Disentangling bottom-up and top-down control during a critical period in the life history of an anadromous fish

Background/Question/Methods

Many populations of Pacific salmon in modified watersheds are not meeting recovery goals, and conservation efforts would benefit from a better understanding of factors limiting productivity. We combined field, laboratory, and modeling approaches to evaluate the relative importance of “bottom-up” (production-limited) and “top-down” (predator-mediated) processes during early marine residence in a population of Chinook salmon (Oncorhynchus tshawytscha) from the upper Columbia River, USA. We examined characteristics (length, mass, and condition) of juveniles (age 0+) collected in the coastal ocean during two periods (June and September) across 11 years (1998 to 2008). Interannual variation in marine growth during the summer was also estimated. The relationships between seasonal and interannual variation in juvenile attributes and environmental conditions within the river, plume, and coastal ocean during early marine residence were determined. Multi-model inference and regression analysis were used to evaluate which factors explained the most variation in subsequent adult returns.

Results/Conclusions

There was no evidence of direct resource limitation, i.e., juvenile body condition displayed a negative relationship with indicators of ocean productivity. Furthermore, mean body condition of juveniles after their first ocean summer was significantly lower during high survival years, which likely indicates top-down effects or competition. A multivariate model that incorporated key physical (plume volume during freshwater emigration) and biological (body condition of juveniles) variables and their interaction hindcast interannual variation in survival with a high level of accuracy (>95%). Our analysis provides evidence that top-down effects, such as selective mortality, or competition influence upper Columbia River summer/fall Chinook salmon juveniles during early marine residence. These observations also indicate that interannual variation in abundance for this population, which migrates >4000 ocean km, may be well-established by the end of their first ocean summer. Future research and management efforts should focus on evaluating potential predators and competitors and understanding how plume structure influences survival.