Consequences of Emergence Timing for the Growth and Relative Survival of Steelhead Fry from Naturally Spawning Wild and Hatchery Parents

<div><p></p><p>For many fishes, reproducing early in the year may present tradeoffs for the offspring: the drawbacks associated with harsh environmental conditions may be offset by advantages in competition for food and space. We investigated this tradeoff in a system where hatchery-origin steelhead <i>Oncorhynchus mykiss</i> were bred to spawn about 4 months earlier than wild-origin fish. Hatchery-origin adults were released into the wild and spawned in common with wild steelhead. Naturally spawned offspring from these matings were assigned to hatchery, wild, and hybrid lineages by using a panel of 96 single-nucleotide polymorphisms. We then evaluated whether fry abundance, body size and condition, and geographic distribution differed based on parental lineage. The observed mixture proportions were 0.007 hatchery-lineage, 0.289 hybrid, and 0.704 wild-lineage juveniles, despite the predominance of hatchery-origin parents. There were significant differences in FL (hatchery > hybrid > wild) but no difference in body condition. We modeled the emergence dates of the sampled fry and estimated that 96.0% of wild-lineage fish emerged after the arrival of temperate spring conditions (reduced flows and warmer temperatures) compared with 80.4% of hybrids and 41.2% of hatchery-lineage fish. We hypothesize that the low abundance of pure hatchery-lineage fry may be due to a mismatch between the timing of breeding by the parents and suitable river conditions, resulting in low survival and physical displacement of hatchery-lineage fry from the system. Thus, the advantages of early emergence and large size associated with hatchery origin did not mitigate the disadvantages of early spawning in this case.</p><p>Received February 5, 2015; accepted May 22, 2015</p>