Incorporating demographic information into spawner–recruit analyses alters biological reference point estimates for a western Alaska salmon population

Abstract

Changes over time in age, sex, and length-at-age of returning Pacific salmon have been widely observed, suggesting concurrent declines in per capita reproductive output. Thus, assessment models assuming stationary reproductive output may inaccurately estimate biological reference points that inform harvest policies. We extended age-structured state-space spawner-recruit models to accommodate demographic time trends and fishery selectivity to investigate temporal changes in reference points using Kuskokwim River Chinook salmon (<i>Oncorhynchus tshawytscha</i>). We illustrate that observed demographic changes have likely reduced per capita reproductive output in an additive manner, for example, models including changes in both length-at-age or age composition showed larger declines than models incorporating only one time trend. Translated into biological reference points using a yield-per-recruit algorithm, we found escapement needed for maximum sustained catch has likely increased over time, but the magnitude further depended on size-selective harvest (i.e., larger changes for reference points based on larger mesh gillnets). Compared to traditional salmon assessments, our approach that acknowledges demographic time trends allows more complete use of available data and facilitates evaluating trade-offs among gear-specific harvest policies.

Chinook salmon diversity contributes to fishery stability and trade‐offs with mixed‐stock harvest

“…There was not strong qualitative evidence of differences in age composition among populations, but it is possible that relatively subtle among‐population differences in age composition would result in bias in our estimates of recruitment (which is the sum of fish that returned across ages for a given spawning cohort), which could in turn influence inferences about the shape of the spawner–recruitment relationship and resulting estimates of population productivity and abundance (Zabel & Levin, 2002). We therefore encourage future efforts to characterize the age structure, as well as other dimensions of “escapement quality” (i.e., per‐capita reproductive output), like size and sex ratio (Ohlberger et al, 2020), and changes in these aspects over time at the population level to better understand what the consequences of changes in reproductive potential may be for the system (Staton et al, 2021).…”

Role of maturation and mortality in portfolio effects and climate resilience

“…Our results focus on mean and variability of escapement and harvest, but there are additional benefits of older, larger fish that we did not consider in this study. Older, larger fish likely increase the productivity of populations through allometric relationships with fecundity and egg mass (Kinnison et al 2001;Ohlberger et al 2020;Staton et al 2021). Larger salmon also transport more marine-derived nutrients to freshwater and terrestrial ecosystems Hocking and Reynolds 2011) and provide more energy to large predators such as Southern Resident Killer Whales (Orcinus orca; Ford and Ellis 2006).…”

A century-long time series reveals large declines and greater synchrony in Nass River sockeye salmon size-at-age

“…Furthermore, since body size is correlated with a suite of reproductive traits (Koch and Narum 2021), the effects of declining fecundity likely underestimate the cumulative population-level impacts. For example, declines in total egg mass, which includes variation in both fecundity and egg size, can have stronger effects on productivity than declines in fecundity alone (Ohlberger et al 2020;Staton et al 2021). Additionally sockeye salmon body size covaries with spawning habitat (Rogers 1987;Hendry and Quinn 1997).…”

“…Declines in size may reduce the total amount of habitat accessible to spawners and therefore the carrying capacity of a system. From a management perspective, declines in per capita reproductive output are important because they indicate that escapement goals (i.e., the target number of adult recruits to arrive on spawning grounds) may need to be adjusted to ensure sustainable harvest rates (Staton et al 2021). Beyond impacts on population productivity, there are also negative socioeconomic effects of declines in body size.…”

“…When declines in body size occur in exploited species, these ecological impacts are often accompanied by negative socioeconomic effects. For example, when declines in the mean body size of harvested fishes reduce population productivity, maximum sustainable yield also declines (Conover and Munch 2002;Marshall et al 2021;Staton et al 2021). In the case of subsistence fisheries, negative impacts on food security occur due to both reductions in abundance and the caloric value of individual fish (Oke et al 2020).…”