Steelhead Population Dynamics: Interactions Between Resident and Anadromous Life-History Types

Steelhead, a name used to describe ocean-going (anadromous) rainbow trout, represent one of many life-history types within Oncorhynchus mykiss populations. Another common life-history tactic is residency, whereby individuals remain in freshwater until fully mature, often traveling only a short distance from their place of origin over the course of their life-cycle. O. mykiss juveniles do not always emulate their parent’s life-history choices. This life-history plasticity is assumed to be reflective of habitat-dependent trade-offs between freshwater survival and increased reproductive output resulting from ocean maturation. We demonstrated that resident rainbow trout play a critical role within steelhead populations in the Yakima Basin, Washington through production of significant numbers of anadromous offspring that survive and return as adult steelhead. Incorporating these estimates of cross-ecotype exchange into a steelhead population dynamics model yielded more accurate predictions of adult steelhead production compared with a model that did not account for cross-ecotype exchange, particularly during years of low marine survival. These results suggest that steelhead population viability and recovery may be dependent on existence of a robust resident trout cohort, which highlights the importance of jointly managing freshwater and anadromous life-histories as units of the same population.