Cooperative Fish and Wildlife Research Units Program: Washington
Education, Research and Technical Assistance for Managing Our Natural Resources

Size-selective mortality (SSM) is a significant force regulating recruitment of salmon. The life stage(s) and habitat(s) when and where SSM occurs can vary considerably among species, stocks, and life history strategies. Moreover, the relationship between size, growth, and condition in freshwater and marine life stages to overall life cycle survival is unclear for most stocks of salmon. The first months of marine growth are commonly regarded as a critical period for growth and survival. For ESA-listed Puget Sound Chinook salmon, an emergent result has been: 1) at least one of the critical growth periods occur during early summer rearing in epi-pelagic marine habitats; 2) that growth is limited mostly by food supply rather than by energetic quality of the diet or thermal regime; 3) higher growth and survival correspond with higher contributions of key prey like crab larvae. Linking stage-specific growth performance and SSM offers a promising avenue for diagnosing the primary factors that affect growth and survival. The timing and relative magnitude of SSM can be determined by serial sampling of size distributions over a progression of life stages using both direct measurements of body size and back-calculated estimates from scales. Disproportionate reductions in the contribution of smaller members to subsequent life stages, especially to adult returns, can be used to determine the timing and magnitude of SSM and identify critical periods of growth and survival. We can then diagnose which factors most affect growth during critical periods through bioenergetics modeling simulations that are linked to directed sampling of diet, growth and environmental conditions. This approach could potentially improve run forecasting and focus restoration efforts.