Age at Release, Size, and Maturation Status Influence Residualism in Hatchery Steelhead

Abstract

Steelhead Oncorhynchus mykiss that fail to emigrate seaward after release from hatcheries, commonly referred to as “residuals,” can have negative impacts on natural populations ranging from competition and predation to interbreeding with returning anadromous adults. We investigated how age at release, size, and maturation status influenced the rate of residual production in hatchery summer‐run steelhead released from the Winthrop National Fish Hatchery (Methow River, Washington) between 2010 and 2015. Migration data from 21,598 individuals implanted with PIT tags identified 1,783 residual steelhead expressing two distinct phenotypes: immature male and female parr; and precociously mature males. We found that age at release significantly affected the predominant residual phenotype. Age‐1 steelhead residuals (S1 rearing strategy) were dominated by smaller parr of both sexes (fish < 146 mm FL), while age‐2 residuals (S2 rearing strategy) were dominated by mature males, although both phenotypes were present in both S1 and S2 residual groups. Collections of residual steelhead in the Methow River indicated that parr phenotype residuals grew as well as natural‐origin juvenile O. mykiss, suggesting potential competition for food resources and habitat. Detections of PIT tags within the Methow River basin indicated that precocious male residuals may overlap both spatially and temporally with previously documented spawning anadromous adults, posing a potential genetic management risk. Both residual phenotypes had poor overwinter survival, and only 1 of the 1,783 residual fish eventually returned to the Methow River as an anadromous adult. We conclude that the ecological and genetic consequences of residual steelhead far outweigh their potential contribution to anadromous production, and measures should be taken to reduce their production by changing hatchery rearing practices.

Temporal Dynamics of DNA Methylation Patterns in Response to Rearing Juvenile Steelhead (Oncorhynchus mykiss) in a Hatchery versus Simulated Stream Environment

“…Genetic males were identified by the presence of the male sex determining gene, sdY [32], following the procedure described in [33]. Briefly, genomic DNA was PCR amplified using the sdY primers as described by [32].…”

Plasticity of Behavioral and Growth Responses to Two Feeding Regimes and Implications for the Domestication of Steelhead Trout

“…Feeding motivation, expressed in the present study as feeding rate, played a greater role than did territorial competitive ability in increasing growth, and it may be an important factor that promotes domestication selection in hatchery environments. Feeding rate was strongly associated with subsequent growth in the hatchery tanks at both ration levels, indicating a plausible link between feeding behavior and size at release, a trait that is likely under selection after release due to low survival (Tatara et al 2017) or residualism (Tatara et al 2019) of smaller fish. Differences in agonistic behavior between wild and hatchery Chinook Salmon O. tshawytscha have been shown to affect growth rate under conditions of contest and scramble competition in aquaria, and the behavioral differences were attributed to a response to domestication selection (Pearsons et al 2007).…”

“…However, the effectiveness of hatchery programs that are directed at supporting the conservation or recovery of natural populations may be hindered by inadvertent genetic selection on fitness‐related traits that are caused by adaptations to hatchery environments (i.e., domestication selection; Price 2002; Araki et al 2008; McClure et al 2008; Lorenzen et al 2012). Domestication selection has been implicated as the most likely cause of fitness loss in hatchery salmonids (Araki et al 2008), and it may result from the unnatural timing of artificial spawning (Thompson et al 2016), rearing density (Brockmark and Johnsson 2010; Thompson and Blouin 2015), and rearing duration (age‐1 versus age‐2 smolts: Berejikian et al 2017; Tatara et al 2017, 2019). In particular, selection for rapid growth rate can occur in as little as one generation of culture (Wilke et al 2015; Horreo et al 2018) and it is presumably caused by size‐selective mortality after release (Clarke et al 2014; Osterback et al 2014; Tatara et al 2017).…”

“…Steelhead conservation hatcheries sometimes produce age‐2 smolts to accommodate natural spawn timing (often later than hatchery‐origin broodstock) and cold hatchery water temperatures that preclude producing yearling smolts, and in this situation slower growth and reduced rations are required to prevent excessive size at release and high rates of precocious male maturation (Berejikian et al 2012). Individuals that do not reach the smoltification size threshold are subjected to size‐selective mortality after release (Tatara et al 2017; Thompson et al 2018) or residualism (failure to migrate after release due to small size or maturation prior to release; Tatara et al 2019). Size variation in hatchery steelhead is heritable, and it is influenced by hatchery rearing strategy (Berejikian et al 2017).…”