Research Document - 2007/019
Status of Birkenhead River Chinook salmon (Oncorhynchus tshawytscha)
By Schubert, N.D., J.R. Candy,.R. Cook, J. Greenbank, D. Lofthouse, R. McNicol, C.K. Parken, D. Sneddon, J.A. Tadey, and K.H. Wilson
Abstract
Birkenhead River Chinook (Oncorhynchus tshawytscha) is a Fraser spring run population that spawns in a tributary of the Harrison-Lillooet River system in southwestern BC. It is a genetically isolated population that possesses local adaptations (e.g., far north marine distribution and very early spawner migration) that are an important component of the evolutionary legacy of the species.
Population status is assessed by evaluating spawner abundance and trends relative to potential benchmarks, fishery harvest and potential limiting factors and threats. This required the reconstruction of escapement and terminal fishery catch estimates based on a reevaluation of historic data and the inclusion of recent data. The spawner population has been trendless over a thirty year period when both enhancement was attempted and conservation actions were applied to the fisheries. The spawner population averaged 480, with an effective population size of about 300; both are below literature estimates for viable, genetically isolated populations. Potential benchmarks are discussed, with the lowest (Ŝmsy = 1,700) over triple current abundances, suggesting that considerable population growth is required.
Small populations are especially vulnerable to threats such as those posed by fishery exploitation, climate change and habitat alteration. The Birkenhead has been harvested at about 50%, with three fisheries predominant: Alaska troll and the First Nations fisheries in the lower Fraser and Lillooet System. The impacts of climate change are already apparent in freshwater and are expected to increase in future decades, while the threat from habitat alteration, geomorphic processes and rapid human population growth remain significant. A comprehensive recovery plan is required.
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