Lipids and Stable Isotopes of Juvenile Salmon and Their Potential Prey Provide Insight into Salmon Feeding Ecology during Early Ocean Residence

Variations in marine prey availability and nutritional quality are hypothesized to affect juvenile salmon growth and survival during early ocean residence. Juvenile salmon and their potential prey were sampled at monthly intervals (May-September) in 2011 and 2012 and their lipid classes, fatty acids, and bulk isotopes of carbon and nitrogen measured to investigate early marine feeding in a population of Chinook salmon from the upper Columbia River, classified by genetics. Using ordination methods, we identified three seasonal prey assemblages occurring in spring (May-June), early summer (July), and late summer (August-September), and determined that the timing of peak biomass for the dominant prey species, Northern anchovy, differed between years. Trophic biomarkers of salmon fatty acids and isotopes changed as salmon presumably became more piscivorous. Biomarkers associated with trophic position (fatty acid ratio of DHA/EPA and δ¹⁵N) and phytoplankton productivity (fatty acid ratio 16/18 PUFA and δ¹³C) increased significantly with salmon length, while fatty acid biomarkers associated with the nearshore (18:3n-3 and 18:2n-6) decreased with length. However, the largest fish did not occupy the highest trophic position. This difference may be related to temporal differences in peak anchovy biomass, indicating the importance of phenological matches between predator and prey resources.