Growth and health responses of juvenile largemouth bass Micropterus salmoides fed practical animal-based and plant-based diets with different levels of vitamin A

João F.A. Koch, Rebecca Lochmann*, Michele Thompson, Ruguang Chen, and Kurt  Rosentrater
Aquaculture/Fisheries Center of Excellence
University of Arkansas at Pine Bluff
Pine Bluff, AR 71603
lochmannr@uapb.edu

Largemouth bass (LMB; Micropterus salmoides) are economically important as sport fish as well as food fish for ethnic markets in the United States. Interest in commercial culture of this species is driven by high demand and market prices. Like other carnivores, LMB perform well on diets high in fish meal. However, environmental and cost concerns of using fish meal have led to diets with reduced levels of fish and other animal proteins. When protein sources are changed, fish performance may be affected by changes in amino acid composition and availability. However, it other nutrient requirements may also be affected. For instance, vitamin A has well-known metabolic interactions with protein in vertebrates. The vitamin A requirement of LMB is unknown, so we identified a range of concentrations that bracket the known requirements of related species. We supplemented these 5 levels of vitamin A into five animal-based diets and five plant-based diets, and tried to equalize the levels of all other nutrients in the diets.

Feed-trained LMB (4 g) were stocked at 20 fish per 4 replicate tanks per treatment in a recirculating system. Fish were fed to satiation twice daily for 8 weeks. Although the goal was to run the trial until the fish had a 500% weight increase, the study had to be terminated at 8 weeks due to low survival in some groups so that we would have enough fish to analyze health parameters and body composition indices. Growth, survival, cannibalism index, body composition indices, and immune parameters were measured to assess diet effects.

Survival (93-100%), feed intake, FCR (1.1-1.2) and cannibalism index did not differ among treatments for the animal-based diets. Weight gain, hepatosomatic index, and hematological parameters were higher in fish fed diets with at least 6060 IU vitamin A/kg diet compared to diets with the basal level (940 IU). Lysozyme was higher in fish fed a slightly higher level (6349 IU/kg diet) of vitamin A compared to the control. In the plant-based diets, weight gain of fish fed diets with 3190 IU or more of vitamin A was higher than that of fish fed diets with 1010 IU or less. Cannibalism was also higher and survival was lower in fish fed diets with higher levels of vitamin A. Feed intake was lower in diets with 3190 IU or more vitamin A compared to fish fed diets with 1010 IU or less. However, feed conversion (1.1-1.6) was similar among treatments. Blood parameters were generally higher in fish fed diets with higher levels of vitamin A. Lysozyme was higher in fish fed the diet with 1010 IU vitamin A/kg diet compared to the control (740 IU/kg diet).

Overall, supplemental vitamin A above basal levels had a few beneficial effects in LMB fed the animal-based diets. However, the more pronounced growth stimulation associated with vitamin A supplementation in fish fed the plant-based diets was offset by increased cannibalism and lower survival. It must be noted that on a commercial farm, LMB of the size we used in this study would have been graded several times over an 8-week period to maintain uniform sizes and reduce cannibalism. Therefore, further studies are needed to determine whether higher levels of vitamin A in plant-based diets would be economically beneficial under industry conditions.