GSI-based Timing Indices Reveal Unexpected Migratory Pattern in Alaskan Bycatch of Fraser River Sockeye Salmon

Abstract Excerpt:
Marine distributions and migration routes of salmon influence when, where, and by whom the salmon may be caught (e.g., Michielsens and Martens 2022). Information regarding the high seas distributions of salmon and their returns to coastal areas during maturation tend to rely on older data with regional resolution (McKinnell et al. 2012), but changes in distributions and migration routes over time are likely complex with intra-specific variation within regions (Shelton et al. 2021). For managers of sockeye salmon (Oncorhynchus nerka) from the Fraser River, near the southern limit of the species range in North America, this is a poor foundation from which to understand, plan for, and adapt to the consequences of climate change. For example, Fraser sockeye bycatch comprises < 1% of Alaska’s District 104 pink salmon (O. gorbuscha) fishery, but episodically constitutes a non-negligible fraction of the total Fraser sockeye recruitment in some years (e.g., a record high estimate of 13% in 2019; Pacific Salmon Commission 2021). Regarding catches of Fraser sockeye in this fishery, the Pacific Salmon Treaty states, “the extent of these incidental catches is unpredictable from year to year” (Pacific Salmon Commission 2020). As the North Pacific Ocean warms, however, the rearing distributions and homeward migrations of Fraser sockeye could shift northward to cooler waters (Healey 2011; McKinnell et al. 2012), resulting in an increased frequency of high exploitation rates in northern fisheries. Understanding the current migration is critical to understanding the consequences of future changes. We used Genetic Stock Identification (GSI) to compare stock compositions in coastal fisheries in southeast Alaska and southern British Columbia to learn about variation among Fraser sockeye stocks in marine distribution and migratory behaviour.