The Life History Attributes that Make a Steelhead a Steelhead: Their Implications for Recovery and Next Steps for Future Research

John McMillan, Northwest Fisheries Science Center of NOAA Fisheries

Anadromous steelhead (Oncorhynchus mykiss) are characterized by perhaps the most diverse life histories of any salmonine ranging from individuals that spend weeks to months in the near shore environment to those that undertake multi-year migrations to the open ocean. Such diversity is essentially what makes steelhead a steelhead among Pacific salmon species and there has been an increasing interest in life history processes. However, traditional management and recovery efforts have focused more strongly on parameters of abundance rather than life history diversity. Consequently, expectations about extent of variability in life histories and their role within the context of population viability are not well understood. Here I review three life history attributes that seem particularly relevant to population viability, including size and age at maturity, resident contributions to anadromy, and iteroparity. I used the review and case studies to highlight three general patterns. First, the continuum of ocean migration produces striking differences in size and age at maturity among and within steelhead populations, with patterns apparently representing evolutionary tradeoffs partly related to environment and sex. Second, some juveniles mature as residents in freshwater, particularly males, where they can mate with female steelhead and sire a significant proportion of offspring. Lastly, females repeat spawn more frequently than males, though rates are highly variable. Iteroparous females may increase the number of spawners without increases in juveniles, but rates appear to have declined. Although there are several gaps in knowledge, the results suggest the variability in life histories is important to steelhead viability because they may buffer against short-term environmental variation, which has important implications for recovery.