Effects of dietary polyunsaturated fatty acid deficiencies on mortality, growth and gill structure in the turbot, Scophthalmus maximus

The preparation of fish oil concentrates containing only ( n ‐3) polyunsaturated fatty acids (PUFA) with different ratios of 20:5 ( n ‐3)/22:6 ( n ‐3) is described. Three groups of turbot were maintained on different diets containing: 1, 10% of the dry weight of the diet as natural fish oil, equivalent to 2.5% ( n ‐3) PUFA and 0–23% ( n ‐6) PUFA; 2, 10% of the dry weight of the diet as palmitic acid, i.e. no PUFA; 3, 8–7% palmitic acid and 1–3% of the dry weight as ( n ‐3 PUFA and negligible ( n ‐6) PUFA. Only the fish on the diet containing natural fish oil showed significant growth over a 15‐week period. In addition there were high mortalities on the two experimental diets (2 and 3). Changing the ratio of 20:5 ( n ‐3)/22:6 ( n ‐3) from 13–8 to 2–2 in the diet containing 1 3% (n‐3) PUFA and negligible ( n ‐6) PUFA markedly decreased the mortalities. Fish fed the two experimental diets (2 and 3) developed gross changes in gill structure involving the disappearance of chloride cells, a ‘sloughing off’ of the epithelium along the primary and secondary filaments and an accumulation of cellular material in the inter‐lamellar spaces. The tissue ultimately disintegrated to leave a skeleton of connective tissue and a mass of cellular material in the inter‐lamellar spaces. It is concluded that 22:6 (n‐3) is an essential fatty acid for turbot and that the gill epithelium is a sensitive indicator of this deficiency in this species.