Evelyn F. Cox Thesis Committee: John S. Stimson, Chairman ... · ACKNOWLEDGEMENTS To John Stimson,...
Transcript of Evelyn F. Cox Thesis Committee: John S. Stimson, Chairman ... · ACKNOWLEDGEMENTS To John Stimson,...
ASPECTS OF CORALLIVORY BY
CHAETODON UNIMACULATUS
IN KANE'OHE BAY, OIAHU
A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
IN ZOOLOGY
By
Evelyn F. Cox
----------
Thesis Committee:
John S. Stimson, Chairman Stephen V. Ralston
S. Arthur Reed
We certify that we have read this thesis and that, in
our opinion, it is satisfactory in scope and quality as a
thesis for the degree of Master of Science in Zoology.
THESIS COMMITTEE
- . .. Chairman
\
ACKNOWLEDGEMENTS
To John Stimson, mahalo nui loa, for all his help
during this project. Many.thanks to Stev.e Ralston, Art
Reed, other member s of the f acul ty and staff of the
Department of Zoology, and my fellow graduate students.
Hawai'i Institute of Marine Biology furnished aquaria,
other supplies and the means of transportation to my
study sites in Kane'ohe Bay. And to Lester Zukeran, a
special thank you for supplying m~ with experimental
animals.
iii
ABSTRACT
Interactions between a corallivore, Chaetodon
unimaculatus, and the two dominant coral species in
Kane'ohe Bay, Montipora verrucosa and Porites ~pressa,
were investigated. Feeding selectivity was tested in
laboratory and field observations, with the fish clearly
selecting H. verrucosa: 39:1 bites in laboratory trials and
284:1 bites in field observations. Using an estimated
bite size of 2.54 mg AFDW and two estimated feeding rates,
4.88 bites min- l during the ndryn season (May to
September) and 7.20 bites min- l during the nwet" season
(October to April), an average sized fish consumes
approximately 4000 g of coral tissue each year, and the
population of ~. unimaculatus on Patch Reef #42 is
-removing approximately 10% of the standing crop of
~. verrucosa each yea~. A series of experiments was
designed to measure the effect of predation by these fish
on growth and ·competition between the two corals. Caged
colonies of ~. verrucosa at Patch Reef #42 had a vertical
growth rate of 9.71 x 10-3cmday-l, and M. verrucosa
killed ~. compressa tissue it came in contact with. In
tincagedc6T6iiles, . .M~ verruca-sa· gtew-ata rate of
3.92 x 10-3cm day-I, about 40% of the caged growth rate,
and several colonies showed a reversal of aggressive
dominance as predicted from previous studies, with
~. &Qmpressa killing branches of M. verrucosa. On Patch
Reefs #42 and #4~ there is a significant increase in the
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percent M. yerrucosa with increased distance from the edge
of the reef, where the fish are normally found, and grazing
pressure is greatest. Because of its selective feeding
behavior in Kane'ohe Bay, ~. unimaculatus appears to have
a significant effect on the growth and distribution of its
preferred poral species, B. verrucosa.
v
TABLE OF CONTENTS
ACKNOWLEDGEMENTS • . . . · . · . · . . · . . . ABSTRACT . . . . . . . · . . • eo.
LIST OF TABLES • • • • • • • • • • • • • • • • •
LIST OF FIGURES • • • • • • • • • • 8 • • • • • &
CHAPTER I. INTRODUCTION
Introduction • • The Study Site • The Corallivore
• • o • 8 • • • •
• • · . . . . . . . . • • · . . . . . . . .
CHAPTER II. FEEDING SELECTIVITY AND THE RATE OF CORAL CONSUMPTION
Introduction • · • .. • • • • • 0
Methods • • • • • .. .. • • • • .. • Results . • • • • • • • • .. • • • Discussion • • • • • • • • • • .. •
CHAPTER III. EFFECT OF PREDATION ON GROWTH AND COMPETITION BETWEEN MONTIPORA VERRUCOSA AND PORITES COMPRESSA
• • • •
• .. • •
iii
iv
vi
viii
1 5 8
11 13 19 24
Introduction • • • • • • • .. • • • •• 28 Methods .. • • • • • • • • • • • • •• 30 Results • • • • • • • .. • • .. • • •• 32 Discussion • • • • • • • • • • • • •• 38
CHAPTER IV. DISTRIBUTION OF FISH AND CORAL
CHAPTER V. --------------- ----- ------- ---- ---- ---
Introduction • Methods • • • Results ••• Discussion • •
· . . . . . . . . . . . · . . . . . . . . . . . · . . . . . . . . . . . · . . . . · . . . . CONCLUSION • • • • • • • • • • • · . .
REFERENCES CITED . . . • • • • · . · . · . . . .
vi
41 42 44 51
53
56
LIST OF TABLES
Table Page
1. Reported diets of ~. unimaculatus •••• • • 10
2. Results of laboratory feeding preference trials on ~. uniroaculatus • • • • • • • • • • • •• 20
3. Observations of field feeding preferences, Patch Reef #42. •••••••••••••• 21
4. Average bite size (mg AFDW) for laboratory held ~. unimaculatus •••••••••••• •• 22
5. Comparison of feeding rates of ~. unimaculatus at different times of year and day at Patch Reef #42. ••••••••••••••••• 23
6.
7.
8.
9.
10.
11.
12.
13.
Standing crop of ~. verrucosa on Patch Reef #42. • ••••••• • eo.
Comparison of morphological and skeletal properties of B. verrucosa and 2. compressa. •••••••••••••
• •
• •
Comparison of coral growth ratef in Kane'ohe Bay (change in radius in cm yr- ) ••••••
Series 1: comparison of vertical grow~h of B. yerrucosa and ~. coropressa on caged and uncaged screens on Patch Reef #42 (cm/160 days). ••••••••••••••
Series 2: comparison of vertical growth of H. yerrucosa and 2. coropressa on caged and uncaged blocks on Patch Reef #42 (cm/130 days), analysis by randomized blocks ANOVA with replication. •••••••••••••••
Series 3:" comparison of vertical growth of H. yerrucosa and ~. coropressa on caged and uncaged blocks at MokuoLo'e (cm/IOO days),
- - analysis--oy---ianaomized bTocKs--ANOVA with -- --replication. •••••••••••••••
Mean no. quanta (integrated over 10 seconds, 4 readings/block) for caged and uncaged treatments at MokuoLo'e (std.dev.) •••••
Comparison of growth rates (cm x 10-3 day-I) for both corals at the two study sites. ••
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23
25
29
33
34
37
38
40
14.
15.
16.
Comparison of coral cover and C. unimaculatus population size on selected patch reefs in Kane'ohe Bay. • ••••••••••••••
Percent cover of ~. yerrucosa with increasing distance from the edge of Patch Reefs #42 and 143. • ••••••••••••••••
Comparison of the time spent by C. unimaculatus in concentric zones on Patch Reef #42 (19 observation periods) =
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" ..
45
50
51
Figure
1.
2.
3.
4.
LIST OF FIGURES
Patch reef locations in Kane'ohe Bay. . . . Map of Patch Reef 142. • • • • • • • • • •
Total coral cover and number of ~. unimaculatus on selected patch reefs in Kane'ohe Bay. • ••••
Percent M. verrucosa and number of ~. unimaculatus on selected patch reefs in K~ne'ohe Bay. • ••••
viii
. . . . .
Page
6
16
46
48
CHAPTER I
INTRODUCTION
Two species of fish, of the genus Scarus, which are common here, exclusively feed on coral •••• Mr. Liesk assured us, that he had repeatedly seen whole shoals grazing wi th their strong bony jaws on the tops of the coral branches: I opened the intestines of several, and found them distended with yellowish calcareous sandy mud. (Darwin [1839] 1972:pp.402)
Small fish swarm about the branching clumps, and when disturbed, seek shelter at once among the branches, where they are safe from pursuit. The author has often witnessed this, and never saw reason to suppose they clustered about the coral for any other purpose. It is an undoubted fact, as stated by Mr. Darwin, that fragments of coral and sand may be found in the stomaches of these animals, but this is not sufficient evidence of their browsing on the coral. Fish so carefully avoid polyps of all kinds because of their power of stinging ••• that we should wait for further and direct evidence on this point. (Dana, 1872: pp.195)
The debate concerning the impact of coral reef
fishes on the growth, survival and competition of corals
'has continued since Darwin's time. Early anecdotal
reports of the impacts of fishes exist. Bertram (1936)
wrote that fishes in the Red Sea could affect the survival
of corals by scraping small amounts of surface area and
exposing the coral to algae and ?t~er boring organisms.
Motoda (1940) experimentally transplanted colonies of a
massive coral~ Goniastrea aspera, from the reef top at
Palau to the reef margin. Within one week, all colonies
suffered damage to the upper portions, presumably due to
fishes. A colony transplanted to mid-channel and
suspended from a float suffered no damage, presumably due
1
to the lack of grazing fishes venturing into deeper water
and off the bottom. He also reported that Porites
somaliensis on the reef margin often had white areas which
he attributed to fish damage.
By the 1960's, it was acknowledged by leading coral
researchers (Wells, 1957; Yonge, 1968; Stoddart, 1968)
that fishes could damage corals, but the impacts of fishes
were downplayed, with the emphasis being placed instead on
the importance of bioerosion due to invertebrates and
other boring organisms. Randall (1974) summarized
observations of fishes feeding on corals, listing members
of the families of Scaridae, Tetraodontidae, Diodontidae,
Balistidae, and Chaetodontidae as coral predators.
However, he writes,
It would seem that these fishes are not fully exploiting the coral resource as food. Part of the answer for some seems to be preference for other kinds of nutriment. Also there may be other reasons for the lack of abundance of these fishes than availability of food •••• Whatever the answer or answers may be, corals have been spared from excessive depredation by fishes. (Randall, 1974:pp.165)
Glynn and co-workers on the Pacific coast of Panama
have quantified the effects of predation on growth of one
species of coral, estimating that 1/3 to 1/2 of the annual
growth of Pocillopora ~icornis is lost to predation by
invertebrate and vertebrate corallivores (Glynn &
Macintrye, 1977). A study of the gut contents of 14
specimens of Arothron meleagris showed that a majority of
items in the guts consisted of coral fragments.
2
A. meleagris kept in experimental aquaria broke off pieces
of coral from colonies, ingesting about 44% of the pieces
broken off, and assimilating about 10% of ,the material
ingested (Glynn et al., 1972).
Neudecker (1979) transplanted colonies of Pocillopora
.da..micornis from the reef flat to the fore reef at Guam and
recorded skeletal weight losses of up to 25% within one
week. He observed balistids and chaetodontids feeding on
the test colonies. Caged colonies in all depth zones grew
equally well, leading Neudecker to hypothesize that fish
predation is the prime factor preventing ~. damicornis
from colonizing the fore reef zone.
Wellington (1982) studied the effects of a herbivorous
damselfish, Eupomacentrus acapulcoensis, on the
distributions of Payona gigantea and Pocillopora
damicornis on reefs off the Pacific coast of Panama.
Unlike chaetodontids which randomly graze on polyps or
balist~ds which can break off branches, this damselfish
nips away large contiguous patches from ~. gigantea,
apparently not as a source of food but to create
suitable bare patches for algal growth. The distribution
of ~. gigantea appears to be controlled by the actions of
the damselfish,' which limits this coral to deeper areas
where there is a lack of suitable shelter for the
damselfish. g. ~icornis is more resistant to damage, as
the damselfish can only remove the tips of branches. In
the deeper areas, however, g. damicornis colonies are
3
subjected to much higher predation pressure from other
corallivores.
Because of the ability of the coral to retract its
polyps into protective calices, the effects of
corallivores are probably similiar to those of herbivorous
browsers or grazers. Most corals are able to recover from
superficial damage, (see e.g., Bak et al., 1977; Bak &
Es, 1980), and colonies are unlikely to be killed by this
type of feeding behavior. However,' there may be secondary
effects of foraging on the coral's ability to compete with
other corals for space on well covered substrates.
Competition for space between corals, by differential
growth rates, overtopping or direct aggression, has been
postulated to be a major community structuring force in
dense coral stands in stable environments (Lang, 1973;
Connell, 1978; Richardson et al., 1979; Sheppard, 1979,
1982). If a corallivore preferred an aggressively,
dominant species, it could prevent monopolization of space
by the dominant species. If the corallivore were
selectively foraging on competitively inferior species, it
could contribute to the rarity of these species.
The purpose of this research was to investigate the
effects of a corallivore on the growth and distribution of
its coral prey, first by identifying any feeding biases
and quantifying the rate of cor"al removal, and then by
assessing the impact of grazing on coral growth, distribution
and the outcome of interspecific competition.
4
THE STUDY SITE
Kane'ohe Bay, located on the windward side of the
island of Olahu at 21 degrees 28 minutes North latitude
and 157 degrees 48 minutes West longitude, is the largest
sheltered body of water in the main Hawaiian Islandse The
bay is approximately 12.8 km in length and 4.3 km wide.
Depth of the lagoon area is generally 12 m. Water
temperatures range from 19.5 0 to 27.80 Centigrade,
with a mean of 21.60 C in January and 27.40 C in August
(Smith et al., 1973).
The north and central sectors of the bay contain a
number of patch ieefs, varying in size, shape, and depth at
low tide (Figure 1). Coral cover on patch reefs is
highest in the more northern areas of the bay, and Porites
compressa is the dominant coral species (Maragos, 1972).
Montipora yerrucosa is also common in the bay. At the
primary study site, Patch Reef 142, the top of the reef is
almost completely covered by large patches of ~. ~mpressa
and M. yerrucosa: both species displaying a blunt
finger-like growth form. Polacheck (1978) states that on
tliisf pat-chi"eeftnere lauric. -apparent patterri to the
relative abundance of ~. ~mpressa and M. verrucosa on the
upper surface" but that the slopes are dominated by
.£. cornpressa.
Fishes, and chaetodontids in particular, are abundant
on Patch Reef 142. Because of the simpliCity of the
5
Figure 1. -- Patch Reef locations in Kane'ohe Bay.
6
N
i PATCH REEFS iN
KANE'OHE BAY
,/ -", I I ! I I I \ , \ , , \ , \ , \
I \
43 ()04Z ,--.... _,: \
.0 0 \ "', .. _,'" " ~ ·'0 ' , o .. I '",,,,
300031 : "
~. ZTt) "',
026 " ' ...
, , , '"\
\ , I o 24 et2' o I •
23" ,\ ~ .. -- ... } \1 10''-........ : ' ....... ""
, ' -'I '0 / , (\ ,-~ • 1. ..... J ... _'
°0
00
o o~ a ~ "[J. .. 0 K U OL 0'£
•
composition of the coral community on this patch reef, the
high coral cover, and the abundance of chaetodontid
corallivores, Patch Reef #42 is an excellent site to study
the interactions between a corallivore and the coral
community.
THE CORALLIVORE
Previous studies of the family Chaetodontidae suggest
that some members of this family are important
corallivores. Gut content studies of seven chaetodontids in
the Marshall Islands indicated that s~x species fed, at
least in part, on corals (Hiatt & Strasburg, 1960). Of 13
species of chaetodontids found on the reefs of Kona,
Hawai'i, four species had coral tissue in their guts
(Hobson, 1974). Reese (1977) observed feeding behavior
and analysed stomach contents of 16 species of Chaetodon
in Hawaii and classified three species as obligate coral
feeders and three species as facultative coral feeders.
Birkeland and Neudecker (1981) compared two species of
Chaetodon found in the Caribbean. One species was
found to specialize on anthozoans, and its abundance was
. ___ .. __ EiJ gIJi fi<::CI. !1t.:J.Y_.9Qrl:'~1_~t; ~g._ w:itll.9Jv. ~l:'f3. :ity Ci1JCl._t. Q1:.Ci:J..':UIl.Q\l11 t. __
of coral cover.
Chaetodon IDulticinctus, ~. ornatissiIDYQ, and
~. trifasciatus were classified as obligate coral feeders
by Reese (1977). Of these, both ~. ornatissiIDYQ and
~. trifaciatus are common in Kane'ohe Bay. Of the three
8
facultative coral feeders, ~. Ynimaculatus,
~. guadrimaculatus, and~. reticulatus, only the first
occurs in K-ane'ohe Bay. ~. Ynimaculatus is the most
common coral feeding chaetodontid on patch reefs in the
north end of Kane'ohe Bay, ~. trifasciatus the next most
common, and~. ornatissim~ relatively rarer.
Reports of the diet of ~. Ynimaculatus include corals
and other invertebrates (Table 1). At Kona, Hawai'i, the
diet of ~. Ynimaculatus included 45% scleractinian corals,
"possibly fragments of. tissue and skeleton from
Pocillopora spp." (Hobson, 1974). Reese (1973) inspected
13 guts from ~. Ynimaculatus speared at Enewetak and found
primarily hard and soft corals, but also algal, sponge,
and polychaete fragments. Boucher (1977), studying the
behavior of this chaetodontid at Enewetak, found that
small individuals «8 cm total length) fed primarily upon
Montipora spp. (75% of the total bites tallied) while
larger individuals (>13 cm total length) took more bites
from soft corals (61% of the bi tes from Lobophyton spp.).
9
TABLE 1. -- Reported diets of ~. unimaculatus.
Area
Africa Africa
Australia
New Guinea Enewetak
Enewetak
Kona
Hanauma Bay
Diet
invertebrates soft corals, crustaceans soft and hard corals soft corals hard and soft corals hard and soft corals hard corals, invertebrates hard corals
Reference
Talbot, 1965 . van den Elst, 1981
Anderson et al., 1981
Tursch & Tursch, 1982 Reese, 1973
Boucher, 1977
Hobson, 1974
Motta, 1980
Motta (1980) attempted to correlate jaw structure in
five chaetodontids with their diets, and proposed that .
several features of ~. unimaculatus adapt it to feeding
upon hard corals: the robust mouth, stout"peripheral
teeth with high iron concentrations, and strongly braced
jaw. He observed feeding of ~. unimaculatus in Hanauma
Bay, O'ahu, and found it took significantly more bites
from Montipora spp. and Leptastrea spp., two uncommon
genera in tlie area, than from any other corals. Motta
suggested that the verrucosities on the surface of
H. verrucosa may supply leverage points allowing
m _~ ____ unJ_ml'£llJ!lEll_s_ ~()_~c:_r_al?~_~i_~~\lE! __ ~Ilcl _ ~~_e~_e_tCll ma 1:~_~~_aJ.
from the surface of the coral more effectively.
10
CHAPTER II. FEEDING SELECTIVITY AND THE RATE OF CORAL CONSUMPTION
INTRODUCTION
In order to assess the significance of corallivory on
the coral community, it is necessary to determine any
feeding selectivity for particular species of corals, the
rate of consumption of coral tissue and/or skeleton, the
standing crop of coral tissue available to feeding fishes,
and the replacement rate of coral tissue. Other effects,
such as a decrease in the competitive abilities of
preferred versus non-preferred corals, may also be
important.
There is a spectrum of ideas concerning the magnitude
of the impact of fishes on corals. There are those who
feel that predation by fishes on corals exerts a significant
impact on the annual growth and survival of corals and
thereby controls the distribution of coral species and the
composition of coral communities (Neudecker, 1979).
Alternately, there are those who believe that predation is
insignificant because of the colonial nature of the coral
animal and its protective. skeleton (Harmel in-Vivien &
Bouchon-Navarro, 1982). At this point, broad
generalizations about the impact of corallivory by fishes
on coral community composition cannot be made, in view of
the plethora of methods of feeding on coral and the
differential abilities of coral species to resist
predation.
11
Indirect measurements of coral consumption by
chaetodontids have been made by Harmelin-Vivien and
Bouchon-Navarro (1982). Based on gut weights and assuming
a twice daily filling of the gut, they concluded that
coral consumption by six species of Chaetodontidae amounted
to a maximum of 1 g m- 2 day-l (wet weight), from which
they concluded that chaetodontids browsing on coral polyps
do so without 'significantly damaging the corals. However,
they fail to provide data on feeding selectivity or coral
standing crop and replacement rates. Glynn and co-workers
(1979) used a similiar method to estimate removal of cor.al
by the sea urchin Eucidaris sp. They admitted that their
use of a 24 hour gut ftlling time was just an estimate,
but using this estimate, they calculated that ea9h individual
could remove 0.40 g day-l of coral. This rate of coral
removal was signficant enough to cause zero net production
by the. coral Pocillopora spp. in areas where coral cover
was less than 30% and consequently grazing pressure was
intense.
Feeding selectivity in- fishes has been studied in both
laboratory and field situations. Reese (1977)
investigated preferences in laboratory trials, by offering
two species of chaetodontids, ~. trifasciatus and
~. ornitissimus, simultaneous choice between three species
of corals, ~. verrucosa, ~. compressa, and 2. ~icornis.
Both chaetodontid species preferred 2. damicornis in these
trials. Birkeland and Neudecker (1981) compared bites
12
taken in the field by two chaetodontids, ~. capistratus
and ~. aculeatus, with abundances of different food
species in the feeding areas. ~. capistratus feeds on
scleractinians and showed some biases in food choice,
although they found that preferences in one location were
not consistant with preferences in other locations.
In order to assess any feeding bias by
~. Ynimaculatus, their feeding preferences in the
laboratory and field were measured. Consumption of coral
was estimated directly by evaluating bite size and feeding
rates, and an estimate of the proportion of the standing
crop removed by the fish was calculated using estimates of
coral removed per fish.
METHODS
LABORATORY FEEDING PREFERENCE TRIALS
Laboratory feeding preference studies were carried out
with five ~. unimaculatus captured from within Kane'ohe
Bay; two came from Patch Reef #42 and three came from the
Sampan Channel near the south end of the bay.
-Experimental anrma-rs-were placed TfidIvIdually inaquarTa
at the Hawai'i Institute of Marine Biology. Each aquarium
measured approximately 1.1 m x 0.6 m x 0.6 mi with a
volume of approximately 350 lof seawater. and a constant
inflow. Each aquarium had a clear window on one long side
permitting observations. Fish were given about 10 coral
13
heads of both species every other day for food. At least
48 hours were allowed for acclimation to aquaria before
trials began. Several small scarids or acanthurids were
added to each aquarium to control algal growth.
Before each trial all corals were removed from the
aquarium. Freshly collected test specimens of
.M. verrucosa and .f. compressa, both approximately 10 to
15 cm in diameter and having the same finger-like branching
form, were placed approximately 30 cm apart in the
aquarium. Left and right positions were alternated with
sequential trials. Data on the number of bites on each
coral species were recorded for 30 minutes, after a 10
minute adjustment period to allow the fish to habituate
to the presence of the observer at the window. Five or
six trials per fish were run on sequential days. Between
trials, fish were given additional corals for feeding. ,
FIELD OBSERVATIONS OF FEEDING PREFERENCES
After laboratory trials were completed, Fish #2, #3,
#4, and #5 were marked with anchor tags, injected
subdermally in the area beneath the dorsal fin. Tagged
fish were released on Patch Reef #42 in October of 1981. - --- -- -- - -- - -- ---- ----- ----------------- - -- -------- -------, - --------------------------- -- -
Fish #4 lost its tag within two days of release. Field
foraging information was collected from the remaining
three tagged fish while snorkeling near them as they fed
on the reef, counting the number of bites on each kind of
food during a 15 minute observation period. Coral cover
14
and the .size of the feeding area were estimated visually
while following the fish. Additional information on
interactions with other fishes was also gathered.
BITE SIZE
Ten fish were captured on Patch Reefs '42 and '43
and kept in flow-through seawater aquaria, approximately
390 1, at the Hawai'i Institute of Marine Biology. After
an acclimation period of at least 48 hours,. during which
H. yerrUCQsa was offered as a food source and replaced
every other day, all coral was removed from the aquarium.
Test colonies of B. yerrUCQsa, which had been stained with
Alizarin Red which colors the newly deposited skeleton
red, were placed in the aquarium. The number of bites
taken by the fish feeding on the test colonies was counted
. for a 30 minute trial period, and the fish was sacrificed.
Because~. unimaculatus takes skeletal material with each
bite (Motta, 1980), it was possible to dissect out the gut
and determine which portion contained the stained
material, usually the stomach and proximal segment of
intestine, approximately 28% of the total intestine - - - -- -- - - - - - -- - -- --------- ------- ------- - -
length. This section was squeezed to remove its contents,
which were then dried at 80 0 C for 24 hours, weighed, and
then ashed at 500 0 C for five hour's to determine the ash
free dry weight (AFDW). Because calcium carbonate will also
break down during the ashing procedure (Paine, 1971),
samples of pure calcium carbonate were ashed separately to
15
provide a correction factor for weight lost due to calcium
carbonate combustion in the gut samples.
FEED! NG RATES
Feeding rates were estimated during February and March
of 1983 by haphazardly selecting a fish in an area on Patch
Reef #42 (Areas A, B & C; Figure 2) and following it for
15 minutes, tallying all bites taken on coral. All fishes
were between 13 and 15 em in total length. The day was
arbitrarily divided into three time periods, morning (0700
to 1000), midday (1000 to 1400), and afternoon (1400 to
1700), to see if differences in feeding rates during
different periods of the day could be detected.
Feeding rates were again estimated during August of
1983, in the same area of Patch Reef" #42. To more
accurately sample periods when feeding rates might be
predicted to be elevated due to inactivity during the
night hours, feeding fish were observed at dawn
(starting approximately 15 minutes before sunrise and
continuing for about 1 hour), one hour around noon, and
dusk (starting approximately one hour prior to sunset and
"-ccnrtiifulfigUfitiT stirfsfetl. Hapnazardly-selecEedfish were
followed for five minute periods, tallying all bites on
coral.
16
Figure 2. Map of Patch Reef #42. A: Grid system, 20 m x 20 m, used to record
time spend in different zones on the reef (Chapter V)
B: Block placement in growth and competitionexperiment (Chapter III).
e: Area of contiguous H. verrucosa.
17
N-
~
PATCH REEF 42
STANDI·NG CROP
Dry weight of B. yerrucosa tissue cm-2 was estimated
at five si tes on Patch Reef 142 by collecting 10 cm x 10 cm
plugs. The plugs, consisting of skeleton and tissue,
were chiseled out to a depth of 10 cm; .below that depth
most coral tissue was dead, and exposed skeleton was
covered with other organisms. Samples were fixed in 10%
formaldehyde in seawater for 24 hours and then decalcified
with 4% nitric acid. The remaining material was dried for
24 hours at 80 0 C and weighed. There is a weight loss
(up to 10%) using this kind of technique (Davis, 1980).
RESULTS
LABORATORY FEEDING PREFERENCE
During the trial period, fish usually investigated
both specimens of coral, but fed almost exclusively on
H. yerrucosa. A simple G test of Goodness of Fit, using \
the extrinsic hypothesis that the number of bites on
~. compressa and H. yerrucosa should be equal, shows that
there is a significant preference for B. verrucosa, both
------ ----f-or--eacn-flsn-ana--poole-d--over-·-alr--inaIvi-dual-sT-ar~fi(Hign
there are differences in the strengths of the response for
different individuals as evidenced by the heterogeneity
component (Table 2). This can be explained in part by
problems with individual fish and disturbances to the
trials; e.g., Fish #4 did not habituate to the presence of
19
the observer and sp~nt most of the trial periods in the
corner of the tank or swimming back and forth in front of
the glass window. Because of the outdoor location of the
aquaria, several of the trials were disturbed by events
such as coconut tree trimming.
TABLE 2. -- Results of laboratory feeding preference trials with ~. unimaculatus.
Fish no.
Bites - Bites -M.yerrucosa P.compressa
1 2 3 4 5
Test
3086 70 770 29
1011 33 35 0
352 0
. Pooled Heterogeneity Total
df 1 4 5
G 6226.7
26.2 6252.9
Total Bites
3156 799
1044 35
352
P<.OOI P<.OOI P<.OOI
G
3703.5 858.4
1154.4 48.6
488.0
To test if mechanical damage resulting from
transport; e.g., abrasion and mucus production, affected
choice of food, additional trials were conducted using
test corals which had been placed in the aquarium at least
24 hours in advance of the trial and were protected from the
fish by cylindrical wire cages. The results of these
tE!Cl~_E; __ appeared n()_dj.:f'~~rent froIll_~e original expE!rimE!lltal
setup. On several occasions, fish were observed to take
a bite of coral with a heavy mucus coat and spit out the
bite, suggesting that this species is not attracted to
excess mucus production by corals.
20
FIELD FEEDING PREFERENCES
Bites tallied during 17 observations periods of the
three tagged fish (Fish 12, 10 periods, Fish 13, two·
periods, Fish 15, five periods) also show a significant
deviation from a hypothesis of no preference for either of .
the two coral species (Table 3). H. verrucosa consisted
of less than 50% of the coral cover in the observation areas,
however it was preferred by a ratio of 284 bites
B. verrucosa to each bite on R. compressa.
TABLE 3. -- Observations of field feeding preferences, Patch Reef 142.
Bites - Bites - Total G M,verrucosa P,compressa Bites
2275 8 2283 3058· P<.OOl
Plankton feeding was sometimes observed, especially
duri.ng periods when dense patches of plankton were visible
in the water column, but it was difficult to quantify, and
even during periods of plankton feeding by all reef
fishes, ~. unimaculatus appeared to continue feeding on
hard coral, with occasional bouts of plankton feeding in
~. unimaculatus collected on Patch Reefs 142 and #43 also
reflect a preference for coral food: contents consisted
almost entirely of coral tissue, with a few algal
fragments and one small whole invertebrate.
21
BITE SIZE
BitE! size was determined for 10 fish, with a mean
of 2.54 mg AFDW bite-l (n=lO, s=1.78, Table 4). There is
some correlation between bite size and fish weight
(r=0.6519, 0.01<P<0.05), but because of the high variance,
it explains only approximately 42% of the variation in
bite size. Part of the variability is probably due to
some imprecision in segregating stained material from the
rest of the contents of the intestine.
TABLE 4. -- Average bite size (mg AFDW) for laboratory held 'c. unimaculatus.
Fish Weight Feeding Rate mg CaC03 Bite Size no. (g) (bi tes/min) per bite (mg AFDW)
1 52.6 9.4 0.3 0.5 2 69.1 3'.6 0.4 1.1 3 94.4 1.0 1.6 3.4 4 95.2 1.6 0.7 1.7 5 97.3 2.8 0.4 0.7 6 103.6 0.9 2.6 5.3 7 104.5 10.8 0.7 1.4 8 123.3 4.2 1.1 2.2 9 125.2 0.8 2.1 4.7
10 139.9 4.7 2.2 4.4
mean 2.54' std. dev. 1.78
FEEDING RATE
During February and March, there were no significant
differences among feeding rates during the three time
periods as determined with a 'Kruskal-Wallis Test (Table
5). Therefore" the data were pooled yielding an ~verage
feeding rate of 7.20 bites min- l (n=50, s=4.l5). During
22
August, a Kruskal-Wallis test again indicated no
significant differences among the three time periods
(Table 5), and the data were pooled for an average rate of
4.88 bites min-l (n=129, s=5.32). There does appear to be
a difference between the "winter" and "summer" feeding
rates, however, they are not comparable statistically due
to the different observation techniques and different
times of the day selected.
TABLE 5. -- Comparison of feeding rates of ~. unimaculatus at different times of year and day at Patch Reef #42.
February/March
min- l Morning Midday Afternoon Pooled
Ave bites 7.96 5.49 7.65 7.20 Std. dev. 2.40 3.77 5.53 4.15 Range 2.5-11.8 0-12.1 0-24.9 No. fish 19 13 18 50
August
min-l Dawn Noon Dusk Pooled
Ave bites 5.51 5.88 3.81 4.88 Std. dev. 4.78 7.03 4.58 5.32 Range 0-21.4 0-27.2 0-14.8 No. of fish 44 39 46 129
STANDING CROP
Five samples of .M. ~.r..w:;,.Q§.a from within 4 m of the
edge of Patch Reef #42 were analysed, resulting in an
estimate of 660 g (tissue dry weight) m- 2 (Table 6). ----------------- --------------- --Bowev-er;---oeca-u-se--ortli-eIr-sTze-an-a--feecUng- behavior,
~. unimaculatus are not able to utilize all of this
tissue. These fishes can only remove the top layer of
tissue and skeleton, probably less than 10% of the
standing crop as estimated using this method.
23
TABLE 6. -- Standing crop of ~. verrucosa on Patch Reef #42.
Sample no.
mean
1 2 3 4 5
std. dev.
dry wt (g)
7.117 6.066 7.570 4.454 6.787
6.599 0.843
DISCUSSION
~. unimaculatus in northern Kane'ohe Bay is clearly
selecting one species of coral, H. verrucosa. Feeding
preferences for two other HawaiIan chaetodontids,
~. ornatissimus and ~. trifasciatus, have been studied by
Reese (1977), by offering captive pairs of chaetodontids a
simultaneous choice of three species of corals. For both
chaetodontids, Pocillopora damicornis was the preferred
'coral, fOllowed by .M. yerrucosa, with of. compressa third.
However, the prefere~ce for .M. yerrucosa over of. compressa
does not seem as strong as I recorded while comparing only
two species. The mean number of bites/50 min observation'
period (n=82 trials) by ~. trifasciatus on of • .aa,m'icornis - ---- -- -- - - - - ----- --
was 112.6, for }1. yerrucosa 80.6, and for of. compressa
55.2, indicating only a weak preference for H. verrucosa
over of. ~mpressa. In the northern end of Kane'oheBay,
~. damicornis is a less abundant and patchily distributed
component of the fauna, representing less than 1% of the
coral cover on Patch Reef #42. ~. trifasciatus feeds on
24
both ~. compressa and H. verrucosa at Patch Reef #42
(personal observations).
Hobson (1974) suggests that Pgcillopora spp. are the
most common food of ~. unimaculatus based on coral tissue
in the gut, but this may reflect the relative abundances
of corals on the leeward coast of the island of Hawaili.
Pgcillopora spp. are more common than Mgntipgra spp., with
~. meandrina representing approximately 16% of the total
coral cover, H. yerrucgsa less than 2% (Dollar, 1980).
Pteferences by large~. unimaculatus for soft corals
as shown by Boucher (1977) at Enewetak and feeding on soft
corals as reported by Anderson et ale (1981) and Tursch &
Tursch (1982) cannot be observed in Kanelohe Bay, as soft
corals are not present. There is a location off the coast
of Molokali where large beds of soft corals and
~. unimaculatus co-occur (J. Maragos, personal
communication), and this may be an ideal site to
investigate preferences for soft corals in Hawaili.
H. verrucosa may be the preferred 90ral for anyone
or a combination of factors. It has a deeper penetration
of tissue into the skeleton and lower skeletal density
- _uum -tnan-.f~-cQmpieSSfc"--{T-aDI e --6r1-factor sU·W-hIC1i -Iliay--allow -
~. un:i;maculatus to remove more tissue per bite. However,
H. verrucosa may have a higher resistance to abrasion than
1:. compressa. Other possibilities included differential
caloric content, nematocyst size and denSity, and/or the
presence of toxic compounds.
25
TABLE 6. -- Comparison of morphological and skeletal properties of ~. verrucosa and R. compressa.
l?roperty
Depth of tissue
Density Displacement
X-ray
% loss in abrasion test
!1.yerrucosa
13.0 mm
0.92 glcc 1.37 g/cc 1.10 g/cc
38.8%
R.compressa
7.5 mm
1.18 g/cc 1.41 g/cc 1.23 g/cc
47.4%
Reference
Polacheck, 1978
White, 1980 Maragos, 1972 Houck, 1979
Jokiel & Cowdin, 1976
The importance of feeding preferences in understanding
the consumer's impact on producer populations has been
underscored by experimental analyses of the effects of
herbivores on marine algal populations (for a review, see
Lubchenco & Gaines, 1981). The.effect of the consumer on
plant diversity depends largely on the selectivity of
grazing and on the competitive abilities of the plant
species. Selective grazing can have opposite impacts on
divers"ity; for example, the effect of the snail Littorina
littorea on algal diversity depends on environmental
conditions: its preferred food· species is competitively
dominant in tide pools but is inferior to other algal
__ ~.E~~Je.§l_ ~n_ ~!llE!~9.~_n! __ ~~~~_t_r_Cl_t:~ __ tr,.~!>!=jl_encQL_19I~1~ ___ '!'lt~ ________________ _
impact of selectivity by~. unimaculatus on growth and
competitive abilities of the two corals was studied in
another series of experiments (Chapter III).
Reproduction by ~. unimaculatus probably occurs
during the months of February, March, and April (Lobel,
1977; personal observations). Increased energetic needs
26
during the reproductive period may be a factor in the
higher feeding rates recorded during the February/March
period, although the pooled means are not directly
comparable due to the different sampling periods used and
the difference in division of the day into sampling
periods.
The magnitude of the consumption of coral by
~. unimaculatus can be estimated by using the feeding
rates as indicators of feeding during the two periods of
the Hawaiian yearly cycle, the dry period or kau (May to
September) with day lengths of approximately 13 hours, and
the wet period or hotoilo (October to April) with day
lengths of approximately 11 hours. Using mean feeding
rates for these two periods and the mean bite size, .a fish
could remove approximately 4000 g per year, or the
population of approximately 168 fishes (with a mean size
equivalent to the average sized fish used in laboratory
bite size calculations) on Patch Reef #42 is removing
670,000 g per year from the reef. Using estimates of the
total amount of H. verrucosa on Patch Reef #42 (see
Chapter IV) and the standing _crop_of c()I'~l 1;i_flsuep~r
square meter, this fish population is removing about 10%
of the standing crop each year.
27
CHAPTER III. THE EFFECT OF PREDATION ON GROWTH AND COMPETITION BETWEEN M. VERRUCOSA AND ~. COMPRESSA
INTRODUCTION
Competition for space between corals, by differential
growth rates, overtopping or direct aggression, has been
postulated·to be a major community structuring force in
dense coral stands in stable environments (Lang, 1973;
Connell, 1978; Richardson et al., 1979; Sheppard, 1979,
1982). However, the outcomes of competitive interactions
may be modified by other factors including the delayed
development of sweeper tentacles (Wellington, 1980), the
site of the interaction (Bak et ale, 1982), or activities
of the epifauna present (Bak et al., 1982).
Corallivores have been found to depress growth rates
of corals (Glynn et al., 1972; Neudecker, 1979), and may
also affect outcomes of competition for space. The impact
of a selective corallivore on the growth and competitive
interactions of corals depends on which coral is the
preferred food species for the corallivore. Following
results based on work with marine herbivore preferences
ClI'l9 __ ~1_g~1 divers.i.i:.y_LfoI' a review,SeELLubchenco .. ~. Gaines,
1981), if the corallivore preferred the aggressively
dominant species, it may prevent monopolization by the
dominant ~pecies in dense coral stands. Selectivity for
subordinate species could make those species extremely
rare.
In Kane'ohe Bay, both ~. verrucosa and ~. compressa
28
have similiar growth rates, in studies of isolated colonies
(Table 8), and colony morphologies. In experimental
tissue transplantation reactions, H. verrucosa displays a
unilateral xenogeneic incompatibility against R. compressa
(Hildemann et al., 1974); i.e., .M. verrucosa was able to
kill tissues of R. compressait came in contact with,
similiar to aggressive encounters of the type described by
Lan g (1 97 3) •
TABLE 8. -- Comparison of prowth rates in Kane'ohe Bay (change in colony radius in cm yr- )
.M. verrucosa
1.85 1.31
.f. compressa
2.43 1.28
Reference
Polacheck, 1978 Maragos, 1972
Other researchers have noted that H. verrucosa has the
ability to grow up and over R. compressa (Branham et al.,
19711 Maragos, 1972; Dollar, 1975). Polacheck (1978)
characterized 51 naturally occuring interfaces between
.M. yerrucosa and R. compressa on Patch Reef 142. He
classified H. yerrucosa as the dominant coral in 14% of
the interactions, a standoff in 84% of the interactions,
and subdominant to R. compressa in 2% of the interactions.
The high proportrons ofu standoffs between l:1.Yerrucosa and
R. ~mpress.a on Patch Reef 142 may resul t from the impact
of a selective corallivore, ~. unimaculatus. The effects
of predation on the outcome of interactions between two
species, with a dominance ranking as determined by
laboratory experimentation and field observations, can be
29
tested because the predator, in this case, is clearly
selective in its feeding preferences.
METHODS
To test for differential growth and competitive
interactions with and without predation, pairs of
~. verruocsa and ~. compressa were collected from MokuoLo'e
reef and artificially placed in contact inside and outside
of predator exclusion cages. To insure genetically
matched protected and unprotected pairs of the two coral
species, coral colonies of each species were cut in half
with a rock saw, which was cooled with salt water during
cutting to minimize tissue damage. Corals normally heal
along such cut surfaces. For the first experimental
series of 12 pairs, the two corals were tied together, cut
sides in contact, with monofilament line, and then each
set of competitors was wired to a screen, in its original
vertical orientation. One pair of each of the matched
sets was protected from chaetodontid grazing by chicken
wire cages, the other pair was left unprotected. Corals
were stained with Alizarin Red to permit measurement of
Tineai-growth, from-t:he--stained sKeleton to the outer edge
of the corallum. These screens were placed at the edge of
the top of Patch Reef *42 on January 12, 1983, at
approximately 2 m depth. Cages were brushed every week to
prevent algal buildup. The screens were removed from the
field after 160 days. In some cases the monofilament line
30
did not hold the two corals together and was replaced with
plastic coated wire. Two halves of ~. verrucosa .were lost
during the trial period, and one set of pairs was removed
from the data analysis because the corals became
separated. For each set, percentage of the colony surface
which had die.d was est1mated, and vertical growth was
calculated by cutting a minimum of ten branches and
measuring skeletal growth in the axial plane beyond the
Al iz ar in Red stain.
A second series of experimental pairs was prepared by
glueing the cut corals to concrete blocks with Sea Goin'
Putty, with cut surfaces flush to the block face and
branch tips in contact. Again, one set of competitors was
prot.ected from chaetodontid grazing by chicken wire cages,
and the other set was left unprotected. This series of
twelve paired blocks was placed in a similiar position on
Patch Reef 142 on March 29, 1983, and removed after 130
days. Cages were brushed at least once per week to
prevent algal buildup. Colony mortality was estimated and
vertical growth was measured b¥ cutting a sample of ten
branches. Observations of direct killing of opposing
---------- -crancfie s -ana -o-vergrowtli--w-ere--aTso--macfe:-· - ------- ------------------------ - - - - --------- --- - -------- -----
Another chaetodontid, ~. trifasciatus, feeds on •
~. verrucosa in laboratory situations (Reese, 1977).
~. trifasciatus occurs in low densities on Patch Reef #42.
In an attempt to separate out the effects of this
chaetodontid on competition between corals from that of
31
~. unimaculatus, the third series of five experimental
sets, prepared identically to the second series, was
placed at approximately 2 m depth off the fringing reef at
MokuoLo'e, an area lacking in ~. unimaculatus, but with a
similiar density of ~. trifasciatus as Patch Reef '42.
Thi s ser ies was put out on June 17, 1983 and removed af ter
100 days. Colony mortality and growth were measured as
previously described. Light levels inside and outside of
cages were measured with a Li-Cor Integrating Quantum,
Radiometer and an underwater quantum sensor'.
RESULTS
SERIES 1
Several of the sets did not survive the experimental
period, either coming apart from each other or coming
loose from the scr,een. Damage from contact of cut
surfaces with other corals seemed to affect both species,
although B. verrucosa consistently had a wider margin of
dead tissue at the interface. On the basis of at-test
for paired comparisons, there was no difference in growth
rates for ~. compressa in the two treatments (t=0.5l2, - ---- -- ------- - -- -- ---- -------
- - -d~=I6; -,TacIe--9r;- but caged B. verrucosa grew significantly
taller than uncaged colonies (t=4.657, df=1'6, Table 9).
32
TABLE 9. -- Series 1: compar~son of vertical growth of ~. verrucosa and ~. compressa on caged and uncaged screens on Patch Reef #42, (cm/160 days).
Pair no.
~. verrucosa caged uncaged
2 0.92 3 0.47 5 0.99 6 1.03 8 1.11 9 1.50
10 1.54 11 0.94 12 1.17
mean 1.07 std.dev. 0.32
SERIES 2
o o
0.30 0.52 0.53 0.40 0.45 o
0.48
0.30 0.23
~. compressa caged uncaged
0.99 1.16 0.86 0.80 0.89 1.11 1.00 1.18
" 1.02
1.00 0.13
0.92 0.98 1.12 1.11 0.94 0.91 1.14 1.08 1.06
1.03 0.09
There appeared to be less mortality following the
cutting procedure than in series 11 in fact most of the
colonies we~e growing over the putty and out onto the
block surface at the end of the experiment. Thr·ee sets of
corals were eliminated from the analysis: H. verrucosa on
the uncaged block of pair #2 came unglue"d and was lost,
corals in pair #11 became partially unglued, and pair #12
was overturned during a period of heavy surge and was
. cl~s_t_r9Y~cl._ J~~I'_tl<:~l_gI'~wt;h_()f~CJ.c::Jl_f3P_ec:iel:iw_a.sanalys_ed ______ _
as a randomized blocks ANOVA, with replication from 10
branches per coral. ~. compressa shows no significant
difference between caged and uncaged treatments, however,
there are some differences among individuals (Table 10).
The analysis of the data for ~. verrucosa indicated a
significant interaction component, i.e. all pairs were not
33
TABLE 10. -- Series 2: vertical growth of M. yerrucosa and ~. ~mpressa on caged and uncaged blocks on Patch Reef #42 (cm/130 days), analysis as randomized blocks ANOVA with replication.
~. compressa
Pair No. Caged Uncaged 1 1.13 1.06 3 0.76 0.98
" 0.96 0.99 5 1.05 1.02 6 1.02 1.00 7 1.05 0.99 8 1.08 0.98 9 1.14 1.06
10 0.96 0.99 .
mean 1.02 1.01 std.dev. 0.12 0.03
ANOVA Source of variation df MS F Treatment 1 0.02405 1.109 n.s. ·Individua1s 10 0.10248 4.726 P<.OOl Interaction 10 0.04524 2.087 .025<P<.0 5 Error 198 0.02168
·B. yerrucosa
Pair No. caged uncaged 1 1.36 0.54 3 1.11 0.09 4 1.28 0.82 5 1.18 0.67 6 0.93 0.91 7 1.74 0.97 8 1.21 0 9 1.21 0.54
10 1.43 0.84
mean 1.27 0.60 - ---- ------ --- -- -- -- ------------
std. dev •. 0.23 0.35
ANOVA Source of variation df MS F Treatment 1 19.39057 475.6 Individuals 7 1.36156 33.4 Interaction 7 0.47313 11.6 P<.OOl Error 144 0.04077
34
responding in a consistent manner. Part of this may be
related to the substrate on which the blocks were placed
(Area B of Figure 2). Sets #5, #6, and ilO were placed.on
the tongue of B. yerrucosa which approaches the edge of
the reef. The two sets with the poorest growth rates, #8
and #3, were at the other end of Area B, placed on
~. compressa. The data were also analysed as a paired
t-test comparing mean growth in uncaged and caged pairs.
Growth of B. yerrucosa was significantly greater in caged
cor al s (t = 4.867, d f = 16, P < • 0 01) •
Where branches of the two competitors came in contact
there was a zone of dead tissue, usually on both species,
with algae covering the exposed corallum. Where branches
did not come in contact, no dead zones occured. Using
predictions of aggressive dominance of ~. yerrucosa based
on immunological work, in which ~. yerrucosa consistantly
killed ~. coropressa tissue that was in contact with it,
there should have been more dead surface on 2. ~pressa
than on M. yerrucosa. However, on the uncaged pairs i8,
#3, and #10, 2. ~mpressa had caused significant mortality
to B. yerrucosa; some branches of B. yerrucosa were dead
tiff to- l~cm. --In contrast, in caged situatiori-s, theEe were
two cases, pairs #8 and #9, where a branch of M. verrucosa
was enveloping the opposing branch of ~. compressa. Ih
the other caged pairs, M. yerrucosa had killed the tissue
of P. ~mpressa, in some cases as much as 1.5 cm of the
contacted branch.
35
SERIES 3
There are no significant differences between treatments
or among individuals for ~. compressa. B. yerrucosa shows
significant effects both between caged and uncaged pairs
anq among individuals, with higher vertical growth in the
caged treatment (Table 11).
36
TABLE 11. -- Series 3: comparison of vertical growth of M. yerrucosa and ~. ~mpressa on caged and uncaged blocks at MokuoLo'e (cm/100 days), analysis by randomized blocks ANOVA with replication.
~. coropressa
Pair No. caged uncaged 1 0.95 0.86 2 0.91 0.88 3 0.94 0.94 4 0.97 0.99 5 0.96 0.96
mean 0.95 0.93 std.dev. 0.02 0.05
Source of variation df MS F Treatments 1 0.01000 0.076 n.s. Individuals 4 0.02585 0.197 n.s. Interaction 4 0.00925 0.070 n.s. Error 90 0.13141
11 •. yerrucosa
Pair no. caged uncaged 1 0.88 0.62 2 1.18 0 .. 65 3 1.37 0.88 4 1.29 0.86 5 1.26 ·0.90
mean 1.20 0.78 std.dev. 0.19 0.14
Source of variation df MS F Treatments 1 4.28490 134.57 P<.OOl Individuals 4 0.48385 15.19 P<.OOl Interaction 4 0.05765 1.81 n.s. Error 90 0.00318
- - - -- - --
37
LIGHT LEVELS
Caging resulted in a decrease of approximately 24% in
Photosynthetically Active Radiation (measured in quanta)
(Table 12) .•
TABLE 12. -- Mean number quanta (integrated over 10 seconds, 4 readings/block) for caged and uncaged treatments at MokuoLo'e (std.dev.).
Pair no.
1 3 5
caged
393 (8.0) 444 (6. 2) 490 (6.5)
DISCUSSION
uncaged
493 ( 8.7) 583 (15.6) 758 (10.1)
'i,.".,. 1\ nU.,er of factors could produce a difference in the
vertical growth rates of ~.yerrucosa inside and out~ide
of predator exclusion cages on Patch Reef #42, including
various cage artifacts and the exclusion of other
corallivores. .It is clear th~t exposure to grazing by
fishes has a detrimental effect on the growth of
~. yerrucosa. The appearence of the surface of the
severely grazed test colonies of ~. yerrucosa from· Patch
Reef #42 resembles the gnawed appearance of colonies left
for several days in aquaria containing ~. un,imaculatus.
~. trifasciatus has a less well developed jaw and tooth
structure (Motta, 1980), which it uses to delicately
remove coral polyps, and presumably its ~ore delicate
mouth is less able to remove skeletal material than
~. YDimaculatus. Other researchers (Motta, 1980;
T. Hourigan, personal communication) indicate that
38
~. trifasciatus takes single polyps or parts of polyps
with each bite, a type of feeding behavior which would not
produce severely gnawed surfaces.
There were distinct changes in the growth morphologies
of H. verrucosa in the two treatments. Uncaged specimens
had thicker, stubbier branches, while the caged colonies
had thinner, finer branches. This undoubtedly affects the
measurement of growth, as thinner branches may show more
vertical growth, but lateral expansion of the stubbier
branches may represent as much mass change as the thinner
branches.
The reduction of light levels within cages may have
an effect on coral growth rates. Studies have shown that
H. verrucosa grows faster in reduced light levels (Houck
et al., 1977; Coles & Jokiel, 1978). However, these
studies indicate a plateau in the increased growth rate
after a reduction to 70 to 50% of the surface incident
solar radiation. 'Typical calculations of natural
attenuation of light with depth suggest that by 2 m depth,
light levels have been reduced to 50% of surface incident
solar radiation (Coles & Jokiel, 1978). Therefore, it
seems unlikely that the additional de'crease in light
levels within the cages would significantly alter growth
rates.
The effects of the cages on water motion are more
difficult to assess. Even if the cages do not appreciably
decrease the total volume of water passing by the corals,
39
they could alter the pattern of flow, perhaps allowing the
coral to construct thinner branches.
There is a between site difference in growth rates
for both coral species (Table 13). Part of this
difference may be due to the additional month of growth of
the corals at MokuoLo'e under elevated summer temperatures,
as temperature has been shown to increase growth rates
(Maragos, 1972; Coles & Jokiel~ 1978). Other physical
parameters may differ between the two sites. However, it
is readily'apparent from these data that the between site
differences'represent an approximate 18% decrease in
growth rates at Patch"Reef #42 for all treatments
excepting the uncaged !1. verrucosa. ,In the presense of
abundant ~. unimaculatus, the selective corallivore,
!1. verrucosa suffered an additional decrease in growth
rate of approximately 32%, presumably due to the effects of
grazing by ~. unimaculatus.
TABLE 13. -- Comparison of growth rates (cm x 10-3 day-I) for both corals at the two study sites.
Site
Patch Reef -#42
MokuoLo'e
!1.yerrucosa uncaged caged
3. 9~- 9-.71
7.82 11.96
40
.f.compressa uncaged caged
1.74·
9~26
7.82 -
9.46
CHAPTER IV. DISTRIBUTION OF FISH ANP CORAL
INTRODUCTION
Specialized foragers are found in areas in which
their preferred food resource occurs, for example the
distribution of the coral feeding gastropod Coralliophila
abbreyiata is positively correlated with the percent of
the coral genus Montastrea (Ott & Lewis, 1972). In
Kane'ohe Bay, ~. unimaculatus is highly selective for the
coral B. yerrucosa, and therefore its distribution would
be predicted to be strongly correlated not with the total
amount of coral cover on a patch reef but with the percent
of B. yerrucosa available to the fishes.
Consumers are often restricted in their foraging area
by a need to remqin close to shelter from predation: the
presence of "haloes" in the distribution of seagrasses around
patch reefs in the Caribbean is generally attributed ~o
the need of herbivorous fishes and urchins to have shelter
from predators (Randall, 1965; Ogden, 1976). Menge and
Lubchenco (1981) suggest that high predation pressure
on marine herbivores in tropical environments creates
-stl'Qnq -9-~aQien-ts-i-n-'-ca-nsumer---EJ-r-az-in(J-pl"-es-su-r-e-on-'a-lEJae.----
If ~. unimaculatus is similiarly limited in its grazing,
needing to remain close to the slope for access to shel ter
in the large blocks of 2. compressa, and~. unimaculatus
has an effect on the growth of B. verrucosa (and its
ability to compete for space with ~. ~mpressa), then the
41
percent of M. verrucosa should increase with increased
distance from the slope.
METHODS
A subset of patch reefs in the north and central
sectors of Kane'ohe Bay were selected for the estimation
of coral cover and~. unimaculatus density. The reefs
were chosen to give a range of total coral cover from low
to high, estimated by visual surveys, and on the basis of
size and simplicity of shape. Reefs differed in depths at
low tide, with some partially exposed at extreme low
tides, whil"e others were always covered with at least 1 m
of water. Because of the unknown extent of utilization of
the reef top, total coral cover and the percent coverage
of the major species were measured along the edge of the
reef top, with meter square quadrats placed just below the
break between the top and slope. The enti're circumference
of each reef was sampled, advancing by a random number of
kicks between quadrats. Preliminary plots of both the
culmulative percent total cover and percent M. verrucosa
against the number of quadrats were used to evaluate the
adequacy of -thesample--size.-
Fish populations were censused from May 1982 through
August 1983, when water visibility permitted. A number of
fish censusing techniques have been developed (for ~
review, see Sale, 1980). Because~. Ynimaculatus
typically swims off the reef top to the slope when
42
disturbed, all ~. unimaculatus seen at the edge or on the
slope to the bottom of the reef were counted when passed
while slowly snorkeling around the perimeter of the reef.
This method was judged to give a reasonable estimate of
the population size, although comparatively low counts on
days of extremely good visibility suggest that fish may
be able to observe the census taker, go into cove~ and
thus are not counted.
The perimeter of each reef was determined using the
formula for the ·circumference of an ellipse, the basic
shape of these reefs, and data on reef size from Roy
(1970). To compare between reefs of differing sizes,
fish populations were expressed as the number of
~. unimaculatus per meter of reef ~erimeter.
Patch reefs 142 and 143 were selected to evaluate
the distribution of B. yerrucosa along concentric
transects in toward the center of the reefs. Both reefs
support large populations of ~. unima¢ulatus. Percent
M. yerrucosa was measured with meter square quadrats at
four distances from the edge. Placement of the quadrats
was accomplished by swimming a random number of kicks at
______________ the __ edq el--- __ dr opping. __ an __ anchor-ed-to-p-e,-and-aw-i-mmin9--the------- --- ------
necessary distance toward a buoy placed in the center of
the reef. Percent B. yetrucosa in each zone was measured
during independent circumnavigations of the reef,
resulting in an unequal number of samples for each zone.
Preliminary plots of the cumulative percent of
43
H. verrucosa against the number of quadrats were made to
evaluate the adequacy of the sample size.
Percent time spent in different zones by
~. unimaculatus was estimated by establishing a grid
system, 20 m x 20 m, delineated at 5 m intervals with
colored bricks, on a portion of Patch Reef #42 (Area A on
Figure 2). Fish that were feeding on the grid system were
followed for a 10 minute period, and time spent in each of
5 zones «5 m from the edge, 5-10 m, 10-15 m, 15-20 m, and
20+ m from the edge) was recorded. This section of Patch
Reef #42 supports approximately 35-40 ~. unjmaculatus,
milling in large aggregations at the slope, and moving up
on the reef top in smaller groups to feed.
RESULTS
As is evidenced by the range in coral cover on
different reefs (Table 14, Figure 3), reefs representing a
gradient in total coral cover were selected. However,
although there is a correlation (r=0.876) between the
number of ~. unjmaculatus per meter of reef perimeter
(transformed by the. natural logarithm) and the percent
H.-yerrucosa on--the-patch I'eef-mars-in (transformed using
the arcsine function), the graph of this association
demonstrates the non-bivariate nature of the data (Figure 4).
44
TABLE 14. -- Comparison of coral cover and ~. unimacu1atus population size on selected p~tch reefs in Kane'ohe Bay (n=quadrat or census)
Reef % total no. coral
42 91.09 std. dev • 16.41
n 64
43 .
30
40
39
31
28
27
22
26
24
21
87.07 18.79· 56
74.54 29.82 92
55.27 31.34 27
40 .. 24 36.12 36
38.22 36.90 59
25.45 17.66 49
18.42 20.17 42
16.78 10.63 20
8.26 6.69
43
6.44 5.18
38
5.61 3.67
19
14.06 29.38' 64
14.72 25.78 56
2.21 9.10
92
o
2 .. 20 , 9.03
.36
0.74 2.09
59
0.41 0.96
49
0.12 0.29
42
0.99 1.38
20
0.04 0.18
43
0.28 0.87
38
0.35 1.15
19
45
no.~. peri- no. fish unimac. meter(m) per m
168.3 25.9 19
152 •. 3 15.7
9
9.4 1.6
10
2.6 1.7
10
8.3 2.1 7
15.4 4.1 7
10.2 4.1
12
2.0 1.4
14
o
14
6.1 2.9
14
3.0 2.9
12
2.5 1.3
15
484
564
437
163
376
478
370
346
94
346
286
66
0.3477
0.2700
0.0210
0.0159
0.0220
0.0323
0.0277
0.0058
o
0.0170
0.0150
o • 0374
Figure 3. -- Total coral cover and number of ~.
unimaculatus on selected patch reefs in Kane10he Bay.
46
N
... •
0 0 0 ..,
., ... ..
0 0 0 N
0 0 0
0 .., •
0:: IJJ
0 >
t-
O
(,)
0 -'
II> «
0 .. 0::
• 0 (,)
..J « I-
10> 0
'" ;;; .
l-•
it!
CD N
• to-
Q
N
N
• ~
CD ~
N
;;; . ... N
• •
Figure 4. -- Percent B. verrucosa and number of ~. unimaculatus on selected patch reefs in K~ne'ohe Bay.
48
.., 'I
* •
o -: o ot
III
0 .. 0 Co)
::a ... ... • :>
ID
~I
at
'"
In spite of ari effort to se~ect reefs ranging from
high to low coral cover, percent ~. verrucosa is high for
two patch reefs, 142 and 143, and low for the remaining
reefs. Although not reflected in the population estimates
for ~. unimaculatus on different reefs, a difference in
size of individuals on different reefs was noted. Patch
reefs.in the central portion of the bay, for example, 119,
120, 121, 123, and 124, characteristically supported small
to medium sized fish «10 cm total length). Patch reefs
#42 and #43 averaged larger fish (>12 cm total. length).
For both Patch Reef #42 and 143 there is an increase in
the percentage of B. yerrucosa with increased distance
from the edge of the reef (Table 15). A Kruskall-Wallis
test shows significant differences in the mean values of
B. yerrucosa for the different distances from the edge of
the reef.
TABLE 15. -- Percent cover of ~. verrucosa with increasing distance from the edge of Patch Reefs #42 and #43.
Reef No.
42 std.dev.
n - - - -- ~------ - -- -- ---- -----
43 std. dev.·
n
Distance from edge (m) o 5 10
14.06 29.65 53.46 29.38 36.20 28.79 64 58 37
--------------- ------ --------------- -
14.72 14.78 25.15 25.78 20.77 27.27 56 45 42
15
64.27 27.92 42
30.56 31.50 37
Time spent in each zone on Patch Reef 142 shows a
pattern of increase to a maximum at 10-15 m in from the
edge, with more than 50% of the time spent less than 15 m
50
from the edge (Table 16). It may be argued that an
observer may prevent fish from moving up slope, but
these fish appeared to have been habituated to my
presence, as many feeding observations of this
aggregation had been made previously. It should also be
noted that several fish were followed approximately 30-
35 m in towards the center of the reef, where they
proceeded to feed in a normal manner on ~. verrucosa.
TABLE 16. -- Comparison of time spent by ~. unimaculatus in concentric zones on Patch Reef #42 (19 observation periods) •
Zone (m from edge) Percent time
<5 20.7 '
5-10 29.0
DISCUSSION
10-15 30.2
15-20 11.4
20+ 8.6
As predicted for a 'specialized feeder, the distribution
of ~. unimaculatus is positively correlated with that of
its preferred food on the basis of comparisons between
reefs. This is similiar to results found for another
corallivorous chaetodontid in the Caribbean. The local
abundance of ~. capistratus was found to be positively
correlated the the index of diversity of coral and the
percent of total coral cover in two areas (Birkeland &
Neudeck~r, 1981). ~. capistratus is not a specialized
corallivore and browses from a variety of coral species;
therefore its abundance would be predicted to follow total
coral abundance.
However, within reefs the distribution of fish and
51
coral are not positively correlated. The increase in
M. verrucosa with increased distance from the edge of
Patch Reefs #42 and #43 and the distribution of activities
of the fish is consistant with the hypothesis that
predation or the perception of danger may have an effect
on time spent by ~. Ynimaculatus further from the reef
slope. It is possible that ~. Ynimaculatus relies on deep
cracks between the blocks of ~. ~mpressa on the slope for
protection: this is also supported by anecdotal
observations on the behavior of ~. unimaculatus in
response to the presence of large predatory fishes. On
one occasion in the early morning, as I was making feeding
observations, all ~. Ynimaculatus upslope from my position
rapidly swam back to the reef margin. Seconds later a
large barracude appeared from the central portion of the
reef, sw imming toward the edge.
52
CHAPTER V. CONCLUSION
It has been shown ••• that there are some species of large fish, and the whole tribe of Holothuriae which prey on the tenderer parts of corals. On the other hand, the polypifers in their turn must prey on some other organic beings •••• The relations, therefore, which determine the formation of reefs on any shore, by the .vigorous growth of the efficient kinds of coral, must be very complex, and with our imperfect knowledge quite inexplicable. (Darwin [1842] 1962:pp.63)
The elements which determine the structure and
species composition of coral reefs are complex. In the
northern end of Kane'ohe Bay, patch reefs are
characterized by high. coral coverage, primarily two
species, .l!. compresa and .M.. verrucosa. In areas where
coral coverage is high and space is at a premium, physical
factors, such as temperature· cycles, water flow
characteristics, salinity and light regimes, are
undoubtedly important in determining coral community
structure. However, because of the high percentage of
space occupied by coral, biological factors, such as
competition and predation, should also playa role in
determining community structure.
On the basis of growth rates and published accounts of
aggressive dominance,-it- would be predicted that
H. verrucosa would be the superior competitor of the two
common species in Ka.ne'ohe Bay and therefore should
monopolize more space on these patch reefs. However,
M. verrucosa comprises only about 15% of the cover at the
edge of patch reefs #42 and #43, which have high coral
53
coverage.
~. YDimaculatus is abundant on these same patch reefs
and is a selective coral feeder, chosing ~. verrucosa in
preference to ~. ~mpressa. This fish is commonly
observed milling around the edges of patch reefs, possibly
to remain near shelter provided by the deep cracks between
large blocks of ~. ~IDpressa. ~. YniIDaculatus makes
feeding forays up onto the tops of these patch reefs but
returns to the edge when a threat is perceived; for
example, the presence of a human snorkeler or a large
predatory fish. This study has shown that grazing by this
fish inhibits the growth of ~. yerrucosa and that
heavy grazing pressure (as observed in uncaged colonies
placed on the reef margin) can cause reversals of the
predicted aggressive dominance. Thus~. Ynimaculatus can
affect the outcome of competition for space on these
crowded patch reefs.
M. yerrucosa can grow at a variety of depths
(Maragos, 1972) and is found at different depths within
Kane'ohe Bay (personal observation); e.g. the slopes of
Patch Reefs #43 and #31 were dredged for the main ship
channel and ~. yerrucosa can be found down these dredged -- ---- ---- - - - --- ----- ~-- -- - -- - - --
slopes. On the slope of Patch Reef #31, ~. yerrucosa
occurs in isolated colonies, possibly not providing a type
of suitable shelter for ~. Ynimaculatus. On Patch Reef
#43, a dense mat of M. yerrucosa without crevices grows
on the dredged slope, again perhaps lacking appropriate
54
shelter for the fish.
The strict preference for M. verrucosa, as evident in
this Kane'ohe Bay study, suggests a possible reason for
low abundances of this fish species in other locations in
Hawai'i (personal observations). A minimum coverage of
the favored species or other acceptable coral food may be
a necessary requirement to support a large population of
~. Ynimaculatus: a condition satisfied by the high coral
coverage in the north end of Kane'ohe Bay.
~. Ynimaculatus is also unique in terms of foraging
behavior of chaetodontids in Hawai'i. It has a
specialized mouth which allows it to remove skeletal
material as well as coral tissue. This is a more
destructive type of feeding behavior than is generally
associated with chaetodontids and may cause more severe
impacts on corals. The pattern of distribution of
.M. ~.I..I..u~..aa.on Patch Reefs #42 and #43 may result from
the presence of a refuge from grazing and good physical
conditions for growth on the interior of these reef tops
combined with the presense of abundant shelter for
~. Ynimaculatus among the crevices on the reef slopes.
55
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