Into the Wild: Life History Tradeoffs in Juvenile Hatchery, Wild, and Hybrid Steelhead Trout

Like many fishes, salmonids face a tradeoff between early and late reproduction: emerging early can contribute to competitive dominance at the risk of being out of phase with environmental conditions. We investigated this tradeoff in a system where hatchery steelhead trout (Oncorhynchus mykiss) were bred to spawn four months earlier than wild fish and released onto natural spawning areas. Offspring were collected 1 – 4 months after emergence (N=1020) and assigned to hatchery, wild, or hybrid lineages using a panel of 96 single-nucleotide polymorphisms. Hatchery females outnumbered wild females 9 to 1, yet hatchery-lineage offspring constituted 0.7% of the sample, compared to 28.9% hybrid and 70.4% wild offspring. There were significant differences in fork length (hatchery > hybrid > wild), but no differences in body condition. We estimated that 72.2% of wild offspring emerged after the arrival of favorable spring conditions, compared to 47.3% of hybrids and 27.0% of hatchery fish. We hypothesize that the low abundance of hatchery juveniles may be due to a mismatch between adult breeding timing and suitable river conditions, resulting in low survival and displacement from the system. The advantages of early emergence associated with hatchery origin did not mitigate the disadvantages of breeding early.