Changes in fatty acid composition and stable isotope signature of Atlantic cod (Gadus morhua) in response to laboratory dietary shifts

Abstract The aquaculture industry supplies coastal waters with large amounts of particulate organic material from the fish cages, which is a source of food for several pelagic and benthic invertebrates and fish. Particularly gadoids, such as cod ( Gadus morhua ) and saithe ( Pollachius virens ), are known to aggregate at fish farms in boreal areas. As a step on the way to investigate the influence of organic fish farm waste on wild fish, we conducted a diet-switch study to determine the extent to which fatty acid composition and stable isotopes (δ 13 C and δ 15 N) of various tissues could be used to trace diet alterations in cod. Initial analysis of the feeds showed that δ 13 C results were non-significant and this tracer was not further pursued. Groups of cultivated cod fry (125–200 g) were fed either salmon feed, herring fillet or cod feed as control feed and sampling was conducted after 26, 41, 69, 106 and 121 days. Fatty acids and stable isotope δ 15 N were analyzed in the various diets and muscle tissue, and δ 15 N was determined in the plasma, liver and heart. All fish groups displayed the fatty acid composition of their diets. The salmon feed group had a significant increase of FAs 18:2n6, 18:3n3 and 18:1n9 by the third sampling (day 69). These FAs are found in rich concentrations in vegetable oils used in salmon feed production. The δ 15 N values showed no significant change in the salmon feed group throughout the experiment whereas the herring group had an increase in all tissues. Linear Discriminant Analysis (LDA) sample distribution plots were identical when FA alone and in combination with the δ 15 N values were analyzed, and both tests correctly classified more than 96% of the samples. Muscle fatty acids were a more precise tracer than δ 15 N since specific FAs could be traced and a combination of fatty acid and stable nitrogen analyses did not improve the robustness of the result.