THE LIPID-COMPOSITION OF SEALOCH SEDIMENTS UNDERLYING SALMON CAGES

The lipid composition of sediments underlying salmon cages in a Scotti sh sealoch was determined along with that of the diet supplied to the fish to examine the influence of lipids originating from fish farm was tes. Sediments were taken at regular intervals along a transect line p erpendicular to the line of cages and extending 50 m on either side. L ipids were extracted from regions of the sediment cores corresponding to different depth layers of sediment and analyzed for lipid class and fatty acid composition. The lipid content of the surface layer of sed iment (0-5 mm) directly under the cages (2 mg/g sediment) was substant ially higher than that of the deeper layers of sediment. The amount of lipid in the surface sediment decreased markedly with the distance fr om the cages, with the decrease being more rapid on one side of the fi sh farm than the other. At the 50-m sampling sites, the lipid content of the surface sediment layer (0.4 mg/g sediment) was closer to that o f the underlying layers. Triacylglycerols, the main lipid class presen t in the diet fed to the salmon, were present in sediments in highest concentration (0.44 mg/g sediment) in surface sediments directly benea th the cages. Free fatty acids, sterols, polar lipids and a combined h ydrocarbons/wax esters/cholesterol esters fraction were all present at similar levels to those of triacylglycerols in sediments beneath the cages. The distribution pattern of the amounts of individual lipid cla sses in sediments followed that of total lipid and showed a decrease w ith the distance from the cages. The lipid component of the fish feed contained a higher level of polyunsaturated fatty acids, particularly 20:5(n - 3) and 22:6(n - 3), than lipid extracted from the sediments, while branched chain and odd-chain length fatty acids were more abunda nt in the latter. The principal fatty acid of the fish diet, 22:1(n - 11) (14.7% total fatty acids) comprised around 9% of the fatty acids o f surface sediment layer directly under the cages, but less than 5% of those at 50 m. The proportions of the other characteristic fatty acid s of the dirt, 20:1(n - 9), 20:5(n - 3) and 22:6(n - 3), showed a. sim ilar decrease with distance. The results show that the lipid compositi on of sediments underlying marine fish cages is influenced by that of waste material from the cages. (C) 1997 Elsevier Science B.V.