Examining Life History Shifts and Genetic Composition in a Hatchery Steelhead Population, with Implications for Fishery and Ocean Selection

Abstract

Steelhead Oncorhynchus mykiss exhibit significant variation in life histories. Dworshak National Fish Hatchery (NFH)‐stock steelhead exhibit a life history that is unique compared to other Columbia River basin hatchery summer steelhead stocks, as it is dominated by older and larger‐length fish relative to conspecifics. We evaluated changes within the Dworshak NFH stock's ocean age composition and length at age from run years 1978 to 2016 using samples collected from the commercial gill‐net fishery, the sport fishery, and the hatchery trap. In addition, we analyzed the genetic composition of the stock by comparing samples collected in 1969 from the wild founding stock to samples from recent returns. We found that Dworshak NFH steelhead became younger across the study, with fewer of the oldest, three‐ocean fish (i.e., rearing in the ocean for 3 years) and increases in the proportion of the population returning as one‐ocean fish (i.e., rearing in the ocean for 1 year). Length at ocean age decreased for all ocean ages; however, the decrease was not significant in one‐ocean fish. In general, samples collected from gill nets and the sport fishery contained fish with older ocean ages and larger sizes at age compared to samples collected from the hatchery trap. Neutral genetic diversity, as measured from a panel of single‐nucleotide polymorphic markers, showed no changes between the sample of fish representing the original founders and the present‐day population. Life history shifts observed in Dworshak NFH‐stock steelhead were likely influenced by a variety of factors, including gear selectivity and changing ocean conditions. Decreases in ocean age composition and length at age have implications for both fishery and broodstock management as well as generating concern that similar trends may be occurring within wild steelhead stocks.

TheGill‐Oxygen Limitation Theoryand size at maturity/maximum size relationships for salmonid populations occupying flowing waters

“…Two of these studies documented increased mean length in the youngest ocean age groups while mean sizes in older age groups declined across multidecadal scales (Ohlberger et al ., 2018; Siegel et al ., 2017). A similar “shrinking” phenomenon has also been reported in populations of Atlantic salmon Salmo salar (Todd et al ., 2008), steelhead Oncorhynchus mykiss (Bowersox et al ., 2019) and most recently for all five species of Alaska salmon (Oke et al ., 2020). Despite the fact that these temporal life history changes could be explained by ocean warming and the GOLT (Cheung et al ., 2013; Pauly & Cheung, 2018), it was only mentioned in a single one of the studies as a possible explanation.…”

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.…”

“…Second, such life-history variation reduces inter-annual variability in adult returns and the frequency of fishery closures, which benefits commercial, recreational, and subsistence fisheries and associated communities (Brown & Godduhn, 2015; Copeland, Ackerman, Wright, & Byrne, 2017; Greene et al, 2010; Schindler et al, 2010). Third, knowledge of age structure is important for modeling the effects of fisheries harvest and developing appropriate restoration strategies aimed at maintaining population diversity in fitness traits (Bowersox, Corsi, McCormick, Copeland, & Campbell, 2019; Hankin & Healey, 1986; Larsen et al, 2019; Ricker, 1980).…”

Steelhead (Oncorhynchus mykiss) lineages and sexes show variable patterns of association of adult migration timing and age‐at‐maturity traits with two genomic regions

“…Because these 2+-ocean steelhead, some of which are returning to ESA-protected Snake River populations, are particularly susceptible to this fishery, the catch limits of steelhead can often restrict the gillnet fishery targeting Chinook salmon (Copeland et al, 2017). However, as the correlation between size, ocean-age, and population composition appears to have become less clear over time (Bowersox et al, 2019;Copeland et al, 2017;Keefer et al, 2018), we encourage continued investigation of the utility of the chromosome 25 markers examined herein, which appear closely tied to the biological mechanisms for predicting size and age composition, to assist in sustaining life history diversity in these populations.…”

“…Steelhead in the Columbia River typically spend 1 or 2 years in the ocean, with more rare occurrences of fish that spend 3 or 4 years in salt water (Busby et al., 1996). While most populations exhibit variation in age‐at‐maturity, certain Snake River tributary populations have historically been predominately 2+‐ocean fish that achieve large body size (Bowersox, Corsi, McCormick, Copeland, & Campbell, 2019; Copeland et al., 2017). However, as with run timing, geographically proximal populations that differ in predominant age‐at‐maturity are nonetheless more genetically similar than more distant populations of the same predominant age (Hess, Ackerman, et al, 2016).…”