Influence of Low Flows on Abundance of Fish in the

Upper San Pedro River, Arizona Jerome A. Stefferud and Sally E. Stefferud1

Abstract.-The relationship between stream discharge and abun­dance of fish in the upper San Pedro River, Arizona, was exam­ined. Backpack electrofishing equipment was used to capture fish at four sites in a 50-kilometer reach between the International Boundary and Fairbank. Surveys were done annually in the spring, 1990 to 1997. Catch per unit of effort (CPUE: number of individuals per 15 minutes shocking) varied between years (mean = 89.1, range= 32.7 to 167.8). Native desert sucker, Catostomus clarki, and longfin dace, Agosia chrysogaster, comprised 57 to 85% of the annual catch. In addition to the two native species, black bullhead, Ameiurus melas, and fathead minnow, Pimephales promelas, were captured in all eight efforts. Seven surveys in­cluded western mosquitofish, Gambusia affinis, green sunfish, Lepomis cyanellus, were in the last six efforts, largemouth bass, Micropterus salmoides, were in two, and common carp, Cyprinus carpio, was taken once. Fish abundance appeared related to ex­treme low flow during the previous year, but not peak or mean flows, or total runoff. Lowest daily mean discharges (mean= 251/ sec, range= 1 to 48) explained 82% of the variation in catch for all species and all sites combined, and 78% and 70% of the variation for native and nonnative species, respectively.

Resumen.-La relaci6n entre descarga de arroyo y Ia abundancia de peces localizados en Ia parte arriba del Rio San Pedro, Arizona se examino. Equipo electrico para pescado en mochila se uso para capturar peces en cuatro sitios entre 50 kilometros de Ia Frontera Internacional y Fairbank. Cada afio se completaron estudios entre 1990 a 1997 en Ia primavera. Captura por esfuerzo de unidad (CPEU: cantidad de individuales por cada 15 minutos de choques electricos) variado entre afios (medio = 89.1, surtido = 32.7 a 167.8). Peces nativo matalote del desierto, Catostomus clarki, y charalito aleta larga, Agosia chrysogaster, constar de 57% asta 85% de Ia captura annual. Ademas de los dos especies nativos, el barge, Ameiurus melas, y carpita cabezona, Pimephales promelas, se capturaron en todo los ocho esfuerzos. En siete estudios incluido

1 315 East Medlock Drive, Phoenix, Arizona 85012

USDA Forest Service Proceedings RMRS-P-5. 1998 167

This file was created by scanning the printed publication.Errors identified by the software have been corrected;

however, some errors may remain.

el guajacon mosquito, Gambusia affinis, pez sol, Lepomis cyanellus) fueron en los ultimos seis esfuerzos. La lobina negra, Micropterus salmoides, fueron en dos, y la carpa comun, Cyprinus carpio, se capturaron en un es fuerzo. Se parese que la abundancia de los peces sera relacionado de las corrientes extremas de agua baja durante el afio pasado, pero de medio o al tiempo mas alta de la corriente, ode escurrimento total. Corrientes extremas de agua baja (medio = 25 1/ seg, surtido = 1 a 48) se explico al 73% de variaci6n en captura de todo los especies y todo los sitios combinadas, y 78% y 70% por los especies nativos o no nativos, respectivo.

INTRODUCTION

Patterns of stream discharge influence population dynamics of fishes in streams of arid southwestern North America, and the retention of a natu­ral hydrograph is vital in sustaining native fishes (Rinne and Minckley, 1991). Extreme floods have been shown to differentially affect native and nonnative fishes (Meffe and Minckley, 1987; Minckley and Meffe, 1987), and during periods of extreme drying of desert watercourses mass mor­talities of fishes occur (Minckley and Barber, 1971; Carpenter and Maughan, 1993). In general, high flood flows are beneficial to native fishes (Stefferud and Rinne, 1997; Rinne and Stefferud, 1997), but low flows, particularly as they approach zero, may have serious consequences for sustainability of the native fishery (Jackson et al., 1987; Neary and Rinne, 1998). Most large streams in this region have been dammed or diverted and their natural hydrograph substantially altered, but a few streams persist where surface flows remain continuous and reflect primarily rain­fall runoff and ground-water discharge (Rinne and Minckley, 1991).

The upper ca. 100-km of the San Pedro River in south-central Arizona and north-central Sonora (figure 1) is essentially unaffected by diversions and dams. Watershed alterations and increased ground water pumping have reduced mean annual flows during the past century, however, the river remains perennial between Hereford and Fairbank (Jackson et al., 1987). Along this reach, the San Pedro Riparian National Conservation Area (NCA) was established in the mid-1980's for the purposes of protec­tion and management of riparian ecosystems, wildlife, and prehistoric and historic resources (Yuncevich, 1993). Although once accommodating 13 native species of fish, currently only two common and widespread native fishes persist in the mainstem: longfin dace, Agosia chrysogaster, and desert

168 USDA Forest Service Proceedings RMRS-P-5. 1998

sucker, Catostomus clarki; about a dozen nonnative fishes are established (Jackson et al., 1987).

In 1989, we initiated a study to observe changes in fish species abundance and occurrence through time and space, and fish densities relative to aquatic microhabitat types in the perennial reach of the upper San Pedro River. The primary objective of our study was to provide the management agency long-term monitoring information on fisheries to achieve the goals of the NCA' s establishment. During our study period record low flows occurred, which appeared to have meaningful consequences on abundance of fish in the river. Here we provide results from eight years of sampling.

Tombstone+

CI'MIIIeaton / Hiway90

SieiT8 V181a +

Hereford

SONORA

0Studyalte 6. Ctwteaton USGS gage

Figure 1. The upper San Pedro River in Arizona and Sonora. Assemblages of fish were sampled at the study sites indicated by closed circles.

STUDY AREA

The San Pedro River arises near Cananea, Sonora, and flows northward about 40 km before entering Arizona. Within Arizona, it continues about 200 km more to its confluence with the Gila River near Winkelman (Minckley, 1985). Our study area encompassed a 40-km reach in the NCA between Hereford and Fairbank, where we established four sample sites (north to south: Fairbank, Charleston, Hiway 90, and Hereford). General location of the study sites was chosen based primarily on reasonable access and linear disposition along the study reach. Specific locations were

USDA Forest Service Proceedings RMRS-P-5. 1998 169

selected during a pre-project survey and were based on channel morphol­ogy and types of habitat present. We hiked much of the 40-km reach to determine how representative our sites were of the overall stream mor­phology. Since we had selected sample sites for habitat heterogeneity in order to increase probability of sampling the entire fish assemblage, our sites were more diverse in terms of habitat type sequencing and occur­rence than what is characteristic of the stream.

Within the study reach, the river flows in a broad channel lined with cottonwood, willow, salt cedar, and mesquite 2 to 10 m below the former floodplain. Presently, active channel widths range from about 60 min straight, confined reaches to nearly 500 m in curved reaches with large point bars. Sinuosity ranges between 1.0 and 1.8, becoming straighter in downstream reaches. Gradient is decidedly convex, ranging from roughly 0.19 percent in the southern reaches to roughly 0.38 percent at the north­ern end of the study area (Jackson et al., 1987). Drainage area at the U.S. Geological Survey (USGS) stream gage at Charleston is 3,196 km2 (USGS 1997).

The annual precipitation cycle is one of a distinct wet season from July through September followed by occasional rainfall from November to late March. Early April through June is the driest time of the year when drought or near-drought conditions prevail (Hereford, 1992). Discharge has averaged 1.6 m3 I sec over 85 years, and instantaneous flows between 0.0014 and 2,700 m3 /sec have been recorded (USGS 1997). During this century, mean annual discharge at Charleston has steadily decreased (Jackson et al., 1987).

METHODS

We sampled about 250 m of stream at each of four sites, but exact length was dependent on habitat complexity. In general, a riffle-pool sequence was selected, and then the study site expanded to include other significant habitat types present. Habitat types were defined by combinations of gradient, subjective descriptions of channel shape and turbulence, and relative position in the stream channel (Bisson et al., 1982; Aadland, 1993). Sampling was done annually during late April to early May, a time when young-of-year specimens were unlikely to be captured by our methods, thus our catch reflected individuals surviving from the previous year.

Habitat types were sampled for fish in sequence, progressing from downstream to upstream. A backpack, DC, electrofishing unit was used to stun fish for capture with hand-held dipnets. Total seconds of elapsed shocking time were recorded by habitat type. All fish captured within a habitat type were identified to species and enumerated before being

170 USDA Forest Service Proceedings RMRS-P-5. 1998

returned to the stream. Measurements of length, width, depth, and ocular evaluations of size of substrate were made at each habitat type. To ensure similarity of effort between years and to document gross changes in chan­nel morphology, each year a map of the entire site was sketched and location of habitat types noted.

Catch per unit effort (CPUE) was determined as number of individuals captured per 15 minutes shocking time. Morisita' s index was used to compare similarity of fish assemblages between sites and years (Morisita, 1959). Discharge records were obtained from the continuous recording gage at Charleston (USGS records). Least-squares regression lines were fit to scatter plots to determine the relationships between CPUE and annual mean discharge, instantaneous peak discharge, highest daily mean dis­charge, and lowest daily mean discharge during the calendar year previ­ous to sampling.

RESULTS

During the study period, daily mean discharges ranged between 0.0014 and 217.8 m3 /sec (table 1), with a median of 0.27 m3 /sec. Annual mean discharges were all below the long-term mean of 1.6 m3 I sec; daily mean discharges of 0.00623 m3 /sec in 1990, and 0.00142 m3 /sec in 1994, were record lows. Daily mean discharges during our annual site visits were between 0.15 (1991) and 0.34 (1993) m3 /sec (USGS records). Peak dis­charges recorded during our study period had a recurrence interval of about 5 years or less, and the two highest occurred in winter and spring.

Fairbank and Hereford sites were similar in habitats, both being wide and shallow with shifting sand substrates, and with times when flow is at or near zero. Hereford included a few pools, whereas Fairbank was virtu­ally absent of pools. Charleston and Highway 90 sites were more diverse in habitat, with deep pools, swift riffles, and runs and glides. Both were strongly influenced by large ephemeral washes entering immediately upstream of the sample area, which during high flows scoured and depos­ited large quantities of sand and gravel into the upper portion of each site.

Mean catch at all sites combined was 397 fish per year (range 161 to 794) with longfin dace and desert sucker decreasing from 87% of the total catch the second year of the study to 57% in 1997 (figure 2); CPUE was between 32.7 and 167.8 (x = 89.1). The ratio of native to nonnative fishes did not appear influenced by flooding that occurred in 1993 and 1994. In addition to the two native species, black bullhead, Ameiurus melas, and fathead minnow, Pimephales promelas, were captured in all efforts. Seven surveys included western mosquitofish, Gambusia affinis, green sunfish, Lepomis cyanellus, were in the last six efforts, largemouth bass, Micropterus

USDA Forest Service Proceedings RMRS-P-5. 1998 171

salmoides, and common carp, Cyprinus carpio, were taken once each. No pattern in longitudinal distribution of any species was apparent, although number of species was higher at Charleston and Highway 90 sites. Over­all, longfin dace was predominant at Hereford and Fairbank, and desert sucker at Highway 90 (table 2). No nonnative species comprised greater than 10% of the total8-yr catch at any site, except fathead minnow at Highway 90, and western mosquitofish at Charleston. The appearance of green sunfish beginning in 1993 is likely the result of that year's flood overtopping gravel ponds near the Highway 90 site and allowing the resident green sunfish there to escape into the river.

Table 1. Discharge data from Charleston USGS gage used in this study.

Lowest Highest Peak Annual Calendar daily mean daily mean discharge mean

year (IIsee) (m3/sec) (m3/sec) (m3/sec) 1989 33.98 9.0 47.6 0.42 1990 6.23 36.8 79.9 0.78 1991 31.15 27.5 55.2 0.56 1992 28.32 20.6 47.6 0.44 1993 48.14 217.8 325.7 1.45 1994 1.42 64.3 276.4 0.99 1995 31.15 38.8 53.4 .78 1996 18.41 22.8 58.6 0.54

100% 398 202 358 458 794 161 587 215 200

~Nonnati ve

80% i'ffii'ii'i'i1' IIIITIINative

- ITTTmTrm ITT'I'1tTTTTtTT

160 Q) ~

0 c:::: m "'0 60%, c:::: ::I

II ~

-rmmTITTTITTT'II

120 .c m Q)

40% > ':.;j m

1\ ~

- v \ I-'

80 Q) 0:::

20°/o - 40

oo/o - 0 1990 1991 1992 1993 1994 1995 1996 1997

Total catch (n) indicated

(") "'0 c m

Figure 2. Relative abundance of native and nonnative fishes, and total CPUE of annual catches, 1990 to 1997. Total catch (n) indicated by numbers atop columns.

172 USDA Forest Service Proceedings RMRS-P-5. 1998

.. •

Table 2. CPUE of fish (number/900 sees) at four sites during 1990 to 1997. AGCH=Iongfin dace, PACL=desert sucker, AMME=black bullhead, CVCA=common carp, GAAF=western mosquitofish, LECV=green sunfish, MISA=Iargemouth bass, and PIPR=fathead minnow.

Year AGCH PACL AMME CYCA GAAF LECY MISA PIPR sees

Hereford site 1990 41.0 61.5 30.8 439 1991 64.6 31.3 3.9 460 1992 13.7 92.1 8.8 919 1993 54.6 5.3 3.5 6.2 0.9 1022 1994 114.6 59.4 1.6 10.6 1.6 1107 1995 3.5 3.5 3.5 6.1 -1033 1996 79.3 14.1 3.2 7.7 5.1 1408 1997 4.8 2.9 7.6 1.0 948 Mean 46.4 34.0 1.8 8.7 2.5 0.11 StDev 40.1 33.4 1.6 9.8 2.8 0.31 Hiway 90 site 1990 24.7 35.1 7.2 0.8 4.0 1128 1991 2.7 23.1 2.7 0.9 1012 1992 25.8 40.2 10.1 0.7 13.6 1254 1993 6.1 26.1 3.9 1.7 5.6 2.2 1619 1994 16.7 120.8 3.3 16.7 5.0 30.8 1080 1995 5.8 21.9 4.0 5.2 1561 1996 21.7 57.2 1.5 7.2 10.1 2.2 13.8 1244 1997 5.4 23.5 3.6 7.2 3.6 0.9 996 Mean 13.6 43.5 4.0 4.4 3.5 0.3 8.8 StDev 9.7 33.5 3.2 5.8 3.5 0.8 10.4

Charleston site 1990 148.2 61.2 4.9 26.9 3.7 735 1991 18.7 7.2 3.6 0.7 2.9 5.0 1252 1992 41.1 14.5 1.6 19.3 0.8 1117 1993 15.1 36.5 16.3 6.9 3.8 1432 1994 26.6 35.4 11.1 15.5 8.9 24.4 1219 1995 20.9 4.7 9.3 1.6 1161 1996 2.2 57.1 17.1 11.9 3.7 0.7 1214 1997 10.4 4.9 25.7 2.8 0.7 1298 Mean 31.5 30.4 5.9 0.1 13.4 5.1 0.5 5.0 StDev 40.3 20.7 5.3 0.3 11.1 4.7 1.3 8.0

Fairbank site 1990 70.4 5.6 3.7 486 1991 88.8 4.6 6.1 588 1992 24.9 1.5 1.5 615 1993 154.0 1.8 0.9 0.9 976 1994 115.9 49.5 3.2 3.2 3.2 854 1995 35.8 5.3 679 1996 139.2 9.7 35.8 653 1997 80.2 1.2 14.0 3.5 774 Mean 88.7 8.8 1.4 7.3 0.5 1.4 StDev 46.0 16.8 2.3 12.4 1.1 2.0

USDA Forest Service Proceedings RMRS-P-5. 1998 173

In terms of similarity of the assemblage between years at each site, Fairbank and Highway 90 were highly consistent, with Morisita index values greater than 0.88 and 0.83, respectively. The assemblage at Charles­ton showed moderate consistency between years, whereas Hereford had index values below 0.60 in five out of seven comparisons. For the eight­year period, all sites were relatively similar, except Fairbank, which was highly dissimilar to Highway 90. Since all sites were generally similar between years and during the study period, we felt it appropriate to combine data from the four sites by year for analysis of discharge relation­ships.

Neither annual mean discharge nor annual peak discharge during the year previous to sampling had significant influence on the variation in total CPUE at any site, nor for all sites combined (figures 3a and b). How­ever, with the exception of Charleston, the general trend was an increase in CPUE with increasing annual mean discharge. Trends in CPUE were neutral or slightly positive with increases in annual peak discharge; again Charleston had a slightly negative trend. When CPUE was plotted against highest daily mean discharge (figure 3c), the trend was again positive at three sites and for all sites combined, and neutral at Charleston. Highest daily mean discharge explained 66% of the variation in CPUE at Highway 90. Annual mean, annual peak, and highest daily mean flows studied were not well-distributed across their respective ranges, in each case, six or seven flows were in the lower 25 to 50%-ile of flows, and one or two in the upper quartile.

Lowest daily mean discharges were well-distributed across the range of flows studied, explaining between 38% and 69% of the variation in CPUE at Highway 90, Hereford, and Charleston sites. When all sites were com­bined, 82% of the variation in CPUE was explained by low discharges during the previous year (figure 3d). Although the relationship between low discharge and CPUE at Fairbank was not significant, that site also showed a positive trend in CPUE with increasing minimum discharges.

Trends of CPUE of individual species at each site generally were posi­tive when plotted against lowest daily mean flow (table 3), although few explained much of the variation or provided significant relationships. Slope of the regression line was generally steeper for the two native spe­cies at each site than for the nonnative species. Regression line slopes for the nonnative species were either negative or close to neutral with increas­ing minimum flows. Common carp and largemouth bass were not consid­ered in this analysis due to their relative rarity in the catch.

174 USDA Forest Service Proceedings RMRS-P-5. 1998

·-·:

c en

0 )>

"T1

0 Ci) ~

en

CD <

cs·

CD

"U

0 0 CD

CD

a.

:;·

co

C/)

::D

s:

::D

en

I "U

I ~

......

co

co

CXI """

(11

w

:::::l

Q.

0 UJ

::::l

0..

(.)

~[

.[ • • •

0.4

200

-1

• •

1001

\ "" 6

•Fai

rban

k (r

•0.0

94;

p<0.

460)

.&

Cha

rtes

ton

(r-0

.027

; p<0

.697

) •H

iway

90

(r-0

.437

; p<

0.07

4)

•Her

efor

d (r

-0.1

53:

p<0.

338)

' •

•Co

mb

ine

d (r

=G.1

48; p

<0.3

47) . ~ ~,.. I

• I

6 •

I I

0.8

1.2

1.6

Ann

ual m

ean

disc

harg

e, m

S/s

ec

•Fa

irb

an

k (r2

=0.0

03; p

<0.8

92)

.&C

harle

ston

(r=

0.01

5; p

<O

.n4)

•H

iwa

y 9

0 (r2

=0.2

29; p

<0.2

31)

•He

refo

rd (,

a=0.

031;

p<0

.675

)

' •C

om

bin

ed

(,a=

0.04

7; p

<0.6

05)

.. --c

o~&.

""--

!!li

i!!C

...::

:-=----

-----·--

---I

• • 04-----------~----------~--------------~

0 10

0 20

0 30

0

Ann

ual p

ea

k di

scha

rge,

m3

/se

c

200r.

•F

air

ba

nk

(r2=0

.160

; p<

0.32

7)

-&C

harle

ston

(r2

=0.0

01; p

<0.9

31)

• H

iway

90

(,a=Q

.662

; p<0

.014

) •H

ere

ford

(r2=

0.29

4; p

<0.1

65)

• •C

om

bin

ed

(r2=

0.34

7; p

<0.1

25)

100 J T

~~-

• ···

······

······

······

·-·-·-

·-·-·.

·

0 0

50

100

150

200

250

Hig

he

st d

aily

me

an

dis

char

ge,

m3

/se

c

•Fa

irb

an

k (,a

=0.2

56; p

<0.2

01)

A C

harle

ston

(r2=

0.38

3; p

<0.1

02)

200

•Hiw

ay9

0 (r

=0.

693;

p<0

.010

)

• H

eref

ord

(r2=0

.613

; p<0

.022

) •

•Co

mb

ine

d (,

a=0.

824;

p<0

.002

)

•

100 0+-------~------~------~------~------~

0 10

20

30

40

50

Lo

we

st d

aily

mea

n d

isch

arg

e,

1/se

c

Fig

ure

3.

Lea

st-s

qu

ares

rel

atio

nsh

ips

bet

wee

n t

ota

l C

PU

E a

nd

an

nu

al m

ean

dis

char

ge

(a),

an

nu

al p

eak

dis

char

ge

(b),

hig

hes

t d

aily

dis

char

ge

(c),

an

d l

ow

est

dai

ly d

isch

arg

e (d

).

Table 3. Least-squares regression (V = a + bX) of lowest daily mean discharge versus CPUE of each species at each site during 1990 to 1997. Acronyms are the same as in table 2.

Species a b r2 p<

Hereford site AGCH 8.324 1.532 0.346 0.125 PACL 4.897 1.170 0.291 0.168 AMME 2.279 -0.018 0.028 0.690 LECY 3.358 -0.035 0.036 0.651 PIPR 0.064 0.002 0.008 0.830 GAAF 1.016 0.308 0.235 0.223

Hiway 90 site AGCH 2.876 0.432 0.474 0.059 PACL 2.286 1.658 0.581 0.028 AMME 1.678 0.094 0.206 0.259 LECY 2.316 0.049 0.046 0.610 PIPR -3.310 0.488 0.524 0.042 GAAF -1.430 0.235 0.393 0.097

Charleston site AGCH 2.202 1.178 0.135 0.370 PACL 1.318 0.768 0.325 0.140 AMME 3.438 0.098 0.081 0.496 LECY 3.947 0.045 0.021 0.730 PIPR -2.296 0.293 0.314 0.149 GAAF 4.147 0.373 0.269 0.188

Fairbank site AGCH 60.078 1.150 0.147 0.348 PACL -8.794 0.708 0.420 0.082 AMME 1.900 -0.021 0.020 0.740 LECY -0.643 0.046 0.407 0.089 PIPR 3.413 -0.081 0.394 0.096 GAAF 3.471 0.153 0.036 0.655

All sites combined AGCH 11.304 1.135 0.640 0.017 PACL 3.440 1.079 0.766 0.004 AMME 2.454 0.051 0.200 0.266 LECY 2.487 0.263 0.019 0.746 ·· . . '

PIPR -0.472 0.191 0.445 0.071 GAAF 1.850 0.248 0.367 0.112

176 USDA Forest Service Proceedings RMRS-P-5. 1998

CONCLUSIONS

The native fishes of the Southwest are adapted to a hydrologic pattern of flood and drought, having high reproductive potential and being stimu­lated to spawn by flood events (John, 1963; Minckley and Meffe, 1987; Meffe and Minckley, 1987; Rinne and Stefferud, 1997). In the San Pedro River, however, floods during 1993 and 1994 had little apparent effect on total abundance, native:nonnative ratios, or presence/ absence of species, as has been found elsewhere. This could be attributed to a number of reasons, including the size of the floods (both <5-year recurrence interval events) not being large enough to displace individuals or species, or occur­ring at a time of the year when the biology of the fishes was unlikely to be affected. In addition, lack of floods that were capable of mobilizing sub­strate materials may have resulted in spawning substrate becoming em­bedded and unsuitable, and nursery habitat being invaded by emergent vegetation. Another explanation could be that the increased channel stability of the San Pedro River as a result of removal of livestock for several years could be ameliorating the effects of floods on the biota.

Whereas floods can significantly affect fish populations, drought condi­tions may be far more perilous (Jackson et al., 1987; Neary and Rinne, 1998). Effects of drought include crowding into reduced habitats with increased potential for disease or starvation, conditions which may also inhibit spawning and reduce reproductive success. Crowding into low water situations also likely increases predation and competition, particu­larly when predacious species are present. Low water conditions can also induce changes in water chemistry that may affect fish, including elevated temperatures and lowered dissolved oxygen concentrations (Lowe et al., 1967).

Eight years of regular and consistent sampling of the fish community at four sites in the upper San Pedro River indicated that discharge during the year previous to sampling had an effect on the abundance of fish there. In general, CPUE for all fish and at all sites decreased after years with lower flows and increased after years with higher flows. This general trend was not particularly strong for peak, annual mean, or highest daily mean discharges, but was strong for lowest daily mean flows. This may have reflected the limited range of high flows that occurred during the study period. However, lowest daily mean flows were well-distributed between the extremes, indicating that the elements that contribute to reduced populations during droughts were affecting the populations.

In particular, the native fishes responded more positively to increases in lowest daily discharges than did nonnative fishes. Longfin dace and desert

USDA Forest Service Proceedings RMRS-P-5. 1998 177

sucker prefer flowing water habitats (Rinne, 1992), whereas the nonnatives are more pool-oriented (Cross and Collins, 1995). Longfin dace becomes most abundant in hot, shallow, sandy-bottomed desert streams. It rarely occupies deep pools, and uses shallow glide and run habitats with sand substrates for spawning. Although it normally spawns in the spring, it will also spawn in the summer in response to freshets. Desert sucker lives in hard-bottomed, shallow streams, but occupies turbulent water more often than longfin dace, and breeds and feeds on riffles; spawning occurs in the spring (Minckley, 1973; Rinne, 1992). These types of habitats de­crease rapidly as flow recedes, thus the amount of habitat available be­coines limited during extreme low flow periods (Neary and Rinne, 1998). Extreme low flows, which occurred in June, could have affected survival and recruitment of young-of-year individuals, and may have diminished adult populations of both species, resulting in limited reproductive effort during subsequent spawning efforts. Additionally, during extreme low flow the small-bodied native fishes are forced into sharing habitat with the larger, predacious fishes, and thus are subject to more predation than in years of higher flows.

All of the nonnative species prefer pool or backwater habitats of calm water over soft substrates, and reproduction takes place in these same habitats (Jenkins and Burkhead, 1994; Cross and Collins, 1995). Even during periods of extreme drought, these areas retain habitat characteris­tics suitable to support the complete life cycles of the nonnative species. The nonnative ictalurid and centrarchids are long-lived species capable of several years of reproductive effort. Thus, even if one or more of their year-classes are lost or diminished due to drought, the adults can survive till conditions for spawning improve.

The upper San Pedro River is a habitat-limited system (Jackson et al., 1987). Where once it was a narrow, unentrenched stream with extensive marshes and beaver ponds characterized by high storage and slow release of water with high habitat heterogeneity, it now has little storage, rapid runoff depletion, and habitats are relatively homogenous. Predominant habitat types are runs, glides, and shallow riffles; deep pools and riffles are limited in extent and occurrence (Velasco, 1993). Some reaches of the study area dry completely during extreme drought, particularly near Hereford and Fairbank (M. Fredlake, Bureau of Land Management, pers. comm.). Jackson et al. (1987) considered that the fishery in the San Pedro River could be maintained under the present flow regime, and likely enhanced for existing fish species if median monthly flows were elevated to long-term norms. They noted that a return to a pre-incision state would permit an increase in diversity of native species, and recommended that higher flows somehow be reestablished. But flows since their study have

178 USDA Forest Service Proceedings RMRS-P-5. 1998

continued to decline, a trend that will presumably continue, thus the fate of the fishery there is problematic.

Obviously, a dry streambed will not sustain fish, but one with some reaches of perennial water can. So long as perennial surface flows remain continuous in a few reaches during a substantial portion of the year, and the hydrograph continues to reflect rainfall runoff and ground-water discharge, the species now present should survive. However, potential for the San Pedro River to ever regain its historic fauna is severely limited under the current conditions. Recovery to pre-incisement habitat condi­tions may also prove disastrous for the native fish so long as nonnative species are present. Before the San Pedro River became incised, habitats were predominantly pools, cienegas, and marshes (Hendrickson and Minckley, 1984), areas that today would harbor nonnative species. Thus management efforts should be focused on removing the nonnative species, or barring that, emphasizing processes that promote formation of riffle and run habitat types, and de-emphasizing those that result in pools.

ACKNOWLEDGMENTS

This work was done under volunteer agreement with the Bureau of Land Management, Huachuca City, Arizona. We greatly appreciate the encouragement and assistance of Mark Fredlake of the Bureau, who se­cured us vehicular access to otherwise restricted sites. Fish sampling equipment was furnished by U. S. Fish and Wildlife Service, Arizona Ecological Services Field Office, and USDA Forest Service, Tonto National Forest. The Spanish translation of the abstract was graciously provided by Gustavo Tellez.

REFERENCES

Aadland, L. P. 1993. Stream habitat types: their fish assemblages and relationship to flow. North American Journal of Fisheries Management 13:790-806.

Bisson, P. A., J. L. Nielsen, R. A. Palmasson, and L. E. Grove. 1982. A sys­tem of naming habitat types in small streams with examples of habitat utilization by salmonids during low streamflow. Pages 62-73 in N. B. Armantrout, editor. Acquisition and utilization of aquatic habitat inven­tory information. Proceedings of a symposium held 28-30 October 1981, Portland, Oregon. American Fisheries Society, Western Division, Bethesda, Maryland.

USDA Forest Service Proceedings RMRS-P-5. 1998 179

Carpenter, J., and 0. E. Maughan. 1993. Macrohabitat of Sonora chub (Gila ditaenia) in Sycamore Creek, Santa Cruz County, Arizona. Journal of Freshwater Ecology 8:265-278.

Cross, F. B., and J. T. Collins. 1995. Fishes in Kansas. University of Kansas Natural History Museum, Lawrence.

Hendrickson, D. A., and W. L. Minckley. 1984. Cienegas-vanishing climax communities of the American southwest. Desert Plants 6:131-175.

Hereford, R. 1992. Geomorphic evolution of the San Pedro River channel since 1900 in the San Pedro Riparian National Conservation Area, south­east Arizona. U. S. Geological Survey Open-File Report 92-339:1-71.

Jackson, W. L., T. Martinez, P. Cuplin, et al. 1987. Assessment of water conditions and management opportunities in support of riparian val­ues: BLM San Pedro River Properties, Arizona. Project Completion Report. U. S. Bureau of Land Management Service Center BLM/YA/ PT88/004+7200, Denver, CO.

Jenkins, R. E., and N. M. Burkhead. 1994. Freshwater fishes of Virginia. American Fisheries Society, Bethesda, Maryland.

John, K. R. 1963. The effect of torrential rains on the reproductive cycle of Rhinichthys osculus in the Chiricahua Mountains, Arizona. Copeia 1963:286-291.

Lowe, C. H., D. S. Hinds, and E. A. Halpern. 1967. Experimental cata­strophic selection and tolerances to low oxygen concentration in native Arizona freshwater fishes. Ecology 48:1013-1017.

Meffe, G. K., and W. L. Minckley. 1987. Persistence and stability of fish and invertebrate assemblages in a repeatedly disturbed Sonoran desert stream. The American Midland Naturalist 117:177-191.

Minckley, W. L. 1973. Fishes of Arizona. Arizona Game and Fish Depart­ment, Phoenix.

Minckley, W. L. 1985. Native fishes and natural aquatic habitats in U.S. Fish and Wildlife Service Region II west of the continental divide. Final Report for U.S. Fish and Wildlife Service, Albuquerque, New Mexico. Arizona State University, Tempe.

Minckley, W. L., and W. E. Barber. 1971. Some aspects of biology of the longfin dace, a cyprinid fish characteristic of streams in the Sonoran desert. Southwestern Naturalist 159:159-464.

Minckley, W. L., and G. K. Meffe. 1987. Differential selection by flooding in stream fish communities of the arid American southwest. Pages 93-104 in W. J. Matthews, and D. C. Heins, editors. Community and evolution­ary ecology of North American stream fishes, University of Oklahoma Press, Norman.

180 USDA Forest Service Proceedings RMRS-P-5. 1998

Morisita, M. 1959. Measuring of interspecific association and similarity between communities. Memoirs of the Faculty of Science at K yushu University, Series E (Biology) 3:65-80.

Neary, D. G., and J. N. Rinne. 1998. Baseflow trends in the upper Verde River relative to fish habitat requirements. Hydrology and Water Re­sources in Arizona and the Southwest 27:57-63.

Rinne, J. N. 1992. Physical habitat utilization of fish in a Sonoran desert stream, Arizona, southwestern United States. Ecology of Freshwater Fish 1 :35-41.

Rinne, J. N., and W. L. Minckley. 1991. Native fishes of arid lands: A dwin­dling resource of the desert southwest. USDA Forest Service General Technical Report RM-206:1-45.

Rinne, J. N., and J. A. Stefferud. 1997. Factors contributing to collapse yet maintenance of a native fish community in the desert Southwest (USA). Pages 157-162 in D. A. Hancock, D. C. Smith, A. Grant, and J.P. Beumer, editors. Developing and sustaining world fisheries resources, 2nd World Fisheries Congress, Sydney, Australia.

Stefferud, J. A., and J. N. Rinne. 1997. Effects of floods on fishes in the upper Verde River, Arizona. Proceedings of the Desert Fishes Council 28:80-81.

U. S. Geological Survey (USGS). Water resources data for Arizona. U. S. Geological Survey Water-Data Report AZ-xx-1, Tucson.

Velasco, A. L. 1993. Summary report of the San Pedro River mesohabitat mapping survey conducted June-July 1993. U.S. Fish and Wildlife Service, Ecological Services, Phoenix, Arizona.

Yuncevich, G. M. 1993. The San Pedro Riparian National Conservation area. Pages 369-372 in B. H. Tellman, J. Cortner, M. G. Wallace, L. F. DeBano, and R. H. Hamre, editors. Riparian management: common threads and shared interests, USDA Forest Service, General Technical Report RM-226, Fort Collins, Colorado.

BIOGRAPHICAL SKETCH

Jerry and Sally Stefferud voluntarily study fishes in the San Pedro River as a break from their day jobs and a reason to get out of the city and back to reality once a year.

USDA Forest Service Proceedings RMRS-P-5. 1998 181

._:;