This study proposed a practical controlled hydrolysis method that 1) utilizes all the endogenous enzymes contained within the fish body to “auto-hydrolyze” its tissue proteins and 2) provides a meal that is tailored to meet nutritional requirements and absorptive capacity of larval fish.
The two main objectives for this experiment were to determine: 1) the effect of the proposed hydrolysis method on tissue protein breakdown level; and 2) the effect of dietary inclusion of obtained hydrolysate on fish growth performance in its larval stages.
Adult Zebrafish (ZB) carcasses were utilized in the hydrolysis process. Four different ZB-meal products were produced for this study, an intact (non-digested) product (control), and three digested products that underwent hydrolysis for 1, 2, and 3 hrs, respectively. The hydrolysate solution was kept at 27oC with a pH of 7-9 during the entire process. The efficacy of the hydrolysis was analyzed using SDS-PAGE. At three days post-hatch, larval ZB were randomly distributed in 15 (3 L) tanks, with 100 larvae per tank. Fish larvae received five dietary treatments with three replicate tanks each; live feed or formulated dry commercial diet (reference groups), intact ZB-meal-based diet (Intact); 50% intact and 50% ZB hydrolysate-based diet (50% Hydro); and 100% ZB hydrolysate-based diet (100% Hydro). The hydrolysate-mix utilized in this study contained equal parts of the three digested products. All fish were fed to satiation from first feeding until the larvae fully metamorphosed to a juvenile stage.
The results from the SDS-PAGE showed that the proposed hydrolysis method was able to efficiently hydrolyze the protein within ZB body. The results from the feeding trial found no significant differences in final weight, total length, or condition factor between the Intact, 50% Hydro, and 100% Hydro groups. However, total length was significantly lower in the experimental groups compared to both reference groups while body weight was lower in both hydrolysate groups compared to the reference groups. The gene expression of Pept1 intestinal transporter showed a significant increase in 50% Hydro group compared to the Intact group but similar expression compared to the remaining groups in 24-hours starved fish. However, 2-hours after feeding the gene expression of Pept1 showed no significant differences between 50% Hydro and the remaining groups. These results found that although the hydrolysis method effectively produced hydrolyzed ZB-meal, the inclusion of the hydrolysates generated the same growth of the larval ZB compared to the intact ZB-meal. The growth results paired with Pept1 gene expression potentially indicate ZB larvae to be highly adapted to dry feeds at first feeding and able to utilize dietary protein in different molecular forms efficiently for growth. Overall, the proposed hydrolysis method provides a practical and cost-effective approach to producing species-specific fishmeal hydrolysates. Further research is necessary to determine whether the produced hydrolysates can improve the growth of larval fish in other fish models.