Environmental Determinants of Residency or Anadromy in O. Mykiss: a Life-Cycle Simulation Approach

Studies from Alaska to California have confirmed that resident and anadromous Oncorhynchus mykiss both produce offspring of the alternate type, and commonly co-occur in the same basin.  We hypothesized that prevalence of either life history was driven by, (1) environmental factors affecting survival, and (2) rates of cross production between the two types. To test these hypotheses, we developed a life-cycle framework to model the complex population dynamics of O. mykiss, including rates of interbreeding between resident and anadromous life histories, competition for freshwater rearing habitat, cross production of life histories, and differences in age at maturity and fecundity.  The model was parameterized with environmental and biological data from Yakima River Basin, Washington where both resident and anadromous types are abundant, but in different portions of the basin.  Carrying capacity and survival were modeled as functions of habitat so the influence of environmental variables on production of each type could be examined.  Proportional production of resident and anadromous types predicted by the model aligned well with know occurrence in different portions of the basin, which supported our first hypothesis.  To test the second hypothesis, carrying capacity and survival were estimated from empirical data on fish abundance rather than from habitat characteristics.  Inputs for smolt-to-adult survival rate followed annual estimates for in-basin steelhead.  The simulated trends for returns of anadromous adults from 1992 to 2006 followed the observed trends only when cross-type production of offspring was accounted for, but not when it was assumed absent.  This corroborated our second hypothesis.  These findings suggest that smolt contributions from resident rainbow trout follow predictable patterns and may strongly influence abundance trends for anadromous O. mykiss.