Effects of dietary fatty acids on juvenile salmon growth, biochemistry, and aerobic performance: A laboratory rearing experiment

- Detected no effect of dietary DHA:EPA (0.5-1.5) on growth of juvenile Chinook salmon over 12 weeks
- Fatty acids and stable isotopes reached equilibrium in muscle tissue in 1 to 2 months.
- Tissue and diet EPA levels were proportional, but DHA was elevated in tissues compared to diet.
- Fasting had a significant effect on salmon size, growth, and lipid stores.
- After fasting, critical swimming speeds were correlated with salmon lipid stores and size.

A three-phase experiment measured the effects of prey quality and fasting on juvenile Chinook salmon (Oncorhynchus tshawytscha) performance. The first phase was designed to evaluate the effect of dietary levels of two essential fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), on salmon growth. Salmon were reared for 12 weeks on three diets varying in proportions of krill (Thysanoessa spinifera and Euphausia pacifica) and northern anchovy (Engraulis mordax). Supplements of DHA and EPA were added to the formulated diets to achieve DHA:EPA ratios (0.6, 0.9, and 1.5) representative of naturally occurring prey. Growth rates over 12 weeks were not significantly different among diet treatments, which may be because DHA and EPA were provisioned at or above minimum requirements. Salmon maintained DHA at high levels (> 20% of total fatty acids) across all treatments and sampling periods, whereas EPA reflected dietary concentrations after 12 weeks. Fatty acids were incorporated into salmon muscle at varying rates but on average reflected diet after 1 to 2 months, similar to bulk stable isotopes of carbon and nitrogen. The second phase of the experiment was designed to evaluate fasting effects on salmon size, growth, and lipid storage over 4 weeks. Fed fish were heavier, grew faster, and had significantly more storage lipids than fasted fish. The third phase was designed to evaluate aerobic performance in fasted fish. Critical swim speeds were found to be positively related to salmon body size and storage lipids, but not prior diet quality, evidence that larger salmon with higher energy reserves may be better suited for overwinter survival due to their ability to swim faster than smaller leaner individuals. These results provide mechanistic support for the idea that body condition prior to the first ocean winter is important for juvenile salmon survival.