The effect of dietary protein and lipid source on dorsal fin erosion in rainbow trout, Oncorhynchus mykiss

A study was conducted to determine the effect of dietary protein and lipid source on dorsal fin erosion in rainbow trout, Seven diets were each fed to four replicate lots of 300 first-feeding fry cultured in 75 1 aluminum tro ughs for 8 weeks. Two basal diets were manufactured with approximately equa l nutrient content, one using krill and squid meals and the other anchovy m eal as the primary protein-containing ingredients. The meals used to manufa cture the diets were separated into two fractions: lipid (ether-extractable ); and protein/ash (non-ether-extractable) using a large soxhlet. The fract ions were then recombined to create two additional diets; one containing an chovy protein/ash with krill/squid lipid, the other krill/squid protein/ash with fish lipid. A fifth diet recombined krill/squid protein/ash with kril l/squid Lipid to evaluate effects of the extraction process. Two additional treatments included a diet with a portion of the krill meal replaced by po ultry by-product meal, and the basal anchovy meal diet supplemented with so dium, magnesium, and copper. Fish consuming diets containing anchovy meal a s the primary protein source gained more weight (P < 0.05) than fish consum ing krill/squid meal-based diets. Dorsal fin index (DFI, measured as mean d orsal fin heightX100/total fish length) was greater (P < 0.05) for fish con suming diets containing krill/squid meal protein/ash fraction (DFI = 9.9%-1 0.0%) than for fish consuming diets containing anchovy meal protein/ash fra ction (DFI = 4.9%-5.3%), regardless of lipid source. Supplementation of the anchovy meal diet with sodium, magnesium, and copper improved (P < 0.05) D FI by approximately 20%, but not to the level supported by the kril/squid m eal protein/ash fraction diets. The cost of the krill meal diet was reduced by inclusion of poultry by-product meal without affecting dorsal fin condi tion. These data indicate that the dietary agent contributing to dorsal fin erosion in rainbow trout is not present in the ether-extractable fraction of the diet, but rather in the protein or mineral fraction. (C) 1999 Elsevi er Science B.V. All rights reserved.