Maintaining a wild phenotype in a conservation hatchery program for Chinook salmon: The effect of managed breeding on early male maturation

Abstract

In many salmonid species, age and size at maturation is plastic and influenced by the interaction between genetic and environmental factors. Hatchery reared salmon often mature at an earlier age and smaller size than wild fish. Modern salmon conservation efforts have focused on managing the level of gene flow between hatchery and natural origin fish to minimize potential genotypic and phenotypic change. In salmonids, maturation probability is dependent on exceeding a genetically set threshold in growth rate and energetic status (and by association, body size) referred to as the probabalisitic maturation reaction norm (PMRN). Over fourteen years, we monitored the frequency of age-2 precocious male maturation (common term: age-2 minijack rate) and the PMRN of natural founder (FNDR), integrated natural-hatchery (INT), and segregated hatchery (SEG) broodlines of spring Chinook salmon, Oncorhynchus tshawytscha . The average age-2 minijack rate (± SEM) of the FNDR, INT and SEG broodlines was 48.2 ± 5.2%, 41.9 ± 3.6% and 30.9 ± 4.7%, respectively. Additionally, the PMRN W P 50 (predicted weight at 50% maturation) of the SEG broodline was significantly greater (20.5 g) than that of the FNDR/INT broodlines (18.2 g). We also conducted a common garden experiment exploring the effects of less than one [INT (0–1)], one [SEG (1)] or two [SEG (2)] generations of hatchery culture on the age-2 minijack rate and PMRN W P 50 . Growth was not significantly different among broodlines, but age-2 minijack rates were significantly lower following two consecutive generations of hatchery culture: [INT (0–1): 68.3 ± 1.7%], [SEG (1): 70.3 ± 1.8%] and [SEG (2): 58.6 ± 0.4%] and the PMRN W P 50 was significantly higher by 6.1 g after two generations of SEG culture. These results indicate that managed gene flow reduces phenotypic divergence, but may serve to maintain potentially undesirably high age-2 minijack rates in salmon conservation hatchery programs.

Heterogeneous genetic basis of age at maturity in salmonid fishes

“…Our study also included both hatchery and wild fish. Hatcheries have been known to truncate age at maturity (Larsen et al, 2019), and different selection regimes and environments of hatcheries might influence the genetic architecture underlying the phenotype (McKinney et al, 2020). We addressed the former by standardizing age at maturity between populations, but acknowledge that the latter may have played a role in our study.…”

“…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).…”

A mobile sex-determining region, male-specific haplotypes, and rearing environment influence age at maturity in Chinook salmon

“…This has been observed in both Pacific and Atlantic salmon (Larsen et al 2004, Good and Davidson 2016). Water temperature and feed rations at hatcheries are often optimized for high growth, which in turn promotes early maturation (Larsen et al 2019); however, hatchery stocks vary in the proportion of males with premature maturation even when raised under identical conditions suggesting genetic differences in susceptibility to early maturation (Spangenberg et al 2015). Hatchery mating practices, which are often random with respect to size and age, mightalso have inadvertently selected for younger fish (Hankin et al 2009).…”

A mobile sex‐determining region, male‐specific haplotypes and rearing environment influence age at maturity in Chinook salmon

“…This has been observed in both Pacific and Atlantic salmon (Good & Davidson, 2016; Larsen et al., 2004). Water temperature and feed rations at hatcheries are often optimized for high growth, which in turn promotes early maturation (Larsen et al., 2019). Hatchery stocks vary in the proportion of males with early maturation, even when raised under identical conditions, suggesting genetic differences in susceptibility to early maturation (Spangenberg et al., 2015).…”