Stable isotopes and fatty acids as tracers of the assimilation of salmon fish feed in blue mussels ( Mytilus edulis)

Stable isotopes and fatty acids were used to trace the assimilation of salmon feed (pellets) into the tissues of the blue mussel, Mytilus edulis. The stable isotope ( δ 13C and δ 15N) and fatty acid signatures of mussel digestive gland and mantle tissue were investigated using a 28 day laboratory feeding experiment. Mussels were fed natural seston and compared to others with the same diet but supplemented with salmon pellets, before the mantle tissue and digestive gland of individuals were analysed. Mussel digestive gland responded more strongly than mantle tissue to both δ 13C and the fatty acid signature of the feed. The δ 15N exhibited a 2‰ change in the direction of salmon pellets in both tissues, and were not significantly dissimilar from salmon pellets after 28 days indicating signature incorporation. There was a large increase in the lipid content of digestive gland, and the amount of fatty acids increased from 4 to 12% after feeding with salmon pellets. The fatty acid profile of the digestive gland reflected the fatty acid profile of the salmon pellets. Several fatty acids could be used as lipid biomarkers of the assimilation of salmon pellets, including 18:1 ( n− 9), 18:2 ( n− 6) and 16:3 ( n− 4). The ratio of n− 3 to n− 6 fatty acids could also be used as a tracer. The mantle tissue responded in a much more conservative manner, but showed some response to the altered fatty acid content of the diet towards the end of the experiment. The results of this study demonstrated that the use of digestive gland δ 13C and fatty acid signatures was valid in tracing the assimilation of salmon pellets into blue mussel populations after 28 days of exposure.