Identifying Critical Life Stages and Processes That Determine Marine Survival of Chinook Salmon in Puget Sound

If we are to increase adult run sizes, salmon recovery programs need to consider factors that improve survival and growth of salmon throughout their life cycle.  Therefore, habitat relations and biological responses should be linked to life stage-specific abundance, growth, and survival performance of populations during potentially critical life stages from eggs through spawning adults.  Over the past 20 years, smolt-to-adult survival for Puget Sound Chinook salmon has generally varied between 0.2% and 1.0% with few exceptions.  Given this 5-fold difference, an understanding of factors that improve marine survival is critical for effectively focusing recovery efforts.  In Puget Sound, ocean-type Chinook salmon O. tshawytscha rely on both freshwater (weeks to months) and marine habitats (months) during their first year of life, but the relative importance of specific habitats and processes influencing growth and survival can be difficult to identify and can vary among populations.  By relating stage-specific growth and feeding to lifetime survival, we can isolate critical periods that determine lifetime survival rates and then focus on the underlying processes that support higher survival and fitness.  For Puget Sound Chinook, overall ocean survival (smolt-to-adult survival) was strongly correlated (r² > 0.80) to the body weight of juvenile Chinook sampled in offshore habitats of Puget Sound during July 1997-2004.  This ocean survival relationship was much weaker (r² < 0.35) for juvenile weight in September.  Bioenergetics modeling indicated that higher ocean survival was strongly associated with higher growth and feeding rates on particular prey assemblages, but not related to inter-annual differences in thermal regime during the critical spring-summer growth period.  Ocean survival over the 1-6 years of adult life was primarily determined by critical periods and sizes (sensu Beamish and Mahnken 1998) during the early marine life history of these populations. These results demonstrate the utility for linking survival and fitness mechanistically to stage-specific feeding and growth.  This approach should be applicable for evaluating energetic and fitness consequences of alternative life history strategies for a wide range of species.  By modeling bioenergetic consequences of different ontogenetic patterns in habitat use, we can project the geographic extent and nature of habitats that could be utilized by these species under different climatic regimes and land use scenarios, and use this as a basis for directing recovery efforts to support these critical life stages in biologically meaningful ways.