Growth, product quality and immune status of Atlantic salmon, Salmo salar L., fed wet feed with alginate

By-products, by-catch and offal from the fishing industry represent valuable resources of high-quality protein and energy. In order to utilize some of these resources, a method for preparing wet feed for Atlantic salmon, Salmo salar L., using a binder produced from seaweed, Ascophyllum nodosum L., was developed. The feed was formulated with offal from lean fish (haddock, Melanogrammus aegleflnus L., saithe, Pollachius virens L., pollack, Pollachius pollachius L., and cod, Gadus morhua L.) and fatty fish [mackerel, Scomber scombrus L., and argentine, Argentina silus (Ascanius)], fish meal extruded wheat, capelin oil, vitamin and mineral premix, astaxanthin, and seaweed binder. The feed was gellified in a solution of calcium chloride. Equipment for the production of such feed was developed and the feed tested on a large scale at a commercial salmon farm. As control, a high-fat, extruded, commercial dry feed was used. On a dry matter basis, the two feeds were comparable with respect to protein, fat and carbohydrate. Growth rates, appetite and feed conversion were similar for the two experimental groups. Fish fed dry feed seemed to be more active when handled before slaughter and had a higher incidence of skin damage than fish given wet feed. Chemical and sensory analyses of the fish showed no difference (P > 0.05) between the two groups. Fifty fish from each group were analysed for haemoglobin, protein, lysozyme, complement and haemolytic activity in the blood. There was a significantly higher level of lysozyme in the wet feed group than in the dry feed group, implying an immunostimulating effect of alginate. It was clearly shown that producers have an excellent opportunity to improve the utilization of the resources and decrease feed production costs in places where the natural conditions are favourable, i.e. locations where there is an integrated production of fish farming and processing of wild catch.