EVALUATION OF SOYBEAN MEAL AS PROTEIN SOURCE FOR NORTHERN AND SOUTHERN LARGEMOUTH BASS

Fish meal (FM) is the first choice as a protein material in aquafeed production for its high quality of protein with a well-balanced amino acid profile. With rapid expansion of aquaculture industry resulting in the limited supply and high price of FM, the alternative protein sources have attracted attention of many researchers and developers. Due to high protein content, balanced amino acid profile and low price, soybean meal (SBM) is widely used as the most cost-effective alternative for FM in many aquatic animals. Several studies showed that SBM could be used well in diets with limiting amino acids supplemented for herbivorous and omnivorous fish. However, the utilization of SBM for carnivorous fish is still limited because of high anti-nutritional factors and lack of sulfur amino acids and micronutrients.

Largemouth bass Micropterus salmoides, typical carnivorous fish, is one of the most important commercial aquaculture species in North American and other countries. Commercial farming has relied primarily on trout diets, which contain approximately 60% of FM with expensive price. Little is known regarding nutritional information for the species, especially on the utilization of plant protein sources. Thus, alternative sources for dietary FM protein urgently need to be verified.

The most largemouth bass currently cultured are from the northern subspecies (N) (M. s. salmoides) and southern subspecies (S) (M. s. floridanus). A 12-week feeding trial in indoor round tank (45 L) was conducted to investigate the effects of dietary SBM levels on growth, survival, feed utilization and body composition of the two types of largemouth bass with initial mean weight of 6.8 g and 5.2 g, respectively.

Four isonitrogenous (crude protein 46%) and isolipidic (crude lipid 11%) diets were formulated with 0, 12, 25, and 40% of SBM, respectively. Each diet was randomly fed to fish in triplicate tanks (each having 20 fish) for each treatment. Fish were fed twice daily (9:00 and 17:30) to visual satiation. Temperatures were kept at 22-25℃. No differences were detected on survival between diet and strain (P>0.05). Significant effects of strain on final weight, daily weight gain and specific growth rate were found (P<0.05). The growth of fish from "N" strain was significantly higher than those from "S" strain (P<0.05). The growth performance of fish decreased with increasing dietary SBM level and no significant differences were observed among all treatments (P>0.05) within strain. There was no interaction of strain by diet (P>0.05) on growth of the two type of largemouth bass. The results indicated that northern subspecies have superior growth compared to southern subspecies in current experimental setup.