An incorporation of protein hydrolysates in larval fish diets as an intact protein replacement has been shown to improve larval performance of several species suggesting that the molecular size of the protein fraction is critical to support proper development of larval fish . However, larval capacity to digest dietary components of different molecular weights changes throughout its development. Consequently, the right balance between different sizes of protein fractions in diets is critical to induce positive growth responses in larval stages . Various protein hydrolysates have been obtained using in vitro methods that have attempted to reproduce the physiological conditions of the digestive tract. However, the practical application of in vitro hydrolysis has not been routinely used by the industry due to complexity and low repeatability. The objectives of this study were: 1) to develop an optimal in vitro methodology for fish muscle hydrolysis using largemouth bass (LMB) endogenous digestive enzymes; and 2) to evaluate the effect of dietary inclusion of the fish muscle protein hydrolysate obtained using methodology in Objective 1 on LMB growth, survival, occurrence of skeletal deformities, and whole-body free amino acid composition.
Briefly, LMB digestive tracts were ground and used as a direct source of digestive enzymes. Each digestive tract homogenate was mixed with muscle originating from bighead carp Hypophthalmichthys nobilis . After temperature and pH were adjusted to the required level muscle homogenates were mixed with digestive tract supernatants (22o C; initial 3-4 pH for the first one hour to mimic stomach digestion followed by 7-8 pH for two hours to mimic intestinal digestion).
At 5 dph larval LM B were randomly distributed into 5 0 L tanks, ~270 larvae per tank. Two diets were tested in the study: control - based solely on an intact fish muscle as the main protein source and Hydro diet - based on 50% intact fish muscle replacement with fish muscle hydrolysate . Both groups received experimental diets in a combination with Artemia nauplii to support larval LMB survival. At the end of the feeding trial LMB in the Hydro group had significantly larger final average weight compared to the control (0.20±0.01 vs. 0.15±0.02 g , p<0.05) . In addition, the Hydro group was characterized by significantly higher weight gain compared to the control (6341±253 vs. 4873±647% , p<0.05) . Finally, the occurrence of skeletal deformities was significantly reduced in the Hydro group co mpared to the control (6 vs 23% , p<0.05 ). These results suggest that in vitro hydrolysis of fish muscle using LMB endogenous digestive enzymes is a practical and cost-effective method to obtain dietary protein hydrolysate that supports LMB performance in its larval stages.