Evaluation of Reduced Fish Meal Diets for Second Year Growout of the Largemouth Bass, Micropterus salmoides

Development of efficient cost-effective diets is a critical component in the refinement of production technologies for the largemouth bass, Micropterus salmoides (LMB). One of the first steps in reducing feed costs can be to decrease the amount of fish meal in the diet. The objective of this study was to evaluate reduced levels of fish meal, and a least-cost formulation diet, for second year growout of LMB under practical pond conditions. Twelve 0.04-ha ponds were stocked with juvenile LMB (210.1 +/- 3.3 g) at a stocking density of 8650 fish/ha (350 fish/pond). Each pond was randomly assigned one of the four dietary treatments with three replicate ponds per treatment. The three experimental diets contained varying levels of fish meal. Diets FM-45, FM-24, and FM-8 contained 45, 23.5, and 8% fish meal, respectively. In diets FM-24 and FM-8, fish meal was replaced by varying levels of poultry by-product meal, soybean meal, and blood meal. The fourth diet was a commercial salmonid diet widely used as a LMB growout feed (Nelson and Sons, Inc., Silvercup T, Steelhead (TM), Murray, UT, USA). This diet served as a commercial control (CC) and contained 46% crude protein. The experimental diets were formulated to be isonitrogenous and isocaloric with the CC diet and were fed once daily to apparent satiation for 180 d. At harvest, there were no significant differences between treatments (P > 0.05) in terms of survival, which averaged 95% overall. Mean weights of fish fed the three experimental diets FM-45, FM-24 and FM-8 were not significantly different (P > 0.05) and averaged 518, 546, and 529 g, respectively, but were all significantly greater (P <= 0.05) than those fed the CC (488 g). Feed conversion ratio (FCR) of fish fed the FM-45 and FM-8 diets (1.43 and 1.46, respectively) was significantly greater (P <= 0.05) than those fed the FM-24 diet (1.34). The FCR of fish fed the CC diet (1.39) was not significantly different (P > 0.05) from fish fed other diets. Feed cost per unit of weight gain ($US/kg) was significantly lower (P <= 0.05) in fish fed the FM-24 and FM-8 diets ($0.73 and $0.72/kg, respectively) than in fish fed other diets. Feed cost per unit gain of fish fed the FM-45 diet ($0.83/kg) was significantly lower (P <= 0.05) than those fed the CC diet ($1.04/kg). There were no significant differences (P > 0.05) in dress-out percentages or proximate composition among fish fed the four diets. This study indicates that fish meal levels in feeds used for the second year growout of LMB can be reduced to >= 8% of the formulation without reducing survival or growth and without negatively impacting body composition.