|
June 1989 Marsh and Brooks-Ictalurid predation on Xyrauchen 189
<br />All stocked fish were produced at Dexter National Fish Hatchery (DNFH), New Mexico. The first
<br />" two groups were held in ponds at DNFH until transported and planted, while the last was transferred
<br />as fry (approximately 10 mm) to AZGFD Page Springs Hatchery for grow-out and held in raceways
<br />for 16 days prior to stocking. Subsequent study showed that raceway holding did not result in acclimation
<br />! to lotic conditions (J. E. Brooks, pers. obser.). Methods of handling, counting, and transport followed
<br />protocols established by the United States Fish and Wildlife Service (1978). Transport temperature
<br />was adjusted to ambient stream temperature, and fish were tempered to site conditions for 1 to 4 h in
<br />a live car; no evidence of thermal or other stress was observed before stocking.
<br />Resident fishes were sampled for 2 days before and after stockings in September 1984 and 1985 and
<br />January 1986. Collections were with hoop nets (O.S m diameter, 5 m long, 0.6-cm mesh) having paired
<br />wings (1.8 by 1.2 m, 1.3 cm-mesh), trammel nets (30.5 to 91,4 m by 1.8 m, 2.5-cm inner and 30.5-
<br />cm outer mesh; all bar measures), and seines of various sizes and meshes. Sampling in 1985 and 1986
<br />also included electroshocking (115 volt AC, approximately 4 amperes, provided by a 900-w, ATV-
<br />mounted gasoline generator and delivered through two, hand-held stainless steel electrodes mounted
<br />on fiberglass probes). Stationary nets were tended at least four times daily, seines were used mostly in
<br />daylight, and electrofishing was during both day and night.
<br />In 1984 and 1986, fishes were identified and enumerated, and representative specimens including
<br />all catfish were preserved in 10% formalin for later examination. In 1985, a multiple mark-recapture
<br />population estimate of channel catfish and flathead catfish was made in a 2.5-km reach extending
<br />downstream from 0,5 km above the stocking site. Catfish were collected, marked by hole-punch in the
<br />caudal fin, and released during the 48-h period before introduction of razorback suckers. Thereafter,
<br />all catfish were killed immediately upon capture. Small specimens were preserved whole; larger ones
<br />were measured, weighed, and dissected to remove digestive tracts, which were preserved for later study.
<br />Stomachs were examined and contents identified, enumerated (where appropriate), and weighed.
<br />Razorback suckers from stomachs were measured, when possible. All fish measurements were standard
<br />length in millimeters to enable inclusion of partially digested specimens. Consumption rates were
<br />combined with population estimates to assess the effects of predation. Size was compared from year to
<br />year for each target species by analysis of variance, ANOVA (a posteriori F-test), and relationships
<br />between sizes of predators and their razorback sucker prey were examined by simple linear regression
<br />(Snedecor and Cochran, 1967),
<br />Population estimates for catfishes followed the method of Schumaker and Eschmeyer (Gerking, 1953;
<br />Ricker, 1975). Estimates were for channel catfish longer than 150 mm and flathead catfish longer than
<br />100 mm, the minimum lengths at which these fishes were observed to eat razorback suckers of the sizes
<br />stocked; total catfish populations were thus not determined. Stream widths were measured throughout
<br />the reach (n = 100) to estimate surface area, and density computations were converted to numbers
<br />(fish/ha) and biomass (kg/ha).
<br />RESULTS AND DISCUSSION-Channel and flathead catfishes made up only a
<br />small proportion of the total fish fauna but were dominant among larger species
<br />and were the only large piscivores collected. Native species were minor com-
<br />ponents of the fish community in the study area. Longfin dace (Agosia chryso-
<br />gaster), typically the most widespread and abundant of desert fishes (Minckley
<br />and Barber, 1971; Minckley, 1973), composed at most 10% of the total number
<br />of fishes collected, and two other indigenous species, desert sucker (Pantosteus
<br />clarki) and Sonoran sucker (Catostomus insignis) each constituted about 0.1%.
<br />Non-native red shiner (Notropis lutrensis) were abundant, making up more than
<br />80% of total fishes caught. Large carp (Cyprinus carpio) were uncommon,
<br />•' contributing about 1.0% of the total fish taken. Forage commonly utilized by
<br />piscivorous catfish (e.g., carp, red shiner, resident native suckers) was available
<br />throughout the study reach.
<br />t Estimated populations in 1985 were 41 fish/ha for flathead catfish (95%
<br />confidence limits of 29 to 71) and 281 fish/ha for channel catfish (99 to 330);
<br />biomass estimates were 20.7 and 56.5 kg/ha, respectively. Stomach contents
|