Can late stage marine mortality explain observed shifts in age structure of Chinook salmon?

Abstract

Chinook salmon (Oncorhynchus tshawytscha) populations have experienced widespread declines in abundance and abrupt shifts toward younger and smaller adults returning to spawn in rivers. The causal agents underpinning these shifts are largely unknown. Here we investigate the potential role of late-stage marine mortality, defined as occurring after the first winter at sea, in driving this species’ changing age structure. Simulations using a stage-based life cycle model that included additional mortality during after the first winter at sea better reflected observed changes in the age structure of a well-studied and representative population of Chinook salmon from the Yukon River drainage, compared with a model estimating environmentally-driven variation in age-specific survival alone. Although the specific agents of late-stage mortality are not known, our finding is consistent with work reporting predation by salmon sharks (Lamna ditropis) and marine mammals including killer whales (Orcinus orca). Taken as a whole, this work suggests that Pacific salmon mortality after the first winter at sea is likely to be higher than previously thought and highlights the need to investigate selective sources of mortality, such as predation, as major contributors to rapidly changing age structure of spawning adult Chinook salmon.

Programmed acoustic tags reveal novel information on late‐phase marine life in Atlantic salmon, Salmo salar

“…The total return of tagged adults to the river Bush (3) in comparison to the potential return to the coast (5-9 fish) implies potential losses of between 40% and 66% from the available returning cohort during the final 45 km of open ocean migration. Recent work on Pacific salmon has also indicated that late-stage marine mortality, defined as mortality after the first winter at sea, was higher than had previously been assumed (Manishin et al, 2021) and, alongside the present work, highlights the need for more focus on the sources of late-stage mortality. In the current study, late-stage marine losses may be linked to predation.…”

Interactions between life history and the environment on changing growth rates of Chinook salmon

“…Furthermore, our analysis focused on potential bottom-up effects on the size-at-age of Chinook salmon and did not account for possible top-down mechanisms (e.g. Ohlberger et al 2019;Manishin et al 2021). Because of these limitations, it is important to consider our findings in the context of prior work on the topic and other forms of evidence.…”

Role of maturation and mortality in portfolio effects and climate resilience

“…However, age 6 fish are rarely observed today, and most fish return to spawn at age 3 (Satterthwaite et al 2017). Shifts in the age structure of Chinook salmon are likely due to a combination of factors, including fisheries, warming environmental conditions, marine predation, competition, and hatchery practices (Barnett et al 2017;Chasco et al 2017;Ohlberger et al 2018Ohlberger et al , 2019Seitz et al 2019;Oke et al 2020;Manishin et al 2021;Munsch et al 2022).…”