Development of an Index to Bird Predation of Juvenile Salmonids within the Yakima River, 2001 Annual Report.
- University of Washington, School of Aquatic and Fishery Sciences, Seattle, WA
Avian predation of fish is suspected to contribute to the loss of out-migrating juvenile salmonids in the Yakima Basin, potentially constraining natural and artificial production. In 1997 and 1998, the Yakima/ Klickitat Fisheries Project (YKFP)--whose goal is increasing natural production within the Yakima River--initiated investigations to assess the feasibility of developing an index to avian predation of juvenile salmon within the river. This research confirmed that Ring-billed Gulls and Common Mergansers were the primary avian predators of juvenile salmon (Phinney et al. 1998), and that under certain conditions could significantly impact migrating smolt populations. Beginning in 1999, the Washington Cooperative Fish and Wildlife Research Unit (WACFWRU) was asked by the YKFP to continue development of avian consumption indices. Monitoring methods developed by Phinney et al. (1998) were adopted (with modifications) and monitoring of impacts to juvenile salmon along river reaches and at areas of high predator/prey concentrations (colloquially referred to as ''hotspots'') has continued each year through 2001. In 2001, piscivorous birds were counted from river banks at hotspots and from a raft or drift boat along river reaches. Consumption by gulls at hotspots was based on direct observations of foraging success and modeled abundance; consumption by all other piscivorous birds was estimated using published dietary requirements and modeled abundance. Seasonal patterns of avian piscivore abundance were identified, diurnal patterns of gull abundance at hotspots were identified, and predation indices were calculated for hotspots and river reaches (for both spring and summer). Changes in survey methods in 2001 included the addition of surveys in the ''Canyon'' reach during spring and altering the method of directly measuring gull feeding rates at hotspots. Primary avian predators in 2001 were ''gulls'' (California and Ring-billed) at hotspots and Common Mergansers within upper river reaches. Consumption on the lower reaches was distributed among a number of species, with slightly more then half of all fish consumption being attributed to American White Pelicans. Estimated consumption by gulls at both hotspots combined (8 Apr-30 Jun) was 169,883 fish. Assuming a worst case scenario (all fish taken were smolts) this represented approximately 4.9% of all smolts estimated passing or being released from the Prosser Dam area during the 2001 smolt migration season. Total gull abundances and estimates of consumption between the two hotspot sites were opposite that seen in 2000. Foraging gulls at Horn Rapids Dam were regressed against flow for the 3 years and found to be significant (alpha = 0.1, P=0.081, r{sup 2} = .2589). A similar 3-year regression vs fish passage through the Chandler Juvenile Fish Facility, however, did not show a significant relationship (alpha = 0.1, P = 0.396, r{sup 2} = .3708). Total estimated take by Common Mergansers across all strata surveyed was 14,777 kg between 8 Apr and 31 Aug, 2001. Approximately 66 percent of that consumption was within the upper river reaches (Stratum 1) where there is a known breeding population of mergansers. Graphical comparisons of merganser abundances over the three years (1999-2001) in the upper reaches of the Yakima River suggest an increase in 2001 from the previous 2 years in both the spring and summer survey periods, but overlapping confidence intervals prevent assumptions regarding upward or downward trends in abundance.
- Research Organization:
- Bonneville Power Administration, Portland, OR (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- 00004666
- OSTI ID:
- 812190
- Report Number(s):
- DOE/BP-00004666-6; R&D Project: 199506424; TRN: US200313%%139
- Resource Relation:
- Other Information: PBD: 1 Aug 2002
- Country of Publication:
- United States
- Language:
- English
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Development of an Index to Bird Predation of Juvenile Salmonids within the Yakima River, 1999 Annual Report.
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