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Influence of Low Flows on
Abundance of Fish in the
Upper San Pedro River, Arizona
Jerome A. Stefferud and Sally E. Stefferud 1
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
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 m 3 I sec over 85 years, and instantaneous flows between
0.0014 and 2,700 m 3 /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 m 3 /sec (table 1), with a median of 0.27 m 3 /sec. Annual mean
discharges were all below the long-term mean of 1.6 m 3 I sec; daily mean
discharges of 0.00623 m 3 /sec in 1990, and 0.00142 m 3 /sec in 1994, were
record lows. Daily mean discharges during our annual site visits were
between 0.15 (1991) and 0.34 (1993) m 3 /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
daily mean
(IIsee)
33.98
6.23
31.15
28.32
48.14
1.42
31.15
18.41
Calendar
year
1989
1990
1991
1992
1993
1994
1995
1996
398
100%
Highest
daily mean
(m 3/sec)
9.0
36.8
27.5
20.6
217.8
64.3
38.8
22.8
202
358
458
Peak
discharge
(m3/sec)
47.6
79.9
55.2
47.6
325.7
276.4
53.4
58.6
794
161
-
60%,
-
ITTTmTrm
215 200
~Nonnati ve
IIIITIINative
160
ITT'I'1tTTTTtTT
~
Q)
0
c::::
m
"'0
c::::
~
II
rmmTITTTITTT'II
::I
.c
m
Q)
>
':.;j
m
40%
-
20°/o
-
oo/o
-
I-'
Q)
120
(")
"'0
~
1\
.. •
587
i'ffii'ii'i'i1'
80%
Annual
mean
(m 3/sec)
0.42
0.78
0.56
0.44
1.45
0.99
.78
0.54
v
\
80
c
m
0:::
40
0
1990 1991 1992 1993 1994 1995 1996 1997
Total catch (n) indicated
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
Hereford site
1990 41.0
1991 64.6
1992 13.7
1993 54.6
1994 114.6
1995
3.5
1996 79.3
1997
Mean 46.4
StDev 40.1
61.5
31.3
92.1
5.3
59.4
3.5
14.1
4.8
34.0
33.4
AMME
CYCA
GAAF
LECY
MISA
PIPR
30.8
3.9
8.8
3.5
1.6
3.5
3.2
2.9
1.8
1.6
10.6
7.7
7.6
8.7
9.8
0.9
6.2
1.6
6.1
5.1
1.0
2.5
2.8
sees
439
460
919
1022
1107
-1033
1408
948
0.11
0.31
Hiway 90 site
1990
1991
1992
1993
1994
1995
1996
1997
24.7
2.7
25.8
6.1
16.7
5.8
21.7
5.4
Mean 13.6
StDev 9.7
35.1
23.1
40.2
26.1
120.8
21.9
57.2
23.5
43.5
33.5
7.2
2.7
10.1
3.9
3.3
0.8
0.9
0.7
1.7
16.7
1.5
3.6
4.0
3.2
7.2
7.2
4.4
5.8
4.0
5.6
5.0
4.0
10.1
3.6
3.5
3.5
2.2
0.3
0.8
13.6
2.2
30.8
5.2
13.8
0.9
8.8
10.4
1128
1012
1254
1619
1080
1561
1244
996
Charleston site
1990 148.2
1991 18.7
1992 41.1
1993 15.1
1994 26.6
1995
2.2
1996
1997
Mean 31.5
StDev 40.3
61.2
7.2
14.5
36.5
35.4
20.9
57.1
10.4
30.4
20.7
4.9
3.6
1.6
16.3
11.1
4.7
4.9
5.9
5.3
0.7
26.9
2.9
19.3
15.5
0.1
0.3
17.1
25.7
13.4
11.1
3.7
5.0
0.8
6.9
8.9
9.3
11.9
2.8
5.1
4.7
3.7
0.5
1.3
3.8
24.4
1.6
0.7
0.7
5.0
8.0
735
1252
1117
1432
1219
1161
1214
1298
Fairbank site
1990 70.4
1991 88.8
1992 24.9
1993 154.0
1994 115.9
1995 35.8
1996 139.2
1997 80.2
Mean 88.7
StDev 46.0
3.7
5.6
4.6
6.1
49.5
1.8
3.2
3.2
1.4
2.3
35.8
14.0
7.3
12.4
1.5
0.9
3.2
1.5
0.9
5.3
9.7
1.2
8.8
16.8
USDA Forest Service Proceedings RMRS-P-5. 1998
0.5
1.1
3.5
1.4
2.0
486
588
615
976
854
679
653
774
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
·-·:
(11
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00
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en
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0
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en
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:::::l
w
•
•
6
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••
I
1.2
!!lii!!C
•Fairbank (r2=0.003; p<0.892)
.&Charleston (r=0.015; p<O.n4)
•Hiway 90 (r2=0.229; p<0.231)
•Hereford (,a=0.031; p<0.675)
•Combined (,a=0.047; p<0.605)
'..
I
I
1.6
~
••
o~&.""----c
...:::-=---------·----- I
Annual mean discharge, mS/sec
0.8
I
6
~,..
.'
Annual peak discharge, m3/sec
04-----------~----------~--------------~
100
200
0
300
1001 \
200 -1
0.4
.[ •
••
•
•Hiway 90 (r-0.437; p<0.074)
•Hereford (r-0.153: p<0.338)
•Combined (r=G.148; p<0.347)
0
J
T
150
•
•
•
250
······················-·-·-·-·-·.·
200
Highest daily mean discharge, m3/sec
100
•Fairbank (,a=0.256; p<0.201)
A Charleston (r2=0.383; p<0.1 02)
•Hiway90 (r=0.693; p<0.010)
• Hereford (r2=0.613; p<0.022)
•Combined (,a=0.824; p<0.002)
50
~~-
•
Lowest daily mean discharge, 1/sec
0+-------~------~------~------~------~
0
10
20
30
40
50
100
200
0
100
200r.
•Fairbank (r2=0.160; p<0.327)
-&Charleston (r2=0.001; p<0.931)
• Hiway 90 (,a=Q.662; p<0.014)
•Hereford (r2=0.294; p<0.165)
•Combined (r2=0.347; p<0.125)
Figure 3. Least-squares relationships between total CPUE and annual mean discharge (a), annual peak discharge
(b), highest daily discharge (c), and lowest daily discharge (d).
(.)
0..
UJ
::::l
0
Q.
~[
.&Charteston (r-0.027; p<0.697)
•Fairbank (r•0.094; p<0.460)
=
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.
a
Species
Hereford site
AGCH
8.324
PACL
4.897
AMME
2.279
LECY
3.358
PIPR
0.064
GAAF
1.016
Hiway 90 site
AGCH
2.876
PACL
2.286
AMME
1.678
LECY
2.316
PIPR
-3.310
-1.430
GAAF
Charleston site
AGCH
2.202
PACL
1.318
AMME
3.438
LECY
3.947
PIPR
-2.296
GAAF
4.147
Fairbank site
AGCH
60.078
PACL
-8.794
AMME
1.900
LECY
-0.643
PIPR
3.413
GAAF
3.471
All sites combined
AGCH
11.304
PACL
3.440
AMME
2.454
LECY
2.487
-0.472
PIPR
GAAF
1.850
176
b
r2
p<
1.532
1.170
-0.018
-0.035
0.002
0.308
0.346
0.291
0.028
0.036
0.008
0.235
0.125
0.168
0.690
0.651
0.830
0.223
0.432
1.658
0.094
0.049
0.488
0.235
0.474
0.581
0.206
0.046
0.524
0.393
0.059
0.028
0.259
0.610
0.042
0.097
1.178
0.768
0.098
0.045
0.293
0.373
0.135
0.325
0.081
0.021
0.314
0.269
0.370
0.140
0.496
0.730
0.149
0.188
1.150
0.708
-0.021
0.046
-0.081
0.153
0.147
0.420
0.020
0.407
0.394
0.036
0.348
0.082
0.740
0.089
0.096
0.655
1.135
1.079
0.051
0.263
0.191
0.248
0.640
0.766
0.200
0.019
0.445
0.367
0.017
0.004
0.266
0.746
0.071
0.112
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
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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
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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.
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._:;
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.
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