Size-selective mortality during freshwater and marine life stages of steelhead related to freshwater growth in the Skagit River, Washington

Wild steelhead trout, Oncorhynchus mykiss, in the Puget Sound are currently in decline, and very little is known about the early life history of these threatened fish. This study evaluated consequences of early growth and survival to smolt or adult stages in different precipitation zones of the Skagit River Basin, Washington. The objectives of this study were to determine whether significant size-selective mortality (SSM) in wild steelhead could be detected between freshwater stages and returning adults; and if so, how the magnitude of SSM varied among juveniles rearing in different precipitation zones (snow and mixed rain-snow). Wild steelhead were sampled as juveniles, smolts, and adults, and scales were measured to compare back-calculated size distributions and growth rates of rearing juveniles with individuals that survived from an earlier life stage to the smolt and adult stages. Back-calculated size-at-annulus comparisons indicated that steelhead in the snow zone were significantly larger at annulus-1 than those in the mixed zone. Steelhead sampled as adults were significantly larger than those sampled as juveniles at annuli-1, -2, and -3, and larger than those sampled as smolts at annuli-2 and -3, Steelhead sampled as smolts were larger than those sampled as juveniles at annuli-1 and -2, but smolts and juveniles were the same size at annulus-3. The disparity in size-at-age-2 and -3 between steelhead sampled at earlier and later life stages suggested that fast growth during the second or third freshwater growing seasons was vitally important for survival to adulthood, and that both freshwater and marine survival could be attributed, in part, to size attained at earlier life stages in freshwater. Efforts for recovery of threatened Puget Sound steelhead could benefit by considering SSM in freshwater environments, and identifying factors that limit growth during early life stages