The reproductive value of large females: consequences of shifts in demographic structure for population reproductive potential in Chinook salmon

Abstract

While conservation and fisheries management are often concerned with changes in population abundance and distribution, shifts in population age–size structure are commonly observed in response to human and environmental stressors. Chinook salmon (Oncorhynchus tshawytscha) have experienced widespread declines in mean age and size throughout their North American range. We investigated the consequences of declines in body size for spawner reproductive potential in terms of total egg mass per female. Our case study is the Yukon River where Chinook salmon have supported subsistence, commercial, and recreational fisheries. Using historical observations on individual body size from throughout the Yukon River and the relationship between female size and total egg mass from the Canadian portion, we estimate a decline in average female reproductive potential of 24%–35% since the 1970s. Because spawner abundances and the population sex ratio have not shown clear trends over time, our results suggest a reduced total population reproductive potential. Changes in spawner quality should be considered when developing management reference points, and conservation of population demographic structure may be necessary to sustain productive Chinook salmon systems.

Heterogeneous genetic basis of age at maturity in salmonid fishes

“…The discovery of genetic variation underlying ecologically important phenotypic traits across species has significant potential to inform their conservation and management, because architectures that predict evolution in one might be applied to the other. Age at maturity has been declining in many species of Pacific salmonids over recent decades (Bowersox et al, 2019; Cline et al, 2019; Lewis et al, 2015; Ohlberger et al, 2018), leading to lower reproductive potential of females (Ohlberger et al, 2020) and decreased population resilience through reduced phenotypic diversity (Greene et al, 2010; Schindler et al, 2010). Although six6 is a candidate gene significantly associated with age at maturity in Sockeye and Steelhead, markers developed from this study need to be validated more broadly to determine population and sex‐specific effects across the geographic distribution of each species, and the relative contributions of other loci to phenotypic variance need to be identified.…”

Genomic prediction of growth in a commercially, recreationally, and culturally important marine resource, the Australian snapper (Chrysophrys auratus)

“…Moreover, reproductive success is often broken into two components: reproductive potential and offspring survival, and in many marine fish species both components are strongly related to body size. Because female fish retain their oocytes internally during their development, maximum reproductive output will be subject to body size constraints (Lambert 2008; Ohlberger et al . 2020).…”

“…Moreover, reproductive success is often broken into two components: reproductive potential and offspring survival, and in many marine fish species both components are strongly related to body size. Because female fish retain their oocytes internally during their development, maximum reproductive output will be subject to body size constraints (Lambert 2008;Ohlberger et al 2020). Alternatively, reduced body size can result in small eggs that maximise the number produced, but with a significant reduction of offspring survival (Einum and Fleming 2000;Wootton et al 2021).…”

Development of a Simple Morphometric Model to Identify Sex in Chinook Salmon Returning to Spawn in the Yukon River

“…Since then, there has been a prolonged period of poor returns resulting in annual harvests in the most recent decade averaging less than 30,000 fish (JTC 2019). During this same period, age and size compositions of the run have also changed, with fewer older and larger fish returning to spawn each year, leading to declines in the reproductive potential of females specifically, and the broader Yukon River stock more generally (Ohlberger et al 2018(Ohlberger et al , 2020b.…”

“…Current management of the Yukon River Chinook Salmon fishery is primarily concerned with the number of fish that escape the fishery and reach the spawning streams (Evenson et al 2009;JTC 2019). The observed declines in recent years, however, have led to an understanding that the demographic qualities of the escapement, age composition, size structure, and sex ratio, may be just as important as the number of spawning fish for sustainable management of this valuable resource (Ohlberger et al 2020a(Ohlberger et al , 2020b.…”