Abundance, Distribution and Estimated Consumption (kg fish) of Piscivorous Birds Along the Yakima River, Washington State; Implications for Fisheries Management, 2002 Annual Report.
- University of Washington, Quantitive Ecology Program, Seattle, WA
This report is divided into two chapters. The abstract for chapter one is--Understanding of the abundance and spatial and temporal distributions of piscivorous birds and their potential consumption of fish is an increasingly important aspect of fisheries management. During 1999-2002, we determined the abundance and distribution and estimated the maximum consumption (kg biomass) of fish-eating birds along the length of the Yakima River in Washington State. Sixteen different species were observed during the 4-yr study, but only half of those were observed during all years. Abundance and estimated consumption of fish within the upper and middle sections of the river were dominated by common mergansers (Mergus merganser) which are known to breed in those reaches. Common mergansers accounted for 78 to 94% of the estimated total fish take for the upper river or approximately 28,383 {+-} 1,041 kg over the 4 yrs. A greater diversity of avian piscivores occurred in the lower river and potential impacts to fish populations was more evenly distributed among the species. In 1999-2000, great blue herons potentially accounted for 29 and 36% of the fish consumed, whereas in 2001-2002 American white pelicans accounted for 53 and 55%. We estimated that approximately 75,878 {+-} 6,616 kg of fish were consumed by piscivorous birds in the lower sections of the river during the study. Bird assemblages differed spatially along the river with a greater abundance of colonial nesting species within the lower sections of the river, especially during spring and the nesting season. The abundance of avian piscivores and consumption estimates are discussed within the context of salmonid supplementation efforts on the river and juvenile out-migration. The abstract for chapter two is--Consumption of fish by piscivorous birds may be a significant constraint on efforts to enhance salmonid populations within tributaries to the Columbia River in Washington State. During 1999-2002, we determined the abundance of fish-eating birds, primarily ring-billed (Larus delawarensis) and California (L. californicus) gulls and monitored their behavior at two man-made structures within the Yakima River in eastern Washington: Horn Rapids Dam, a low-head irrigation dam, and the return pipe for the Chandler Juvenile Fish Handling Facility. Earlier observations of congregations of gulls at these structures suggested an increased likelihood of predation of out-migrating juvenile salmonids. We estimated the number of fish consumed and examined the relationship between river flow and gull numbers and fish taken. Numbers of gulls at the structures varied daily between their arrival in Late March-early April and departure in late June (mean ({+-}SE) - Horn Rapids: 11.7 ({+-}2.0), Chandler: 20.1 ({+-}1.5) ). During the 4-yr study, numbers at Horn Rapids peaked dramatically during the last 2 weeks in May (between 132.9 ({+-}4.2) to 36.6 ({+-}2.2) gulls/day) and appeared to the associated with the release of > 1-mil hatchery juvenile fall chinook (Oncorhynchus tshawytscha) above the 2 study sites. A comparable peak in gull abundance was not observed at Chandler. Diurnal patterns of gull abundance also varied among years and sites. The relationship between foraging efficiency and gull numbers was not consistent among years or sites. Gull numbers were not correlated with river flow when year was considered. However, variations in flow among years appeared to be associated with average gull numbers at each site, but trends were not consistent between sites. Low seasonal flows were associated with increased predation at Chandler, whereas high seasonal flows were associated with increased predation at Horn Rapids. Assuming all fish taken were salmonids, we estimate gulls consumed between 0.1-10.3 % of the juvenile salmonids passing or being released from the Chandler Juvenile Fish Monitoring Facility located above the two structures. Staggered releases of hatchery fish, nocturnal releases of fish entrained in the Chandler facility, changes in the orientation of the outflow from the facility, and physical deterrents (e.g., sprinklers or overhead wires) may significantly reduce take by gulls at these sites.
- Research Organization:
- Bonneville Power Administration (BPA), Portland, OR (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- 00004666
- OSTI ID:
- 887179
- Report Number(s):
- DOE/BP-00004666-11; R&D Project: 199506424; TRN: US200617%%583
- Country of Publication:
- United States
- Language:
- English
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