Life History Diversity, Marine Survival, and Viability of Pacific Salmon

The hypothesis that ocean productivity to support salmon populations fluctuates between "good " and "bad" cycles that can span 2–3 decades, and that these fluctuations are negatively correlated between waters off Alaska and the Pacific Northwest, is gaining increasing acceptance. However, not all populations within a geographic area respond to common environmental conditions in the same way. Genetic and life history diversity among salmon and steelhead populations leads to considerable variability in their response to the marine environment. Conversely, events that occur during the marine phase of the life cycle can profoundly affect metapopulation structure and diversity of salmon and steelhead populations in fresh water. Some empirical examples illustrate that these patterns of genetic and life history diversity can be important on a variety of spatial and temporal scales. Figure 1 shows the timing of arrival at a mainstem Snake River dam of outmigrating chinook salmon (Oncorhynchus tshawytscha) smolts that had been implanted with Passive Integrated Responder tags (PIT tags) as wild parr or as presmolts in hatcheries (Achord et al. 1996). The natural populations show a protracted outmigration timing over a more than 3-month period that reflects diversity within populations as well as differences among populations in mean outmigration timing. In contrast, hatchery smolts are typically released during a short temporal window in early spring. More recent research (J. Williams, Northwest Fisheries Science Center, personal communication) has shown that just a few days difference in date of ocean entry of spring/summer chinook salmo