Most mortality of salmon during the ocean residence period is thought to occur during the critical first summer in the ocean. The performance of juvenile salmon (e.g., size, diet, growth rate, energy content, etc.) during this critical first summer is hypothesized to reflect the quality of marine habitats occupied during this period, and may serve as a predictor of marine survival. To test this hypothesis, I examined a variety of performance measures for juvenile chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon collected from marine waters of SE Alaska as part of NMFS Auke Bay Southeast Coastal Monitoring Study during 1997-2000. This Alaskan system was selected because coho salmon marine survival rates have been phenomenal, averaging 20% in the last decade, while chinook survival rates were far lower (< 5%), even though both species enter marine waters at approximately the same size and time.
I compared the distributions, size, growth rates, diets, and energy content of chinook and coho salmon in order to determine whether they might reflect higher coho survivals, and to elicit possible mechanisms to explain the survival differences. This comparison revealed that juvenile coho salmon likely benefit from a predation buffer due to their overlap with abundant juvenile pink and chum salmon, a buffer that does not extend to chinook salmon. Other performance measures suggested either no difference between the species or values that were only marginally lower in chinook salmon. This study highlights the challenges of documenting and interpreting salmon marine ecology, but also the fascinating patterns that are often uncovered.