width. J. N. Am. Benthol.

Soc., 23: 542-555.

Lehane, B. M., Giller, P. S., O'Halloran, J., Smith, C. and Murphy, J. (2002).

Experimental provision of large woody debris in streams as a trout

management technique. Aquatic conservation: Marine and Freshwater

ecosystems, 12: 289-311.

Lisle, T. E. (1986). Effects of Woody Debris on Anadromous Salmonid habitat,

Prince of Wales Island, Southeast Alaska. North American Journal of

Fisheries Management, 6: 538-550.

Mason, C. F. and MacDonald, S. M. (1982). The input of terrestrial

invertebrates from tree canopies to a stream. Freshwater Biology, 12:

305-311.

Montori, A., De Figueroa J. M. T. and Santos X. (2006). The diet of the brown

trout Salmo trutta (L.) during the reproductive period: Size-related and

sexual effects. International Review of Hydrobiology, 91: 438-450.

Murphy, M. L. and Koski K. V. (1989). Input and Depletion of Woody Debris

in Alaska Streams and Implications for Streamside Management. North

American Journal of Fisheries Management, 9: 427-436.

Mäki-Petäys, A., Muotka, T., Huusko, A., Tikkanen, P. and Kreivi, P. (1997).

Seasonal changes in habitat use and preference by juvenile brown trout,

Salmo trutta, in a northern boreal river. Canadian Journal of Fisheries

and Aquatic Sciences, 54: 520-530.

Naiman, R. J. and Decamps, H. (1997). The ecology of interfaces: Riparian

zones. Annual Review of Ecology and Systematics, 28: 621-658.

39

Nakano, S. (1995). Individual differences in resource use, growth and

emigration under the influence of a dominance hierarchy in fluvial red-

spotted masu salmon in a natural habitat. Journal of Animal Ecology, 64:

75-84.

Nakano, S., Kawaguchi, Y., Taniguchi, Y., Miyasaka, H., Shibata, Y., Urabe, H.

and Kuhara, N. (1999a). Selective foraging on terrestrial invertebrates by

rainbow trout in a forested headwater stream in northern Japan.

Ecological Research, 14: 351-360.

Nakano, S., Miyasaka H. and Kuhara, N. (1999b). Terrestrial-aquatic linkages:

riparian arthropod inputs alter trophic cascades in a stream food web.

Ecology, 80: 2435-2441.

Nakano, S. and Murakami, M. (2001). Reciprocal subsidies: dynamic

interdependence between terrestrial and aquatic food webs. Proceedings

of the National Academy of Sciences USA, 98: 166-170.

Northcote, T. G. and Hartman, G. F. (2004). Fishes and Forestry – Worldwide

Watershed Interactions and Management. Blackwell publishing.

O’Brien, W. J. and Showalter, J. (1993). Effects of current velocity and

suspended debris on the drift feeding of Arctic grayling. Trans. Am.

Fish. Soc., 122: 609–615.

O’Connor, N. A. (1992). Quantification of submerged wood in a lowland

Australian stream system. Freshwater Biology, 27: 387-395.

Polis, G. A., Anderson, W. B. and Holt, R. D. (1997). Toward an integration of

landscape and food web ecology: the dynamics of spatially subsidized

food webs. Annual review of Ecology and Systematics, 28: 289-316.

Power, M. E., Matthews, W. J. and Stewart, A. J. (1985). Grazing minnows,

piscivorous bass, and stream algae: dynamics of a strong interaction.

Ecology, 66: 1448-1456.

Power, M. E. (1987). Predator avoidance by grazing fishes in temperate and

tropical streams: importance of stream depth and prey size. In Predation

(Ed. Kerfoot, C. W. and Sih, A.), University Press of New England.

Quinn, T. P. and Peterson, N. P. (1996). The influence of habitat complexity

and fish size on over-winter survival and growth of individually marked

juvenile coho salmon (Oncorhynchus kisutch) in Big Beef Creek,

Washington. Canadian Journal of Fisheries and Aquatic Sciences, 53:

1555-1564.

Reinhardt, U. G. and Healey, M. C. (1997). Size-dependent foraging behaviour

and use of cover in juvenile coho salmon under predation risk. Can. J.

Zool., 75: 1642–1651.

40

Richmond, A. D. and Fausch, K. D. (1995). Characteristics and function of

large woody debris in subalpine Rocky Mountain streams in northern

Colorado. Canadian Journal of Fisheries and Aquatic Sciences, 52: 1789-

1802.

Riley, S. C. and Fausch, K. D. (1995). Trout population response to habitat

enhancement in six northern Colorado streams. Canadian Journal of

Fisheries and Aquatic Sciences, 52: 34-53.

Roni, P. and Quinn, T. P. (2001). Density and size of juvenile salmonids in

response to placement of large woody debris in western Oregon and

Washington streams. Canadian Journal of Fisheries and Aquatic

Sciences, 58: 282-292.

Roberts, S., Sabater, S. and Beardall, J. (2004). Benthic micro algal colonization

in streams of differing riparian cover and light availability. Journal of

Phycology, 40: 1004-1012.

Rosemond, A. D. (1993). Interactions among irradiance, nutrients, and

herbivores constrain a stream algal community. Oecologia, 94: 585-594.

Rosemond, A. D., Mulholland, P. J. and Brawley, S. H. (2000). Seasonally

shifting limitation of stream periphyton: response of algal populations

and assemblage biomass and productivity to variation in light, nutrients

and herbivores. Can. J. Fish. Aquat. Sci., 57: 66-75.

Savino, J. F. and Stein, R. A. (1982). Predator-prey interaction between

largemouth bass and bluegills as influenced by simulated, submersed

vegetation. Trans. Am. Fish. Soc., 111: 255-266.

Saunders, J. W. and Smith, M. W. (1962). Physical Alteration of Stream Habitat

to Improve Brook Trout Production. Transactions of the American

Fisheries Society, 91: 185-188.

Schlosser, I. J. (1987). The role of predation in age- and size-related habitat use

by stream fishes. Ecology, 68: 651-659.

Schutz, D. C. and Northcote, T. G. (1972). Experimental study of feeding

behavior and interaction of coastal cutthroat trout (Salmo clarki-clarki)

and Dolly Varden (Salvelinus malma). Journal of the fisheries Research

Board of Canada, 29: 555-565.

Shirvell, C. S. (1990). Role of instream rootwads as juvenile Coho salmon

(Oncorhynchus kisutch) and Steelhead trout (O. mykiss) cover habitat

under varying streamflows. Canadian Journal of Fisheries and Aquatic

Sciences, 47: 852–861.

41

Shuler, S. W., Nehring, R. B. and Fausch, K. D. (1994). Diet Habitat Selection

by Brown Trout in the Rio Grande River, Colorado, after Placement of

Boulder Structures. North American Journal of fisheries Management,

14: 99-111.

Speaker, R. W., Moore, K. and Gregory, S. (1984). Analysis of the process of

retention of organic matter in stream ecosystems. Verh. Internat. Verein.

Limnol., 22: 1835-1841.

Spänhoff, B., Alecke, C. and Meyer, E. I. (2000). Colonization of submerged

twigs and branches of different wood genera by aquatic

macroinvertebrates. Internat. Rev. Hydrobiol., 85: 49-66.

Stockner, J. G. and Shortreed, K. R. S. (1978). Enhancement of autotrophic

production by nutrient addition in a coastal rainforest stream on

Vancouver Island. J. Fish. Res. Board. Can., 35: 28-34.

Stott, T. and Marks, S. (2000). Effects of plantation forest clearfelling on stream

temperatures in the Plynlimon experimental catchments, mid-Wales.

Hydrology and Earth System Science, 4: 95-104.

Sundbaum, K. and Näslund, I. (1998). Effects of woody debris on the growth

and behaviour of brown trout in experimental stream channels. Canadian

Journal of Zoology, 76: 56-61.

Sweka, J. A. and Hartman, K. J. (2006). Effects of large wood debris addition

on stream habitat and Brook trout populations in Appalachian streams.

Hydrobiologia, 559: 363–378.

Sweka, J. A. and Hartman, K. J. (2008). Contribution of Terrestrial

Invertebrates to Yearly Brook Trout Prey Consumption and Growth.

Transactions of the American Fisheries Society, 137: 224-235.

Teixeira, A. R., Cortes M. V. and Oliveira, D. (2006). Habitat use by native and

stocked trout (Salmo trutta L.) in two Northeast streams, Portugal. Bull

Fr Peche Pisc., 382: 1-18.

Twomey, H. and Giller P. S. (1990) Stomach flushing and individual Panjet

tattoing of salmonids: an evaluation of the long-term effects on two wild

populations. Aquaculture and Fisheries Management, 21, 137-142.

Urabe, H. and Nakano, S. (1998). Contribution of woody debris to trout habitat

modification in small streams in secondary deciduous forest, northern

Japan. Ecological Research, 13: 335-345.

Utz, R. M. and Hartman, K. J. (2006). Temporal and spatial variation in the

energy intake of a brook trout (Salvelinus fontinalis) population in an

Appalachian watershed. Can. J. Fish. Aquat. Sci., 63: 2675-2686.

42

Vannote, R. L., Minshall, G. W., Cummins, K. W., Sedell, J. R. and

Cushing, C. E. (1980). The river continuum concept. Canadian Journal

of fisheries and Aquatic Science, 37: 130-137.

Videler, J. J. (1993). Fish Swimming. Chapman and Hall, London.

Wallace, J. B. and Benke, A. C. (1984). Quantification of wood habitat in

subtropical coastal plain streams. Canadian Journal of Fisheries and

Aquatic Sciences, 41: 1643-1652.

Webster, J. J. and Hartman, K. J. (2005). The role of terrestrial invertebrates in

allopatric brook trout headwater streams in the central Appalachian

Mountains. Journal of Freshwater Ecology, 20: 101-107.

Werner, E. E., Mittelbach, G. G., Hall, D. J. and Gilliam, J. E. (1983). An experimental test of predation risk on habitat use in fish. Ecology, 64: 1540-1548.

Wilzbach, M. A., Cummins, K. W. and Hall, J. D. (1986). Influence of habitat

manipulations on interactions between cutthroat trout and invertebrate

drift. Ecology, 67: 898-911.

Wipfli, M. S. (1997). Terrestrial invertebrates as salmonid prey and nitrogen

sources in streams: contrasting old-growth and young-growth riparian

forests in southeastern Alaska, U.S.A. Canadian Journal of Fisheries and

Aquatic Sciences, 54: 1259-1269.

Zadorina, V. M. (1988). Importance of adult insects in the diet of young trout

and salmon. Voprosy Ikhtiology, 2: 259-265.

Karlstad University StudiesISSN 1403-8099

ISBN 978-91-7063-335-5

Forest – stream linkages

The five papers in this thesis focus on woody debris, terrestrial invertebrates and light, three factors influenced by riparian zone structure, potentially affecting streams and brown trout (Salmo trutta). Using a combination of laboratory and field experiments, I examined the effects of these factors on prey availability and on the growth rates, diets and beha-vior of different size-classes of trout. My field experiments showed that addition of high densities of large wood affected trout growth positively. This positive effect of large wood on trout growth may be related to prey abundance, as indicated by the high standing crop of aquatic macroinvertebrates on the wood. The positive effects on trout may also be related to decreased energy expenditures in wood habitats, as trout increased the ratio between numbers of prey captured and time spent active and that swimming activity and level of aggression decreased as wood densities were increased in a laboratory experiment. Terrestrial invertebrates are generally assumed to be a high quality prey resource for fish and my field experiments showed that a reduction of terrestrial invertebrate inputs had a negative effect on trout growth. The availability of terrestrial prey in the stream was also coupled to trout diet and linked to growth, as fish with high growth rates had high proportions of terrestrial prey in their diets. Light, measured as PAR, did not have an effect on chlorophyll biomass, nor was there an effect on aquatic macroinvertebrates or trout. This result may be explained by that other factors, such as low nutrient content, may have limited the effects. Many of my results were dependent on fish-size. I observed, for example, that large trout had higher capture rates on surface-drifting terrestrial prey than small trout when prey densities were intermediate or high, but at low prey densities, the consumption of terrestrial prey by large and small trout were similar. Moreover, although large wood and terrestrial invertebrates affected growth of both small and large trout, the effects were generally more consistent for large trout.

Changes in riparian forests typically induce an array of interacting effects that call for further research. However, this thesis show that some factors may have profound effects on stream biota also as single stressors and that positive effects from wood, by adding trees to streams, may partly compensate for negative effects associated with riparian deforestation.