1. Replicate groups of rainbow trout (Salmo gairdneri) were given one of five experimental diets (diets 1-5) for 16 weeks. The diets contained different amounts of calcium, phosphorus and magnesium and were prepared so that there were three levels of Ca (g/kg): 14 (diet l), 26 (diets 2 and 3) and 40 (diets 4 and 5), Ca:P being approximately 1 : I in all diets. Diets 1, 2 and 4 had basal Mg levels (not more than 0.063 g/kg) whereas diets 3 and 5 contained supplementary Mg (1.0 g/kg).2. Weight gains of the trout given diets containing supplementary Mg were twice those of trout given diets with basal levels of Mg. At both dietary Mg concentrations weight gain was unaffected by the dietary Ca level.3. Serum Mg levels were significantly reduced in those trout given diets without supplementary Mg. The serum Ca level in those trout given the lowest concentration of Ca in their diet (14 gjkg, diet 1) was significantly greater than in those given higher amounts of Ca in their diets. Serum P levels were not significantly different with any of the experimental diets. 5. Renal calculi were demonstrated by light and electron microscopy in tubules of those trout given diets 3 and 5 (basal Mg; 26 and 40 g Ca/kg respectively). Electron-probe micro-analysis showed that these calculi contained or comprised tricalcium phosphate.6. The skeletal muscle of Mg-deficient trout contained significantly more sodium than that of normal trout. It is suggested that this is indicative of an increase in extracellular fluid in the muscle of Mg-deficient trout.
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.
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