Food quantity does not compensate for poor food quality in fitness of juvenile Chinook salmon (Oncorhynchus tshawytscha)

Changes in marine prey availability and nutritional quality can have effects on juvenile salmon fitness (i.e., growth, condition, and mortality) during the early marine phase. There is limited knowledge of the interplay between prey availability and prey quality, and the importance of food quality under food satiated conditions. Here, a four-phase and 11-week long feeding experiment measured the effects of nutritional quality (fatty acid composition and ratios) juvenile Chinook salmon ( Oncorhynchus tshawytscha ) fitness. Experimental diets were chosen based on the ratio of two essential fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). We tested the effects of three diets with different DHA/EPA ratios representing different naturally occurring prey species (DHA/EPA: Artemia = 0.05; Aquafeed = 0.79; Krill ( Euphasia pacifica ) = 0.99). The four experimental phases were: 1 - acclimation; 2 - weaning onto treatment diets; 3 - experimental feeding phase; 4 - Artemia-reared fish switching to commercial aquafeed. Fish were sampled weekly for all treatments and replicates, and growth rates, condition (RNA/DNA and Fulton K), fatty acid composition and mortality rates were measured. Fatty acids were incorporated into salmon muscle at varying rates but on average reflected dietary concentrations. High dietary concentrations of DHA, EPA and high DHA/EPA ratios resulted in increased fish growth and condition. In contrast, low concentrations of DHA and EPA and low DHA/EPA ratios in the diets were not compensated for by increased food quantity. This result highlights the importance of food quality being considered when assessing fish response to changing ocean conditions. Highlights ### Competing Interest Statement The authors have declared no competing interest.