BACKGROUND
Protein is the major organic component of fish tissue and a key source of amino acids for body protein synthesis, enzymes, and hormones. In aquaculture, protein is also the most expensive component of most feeds, so defining an adequate but not excessive dietary protein level is important for growth, feed efficiency, cost control, and reduction of nitrogen waste. The dotted gizzard shad, Konosirus punctatus, is a commercially important small nearshore pelagic fish in China, South Korea, and Japan, but its dietary protein requirement had been poorly defined. To address this, the authors evaluated dietary protein requirement in juvenile K. punctatus using fish meal as the sole protein source, with attention not only to growth but also to feed utilization, body composition, digestive enzyme activity, and energy, nitrogen, and lipid retention.
METHODS
An 8-week feeding trial was conducted using five semi-purified, isoenergetic diets containing graded crude protein levels of 22.52%, 28.69%, 34.85%, 38.84%, and 45.78%, designated CP1 through CP5. Fish meal was used as the only protein source, while fish oil and soybean oil were the main lipid sources. A total of 300 juvenile fish with an initial body weight of 3.61 ± 0.20 g fish−1 were randomly allocated to five dietary groups with three replicate tanks per treatment, 20 fish per 300 L tank. Fish were fed to apparent satiation twice daily, at 09:00 and 17:00, under natural photoperiod. Water quality during the trial was maintained at approximately 27.7 ± 1.4 °C, salinity 26.10 ± 0.9 g L−1, pH 7.5 ± 0.1, dissolved oxygen above 6 mg L−1, and ammonia nitrogen below 0.05 mg L−1. Growth performance, feed utilization, proximate body composition, liver digestive enzymes, and glutamyl pyruvic transaminase (GPT) were measured. Data were expressed as mean ± SD with n = 3, analyzed by one-way ANOVA and Duncan’s multiple-range tests, with p < 0.05 regarded as significant. A second-order polynomial regression model based on weight gain (WG) and feed conversion ratio (FCR) was used to estimate protein requirement.
KEY RESULTS
Survival was not significantly affected by dietary crude protein level (p > 0.05). In the discussion, the authors reported survival ranging from 92.42% to 96.97%, with no mortalities attributed to dietary protein treatment. Weight gain and specific growth rate (SGR) showed a general increasing trend as dietary protein rose and then declined slightly at higher protein levels, although these differences were not statistically significant (p > 0.05). Feed utilization followed a similar pattern: FCR improved as dietary protein increased up to 34.85% and then worsened beyond that level, again without statistical significance (p > 0.05). The optimal FCR was observed in fish fed the CP3 diet. Based on second-order polynomial regression analysis of WG and FCR, the optimal dietary protein level for juvenile K. punctatus was estimated at 31.75–33.82%.
Daily feed intake (DFI) and protein efficiency ratio (PER) were significantly affected by treatment (p < 0.05). The abstract states that increasing dietary crude protein from 22.52% to 45.78% enhanced DFI and PER, but the Results section specifically reports that fish fed the CP1 diet had the highest DFI and PER, significantly higher than fish fed CP3, CP4, and CP5 (p < 0.05). The Discussion similarly states that the highest DFI value was observed in fish fed the lowest protein diet, CP1. The authors interpreted this as compensatory feeding in response to insufficient dietary protein. However, despite high DFI, the CP1 diet was associated with reduced WG, daily nitrogen intake (DNI), and daily nitrogen gain (DNG), suggesting that greater intake did not fully compensate for inadequate protein supply.
Retention data showed a mixed pattern. There were no significant differences in daily nitrogen gain, daily energy gain, or daily lipid gain among treatments (p > 0.05). In contrast, DNI, energy retention (ER), and lipid retention (LR) increased significantly with increasing dietary protein (p < 0.05). Increasing dietary protein from 22.52% to 45.75% reduced daily energy intake (DEI), daily lipid intake (DLI), and nitrogen retention (NR) (p < 0.05). The authors emphasized that low-protein diets resulted in the highest DEI and DLI but the lowest ER and LR, suggesting poor nutrient balance and likely poorer digestibility, potentially related to higher corn starch and cellulose inclusion in the lowest protein diet.
Whole-body composition was relatively stable across diets. Moisture, crude protein, and crude lipid contents did not differ significantly among treatments (p > 0.05). In the discussion, the authors noted that the crude lipid content in the CP4 group was higher than in the CP3 group, suggesting that some excess dietary protein might have been converted to lipid rather than used for protein deposition, although formal statistical significance for whole-body lipid was not reported.
Liver enzyme activities supported an effect of dietary protein on digestion and amino acid metabolism. Lipase activity in CP3 and CP4 was significantly higher than in CP1 (p < 0.05). Amylase activity did not differ significantly among CP1 through CP4, but fish fed CP2 and CP3 had significantly higher amylase activity than fish fed CP5 (p < 0.05). GPT activity was highest in fish fed CP3 and was significantly higher than in CP1, CP2, and CP5 (p < 0.05). The authors interpreted rising GPT activity from 22.52% to 34.85% as evidence that dietary protein level influenced amino acid metabolism.
CLINICAL IMPLICATIONS
Although this is an aquaculture nutrition study rather than a human clinical study, it has clear practical implications for fish production. A dietary crude protein target of 31.75–33.82% appears to provide the best overall balance between growth and feed efficiency in juvenile K. punctatus when fish meal is the sole protein source. Lower protein diets may drive higher feed intake without adequate growth, while higher protein diets may reduce protein utilization efficiency, increase metabolic burden, and alter digestive enzyme activity and amino acid catabolism. For commercial feed formulation, the findings support avoiding both underfeeding and overfeeding of protein to improve performance and potentially reduce unnecessary nitrogen excretion and feed cost.