An Investigation of the Potential Effects of Selective Exploitation on the Demography and Productivity of Yukon River Chinook Salmon
Dates
Year
2008
Citation
Bromaghin, Jeffrey F., Nielson, Ryan M., and Hard, Jeffrey J., 2008, An Investigation of the Potential Effects of Selective Exploitation on the Demography and Productivity of Yukon River Chinook Salmon: US Fish and Wildlife Service: Anchorage, AK, v. Alaska Fisheries Technical Report Number 100.
Summary
Yukon River Chinook salmon (Oncorhynchus tshawytscha) have been targeted in large-mesh gill net fisheries for over 100 years. Recent reductions in productivity and perceptions of reduced size and earlier age at maturation have elevated concerns regarding the potential consequences of the selective exploitation of large fish. Investigations associating changes in population productivity and demography, particularly size and age composition, with overharvest and selective exploitation are common in the fisheries literature. However, most such investigations have considered long-lived species repeatedly subject to exploitation, often prior to maturation. Similar investigations concerning semelparous Pacific salmon are comparatively limited [...]
Summary
Yukon River Chinook salmon (Oncorhynchus tshawytscha) have been targeted in large-mesh gill net fisheries for over 100 years. Recent reductions in productivity and perceptions of reduced size and earlier age at maturation have elevated concerns regarding the potential consequences of the selective exploitation of large fish. Investigations associating changes in population productivity and demography, particularly size and age composition, with overharvest and selective exploitation are common in the fisheries literature. However, most such investigations have considered long-lived species repeatedly subject to exploitation, often prior to maturation. Similar investigations concerning semelparous Pacific salmon are comparatively limited in number, and largely inconclusive with respect to cause. Exploratory analyses of data collected during fishery sampling or stock assessment activities may be hampered by the lack of pre-fishery data to provide a baseline, short time series of available data, biased samples obtained via selective gear, and high levels of natural variation. In addition, retrospective analyses of observational data are insufficient to ascribe cause. For these reasons, we chose to investigate the potential long-term effects of large-mesh gill net fisheries on Chinook salmon by stochastic modeling. We constructed an individual-based model integrating population dynamics and the heritability of traits, using information from Yukon River Chinook salmon to guide model construction when possible, and simulated the effects of selective exploitation under a variety of productivity and fishing scenarios. In most cases considered, the mean size and age at maturation declined rapidly for approximately 50 years and stabilized at reduced levels after approximately 100 years. In these cases, subsequent adoption of gill nets with moderately reduced mesh size was not effective in reversing prior declines in mean size and age unless exploitation rates were also reduced. Our results suggest that long-term, selective exploitation of large Chinook salmon is likely to cause reductions in fish size and maturation age, and impair population productivity. The effectiveness of management strategies to reverse prior effects of selective exploitation appears to be enhanced by the concurrent reduction of both exploitation rates and selectivity for large individuals, especially if implemented before large declines in mean size and age have been induced.
