Expansion of the American white pelican Pelicanus erythrorhynchos colony on Blackfoot Reservoir, Idaho, and the associated declines in adfluvial Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in the upper Blackfoot River drainage has generated concern about the impact of pelican predation on this native trout stock. During a 4-year study, 4,653 wild Yellowstone Cutthroat Trout were tagged using a combination of radiotelemetry and PIT tags. Annual predation rate estimates were made by recovering Yellowstone Cutthroat Trout tags from the nesting islands of American white pelicans. On-island tag recovery rates were corrected for ingested tags that went undetected during island searches and for tags that were deposited away from the nesting islands. American white pelicans consumed tagged Yellowstone Cutthroat Trout ranging from 150 mm to 580 mm TL and showed no size selection within that range for their prey. Predation rates on adult and juvenile Yellowstone Cutthroat Trout generally exceeded 20%, and the highest values were above 60%. Our independent methods (telemetry and PIT tagging) for estimating pelican predation on adult Yellowstone Cutthroat Trout produced similar results. Annual river flow conditions varied markedly and may have contributed to some of the observed range in predation rate estimates. Predation by the pelican colony appears to be a likely contributor to the recent collapse of Yellowstone Cutthroat Trout in the upper Blackfoot River drainage. In the past, overexploitation by anglers severely reduced the trout population and was remedied by implementing catch-and-release regulations. The current predation impact poses a greater management challenge, namely, finding a balanced approach for conserving both the native trout stock and the pelican colony.
In southern Idaho, population growth of American white pelicans Pelecanus erythorhynchos at the Blackfoot Reservoir and Lake Walcott colonies since the early 1990s has generated concerns about whether pelican predation is impacting angler catch of hatchery trout stocked in Idaho waters. To evaluate this concern, we estimated rates of pelican predation (i.e., the proportion of fish consumed by pelicans) and angler catch (i.e., the proportion of fish caught by anglers) for 19 unique springtime fish stocking events over 3 years across 12 study waters; where feasible we also estimated double‐crested cormorant Phalacrocorax auritus predation. Stocked Rainbow Trout Oncorhynchus mykiss averaged 247 mm in length and were internally PIT‐tagged (to monitor bird predation) and externally anchor‐tagged (to monitor angler catch) before stocking. Additional hatchery trout were PIT‐tagged, euthanized, and fed directly to pelicans to estimate PIT tag deposition rates at the colonies; feeding was unsuccessful for cormorants. After the juvenile pelicans and cormorants fledged in the fall, we recovered PIT tags from stocked and fed fish that were deposited at the two colonies. Deposition rates for pelican‐consumed tags averaged 21% and declined exponentially as distance increased from the colonies. Pelican predation on hatchery trout averaged 18% and ranged from 0 to 48%, whereas angler catch averaged 21% and ranged from 0 to 82%. Mean angler catch was nearly four times higher when pelican predation was low (i.e., <25%) than when pelican predation was high (≥25%). Cormorant predation estimates (available for seven stocking events) were minimum estimates only (i.e., they assumed 100% of tags consumed by cormorants were recovered) and averaged 14% (range, 2–38%). Our results suggest that predation by American white pelicans and double‐crested cormorants on catchable‐sized hatchery Rainbow Trout stocked in southern Idaho waters often exceeds the total catch of those fish by anglers who compete directly with avian predators for use of stocked trout.Received June 23, 2015; accepted November 8, 2015 Published online March 30, 2016
Introductions of fertile nonnative hatchery trout have led to interspecific and intraspecific hybridization of native salmonid stocks throughout North America. Use of sterile triploid hatchery trout in stream-stocking programs could reduce genetic risks to native stocks while addressing public demand for consumptive fishing opportunity. Techniques to produce triploid salmonids are well developed, and triploid rainbow trout Oncorhynchus mykiss are readily available from commercial sources. However, there is no published information on the return to creel of triploid trout in stream recreational fisheries. We purchased mixed-sex triploid and diploid rainbow trout eggs from a commercial supplier and reared the resulting fish to catchable size. Flow cytometry was used to verify triploid induction rates in the triploid group. Estimated cost to produce a triploid catchable rainbow trout was about 15% higher than for a diploid fish. We jaw-tagged and stocked 300 triploid and 300 diploid fish into each of 18 streams throughout Idaho and used tag returns to assess relative return to creel and timing of returns for the two groups. In all, 1,849 tags were returned by anglers, 931 from triploid fish and 918 from diploid fish. Overall returns were not significantly different between groups (paired t-test; P ϭ 0.80). Mean time to harvest also did not differ between groups (paired t-test; P ϭ 0.35). These results suggest that triploid rainbow trout can provide stream angling opportunity equal to that provided by fertile diploid fish. Although there are other concerns regarding the stocking of hatchery trout in streams containing native trout, we suggest that using triploid rainbow trout in stream-stocking programs can help balance the demands for both consumptive fishing opportunity and conservation of native stocks.
Kokanees Oncorhynchus nerka (lacustrine sockeye salmon) and Utah chub Gila atraria feed extensively on similar sizes and species of zooplankton in Flaming Gorge Reservoir. We measured the effects of Utah chub on the growth of kokanees in six 80-m 3 enclosures containing ambient zooplankton populations. Three kokanees and 0, 3, 6, 12, or 24 Utah chub were weighed and placed in each enclosure. After 21 d, fish were removed and kokanee growth was compared among treatments. Zooplankton biomass, chlorophyll-^ concentrations, and temperature profiles were monitored during each test. In May 1992, no competitive interactions were observed; in June 1992, however, zooplankton biomass and kokanee growth declined significantly as Utah chub densities increased. The best predictor of kokanee growth was Daphnia pulex density, which explained 76% of the variance in kokanee growth. Differences in temperature and initial zooplankton biomass affected the strength of observed competition. Our results suggest that monitoring forage resources rather than nongame fish densities may provide a better framework for assessing competition among pelagic fish species.
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