ATLANTIC DOGFISH SILAGE VS HERRING SILAGE IN DIETS FOR ATLANTIC SALMON (SALMO-SALAR) - GROWTH AND SENSORY EVALUATION OF FILLETS

The preparation of silage-based diets from dogfish waste suitable for Atlantic salmon was studied because of a concern that the urea in the dogfish would affect either diet acceptability by salmon and/or consumer-sensory evaluation of the salmon muscle. The dogfish liver was not included in the waste utilized. The protein content of dogfish silage was adequate (approximately 14.9%, Kjeldahl nitrogen corrected for urea nitrogen). The fat in dogfish silage included triglycerides at approximately 81% of total lipid. The high level of polyunsaturated fatty acids (approximately 28% of total fatty acids) in this lipid was comparable to that of other fish lipids employed in salmonid diets. Two sets of experiments were conducted with Atlantic salmon to evaluate the suitability of dogfish-silage-based diets. In both, three moist diets based on ground herring (control), on herring silage, and on dogfish silage were tested for 9 weeks. The studies were conducted in two phases based on salmon averaging respectively 190 or 490 g. These were fed twice daily to apparent satiation (water temperature range 5-14-degrees-C, 12 h photoperiod). Although the urea content was nearly 0.5% in the dogfish silage diet, there was no apparent decrease in palatability for salmon in the experiment with larger fish. However, there was a decrease in the palatability of both herring and dogfish silage diets fed to the smaller fish, probably due to their being stored for 8 weeks at room temperature prior to diet formulation. The addition of natural tocopherols in the preparation of silage for the larger fish kept the lipid oxidation levels acceptable (peroxide value < 5 mEq/kg) for the 15 days of storage before making the diets. In the second study with larger fish, there were no significant differences among the diets in weight gain, feed efficiency, and protein efficiency ratio. These parameters had been significantly higher in the smaller fish fed the ground herring control diet. A sensory evaluation was conducted with the salmon fillets from both studies where the fish fed on different diets were compared. No significant differences were detected in either experiment. The lipid, protein and n-3 fatty acid levels of the muscle of the larger salmon fed the three different diets did not exhibit any significant differences (P > 0.05). Our results show that dogfish offal, an environmental waste problem, can be made into a fish silage acceptable for salmon farming if proper storage conditions are employed.