Throughout the northern extent of its range, Chinook salmon (Oncorhynchus tshawytscha) adult returns have been in decline for over a decade, leading to severe harvest restrictions on subsistence, commercial, and recreational fisheries. In addition to these overall declines in abundance, changes in size structure and age structure, including a proportional decrease of older age classes returning to spawn, suggest that late-stage marine mortality for this species may be more frequent than currently assumed. To examine this late-stage mortality hypothesis, we examined diagnostic evidence of predation on large (57–100 cm fork length) Chinook salmon (n = 33) from depth, temperature, and light records collected during recent satellite tagging research. Satellite tags provided evidence of predation on tagged Chinook salmon by salmon sharks (Lamna ditropis) (n = 14), marine mammals (n = 2), ectothermic fish(es) (n = 3), and unidentified predators (n = 5) in the Bering Sea and Gulf of Alaska. High mortality rates in this study suggest that fisheries scientists should consider that late-stage mortality by marine apex predators may be shaping this species’ abundance and demographics.
Top predators, such as salmon sharks (Lamna ditropis), can influence the abundance and population structure of organisms at lower trophic levels through direct effects, such as predation mortality, and indirect interactions. As a first step towards better understanding the average annual prey consumption for individual adult salmon sharks, we bracketed consumption estimates using three methods: (1) daily ration requirement; (2) bioenergetic mass balance; and (3) a Bayesian model of shark growth. In the first method, we applied ration estimates for related lamnid shark species that yielded salmon shark estimates of 1461 and 2202kgyear–1. The second method used a mass–balance technique to incorporate life history information from salmon sharks and physiological parameters from other species and produced estimates of 1870, 2070, 1610 and 1762kgyear–1, depending on assumed diet. Growth modelling used salmon shark growth histories and yielded estimates of 16900 or 20800kgyear–1, depending on assumed assimilation efficiency. Of the consumption estimates, those from the mass–balance technique may be the most realistic because they incorporated salmon shark life history data and do not produce extreme values. Taken as a whole, these estimates suggest that salmon sharks have similar energetic requirements to piscivorous marine mammals.
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.
Steelhead Oncorhynchus mykiss exhibit substantial life history variation throughout their range, but the “half‐pounder” life history is limited to several rivers in northern California and southern Oregon. Half‐pounders first return to freshwater as immature fish after spending just 3–5 months at sea and support valuable freshwater fisheries. In the main‐stem Klamath and Rogue rivers, historically and at present, half‐pounder expression rates were and remain high for fall‐run steelhead. We used visual classifications of the half‐pounder life history from scales collected from the Trinity River to estimate half‐pounder expression rates and investigate the role of size and age prior to ocean entry on the probability that adult steelhead expressed the half‐pounder life history. Half‐pounder expression rates among both wild and hatchery steelhead in the Trinity River have substantially declined since 1982. Among the most common age‐2 wild smolt type, 71% of adults returning in 1982 had previously made a half‐pounder run, whereas estimated rates for adults returning in various years from 1994 to 2013 averaged just 10% (range = 2–25%). Estimated returns of half‐pounders and adults from Trinity River Hatchery releases made prior to 1982 are consistent with a historically high half‐pounder expression rate. Among age‐1 hatchery steelhead smolts, a similar decline seems evident and seems in part related to a large increase in size at release in 1992. Strong negative associations were found between the size prior to ocean entry (wild and hatchery fish) and smolt age (wild fish) and the probability that an individual steelhead expressed the half‐pounder trait; smaller‐sized and younger smolts were more likely to express the trait. Further research focusing on the heritability of this life history trait should be conducted to develop strategies intended to prevent further loss of this unique life history among Trinity River fall‐run steelhead.
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