Dietary Protein Requirement of Juvenile Dotted Gizzard Shad Konosirus punctatus Based on the Variation of Fish Meal

SIMPLE SUMMARY: The dotted gizzard shad Konosirus punctatus is a typical representative of small commercial fish. However, the dietary protein requirement of this species is still unknown, which has brought difficulties to culture and reproduction as well as artificial release. Fish meal has been the preferred protein source for aquafeeds. For this reason, fish meal was used as the single protein source in this study. Juvenile dotted gizzard shad were fed diets with different protein levels based on the variation of fish meal and were tested for growth, survival, feed utilization, body composition, digestive enzymes, and immunity, especially on the utilization of protein from the perspective of energy retention. The results of this study demonstrated that appropriate protein levels could improve growth and feed utilization of dotted gizzard shad. Excessive dietary protein inclusion alternates the digestive enzyme activities and metabolism of amino acids. The research outcomes will have good commercial potential for the development of formula feed for dotted gizzard shad. ABSTRACT: An 8-week feeding trial was conducted to investigate the effects of dietary protein levels on growth performance, feed utilization, and energy retention of juvenile dotted gizzard shad Konosirus punctatus based on the variation of fish meal. Fish meal was used as the sole protein source; five semi-purified diets were formulated with varying crude protein (CP) levels of 22.52%, 28.69%, 34.85%, 38.84%, 45.78% (CP1-CP5 diets). A total of 300 uniform juveniles with initial body weight 3.61 ± 0.20 g fish(−1) were randomly divided into five groups with three replicates in each group. The results showed that different CP levels did not significantly affect the survival of juvenile K. punctatus (p > 0.05). The values of weight gain (WG) and specific growth ratio (SGR) showed a general enhancing trend and then weakened with increasing dietary CP levels (p > 0.05). Feed utilization also improved with increasing dietary CP levels (p > 0.05), and the optimal feed conversion ratio (FCR) value was found in fish fed the diet with CP3 (p > 0.05). The rise of dietary CP from 22.52% to 45.78% enhanced the daily feed intake (DFI) and protein efficiency ratio (PER) values of K. punctatus (p < 0.05). With the increase of dietary CP levels, daily nitrogen intake (DNI), energy retention (ER), and lipid retention (LR) elevated, while retention (NR), daily energy intake (DEI), and daily lipid intake (DLI) reduced (p < 0.05). No statistical differences in the content of water, crude protein, and crude lipid were observed among different treatments (p > 0.05). The activity of lipase in CP3 and CP4 diets was significantly higher than that of the CP1 diet (p < 0.05). Fish fed CP2 and CP3 diets had significantly higher amylase activity than that of the CP5 diet (p < 0.05). The levels of alanine aminotransferase (GPT) first enhanced and then decreased as dietary CP levels raised. The second-order polynomial regression model analysis of the WG and FCR indicated that the optimal dietary protein level for K. punctatus is about 31.75–33.82% based on the variation of fish meal.