Life History Diversity of Steelhead Oncorhynchus mykiss in Two Coastal Washington Watersheds

Understanding life history diversity and factors influencing expression of resident (rainbow trout) and anadromous (steelhead) Oncorhynchus mykiss life histories are essential pre-requisites for O. mykiss population conservation and recovery. To this end, we monitored migration, growth, and survival of Oncorhynchus mykiss in two coastal watersheds using passive integrated transponder (PIT) tags. Our results indicated that these populations expressed a diversity of anadromous life histories with no evidence of resident life history types. A majority of juveniles migrate to the ocean in their first fall or winter, although these parr migrants do not produce adults. Adults were only produced from fish that reared in the river for at least one winter, with rearing at least two winters producing most adults. Our results indicated that body size early in life affected the probability of fish expressing an early or late migration strategy. Fish that attained larger body size early in life were more likely to migrate early and ultimately fail to produce adults. In contrast, smaller fish were more likely to rear in the river at least one winter to produce smolt migrants that ultimately had a greater chance of producing an adult.