Nutritional assessment of linseed meal (Linum usitatissimum L.) protein concentrate in feed of silver catfish

Linseed protein concentrate (LPC) was produced in the laboratory and subsequently a feed assay was performed to evaluate the inclusion of increasing levels of LPC in replacement of fish meal on growth, nutrient utilization, metabolic responses and goblet cells of silver catfish (Rhamdia quelen). Five isotopic and isocaloric diets were formulated, including 0, 100, 200, 300 and 400 g/kg of LPC protein replacing the fish meal, respectively. Each diet was randomly distributed to quadruplicate groups of 25 fish (initial average weight of 6.13 g) per tank, totaling 20 tanks. The crude protein of LPC was lower (P < 0.05) than that of fish meal. However, its in vitro digestibility was higher (P < 0.05). Fish fed with LPC presented the same growth and nutrient utilization (P > 0.05) than animals submitted to the 0 g/kg LPC diet. Diets of 300 and 400 g/kg LPC provided higher (P < 0.05) free amino acid content in plasma. Hepatic protein was higher (P < 0.05) in the 300 g/kg LPC treatment, differing from the 0 g/kg LPC diet. Hepatic ammonia was higher (P < 0.05) in fish submitted to 0% LPC diet, differing from 20 and 30 % treatments. Fish fed with 300 g/kg LPC presented more (P < 0.05) goblet cells, differing from the 0% LPC group. LPC presents equivalent nutritional quality and can replace fish meal protein by up to 400 g/kg without causing metabolic and histological injuries that affect growth and nutrient utilization.