Sex reversal, selection against hatchery females or wild males does not explain differences in sex ratio between first generation hatchery and wild steelhead, Oncorhynchus mykiss

Abstract

Wild steelhead (Oncorhynchus mykiss) returning to the Hood River in Oregon, USA, show a strongly female-biased sex ratio (average of 63 % female from brood years 1992 to 2004) while first-generation hatchery steelhead (created using local, wild broodstock) display an even sex ratio (51 % female). We considered four hypotheses to explain the difference in sex ratio between populations. First, it is well established that wild male O. mykiss adopt the resident (non-anadromous) life history phenotype at a higher rate than do females, and that the propensity to become resident is under genetic and environmental influence. Therefore, the simplest explanation for the difference in sex ratio between anadromous wild and hatchery fish is that hatchery males adopt the resident life history at a lower rate than do wild males. However, alternative explanations include (1) sex reversal of female hatchery fish to phenotypic males, (2) selection against hatchery females or (3) selection against wild males. The possibility of sex reversal in the hatchery was of particular interest given increased temperature has been shown to skew sex ratios to a male bias. Using a Y-chromosome specific marker (OmyY1 locus) and samples of wild and hatchery fish from various life stages, we were able to reject alternative hypotheses 1, 2 and 3. Hatchery fish were sampled at three life stages (onset of exogenous feeding, 1-year of age and returning adult). Phenotypic and chromosomal sex matched in all 1-year old and adult hatchery samples. Therefore, we see no evidence for sex reversal in the hatchery population. Furthermore, hatchery fish at all three life stages exhibited a 50:50 chromosomal (OmyY1 marker) sex ratio. Therefore, selection against hatchery females while in captivity or after release can be ruled out. The chromosomal sex ratio in a sample of wild smolts was female-biased and matched the sex ratio in returning adults from the same cohort. Therefore, we can also rule out selection against wild males at sea. Given no evidence for sex reversal or selection against either sex it seems most plausible that the greater female bias in wild, compared to hatchery, steelhead from the Hood River results from differential life history expression in males. Wild males appear to become resident (non-anadromous) at a higher rate than do hatchery males.