The effect of protein to lipid ratios on growth, digestibility, and feed utilization of striped bass (Morone saxatilis) raised in seawater at 21 °C

A 12-week feeding trial assessed the impact of protein-to-lipid ratios (P:L) on the growth, feed and protein utilization, digestive enzyme activity, and fillet composition of juvenile striped bass (Morone saxatilis) raised at summer local seawater temperature conditions (21 °C). A 3 × 2 factorial design, using three protein levels (40, 44, and 48%) and two lipid levels (12 and 16%) resulting in six diets (40:12, 44:12, 48:12, 40:16, 44:16, and 48:16) was used. Hatchery-reared fish with an initial weight of 11.4 ± 0.2 g were used. Laboratory-formulated experimental diets were randomly assigned to triplicate groups of 22 fish in 500-L tanks connected to a recirculating seawater (35.4 ± 3 ppt) system, and fish were fed manually to apparent satiation four times a day. Survival rates were higher than 96% across all treatments, remaining unaffected by dietary treatments. Fish fed the 44:12 and 48:12 diets resulted in significantly higher weight gain (WG) (274.1 ± 18.4 and 306.8 ± 19.8, respectively), final body weight (FBW) (42.4 ± 1.2 g and 47.1 ± 2.4 g, respectively), and specific growth rate (SGR) (1.6 ± 0.06 and 1.7 ± 0.06, respectively). Diets with 12% lipid resulted in feed conversion rate (FCR) values closer to 1, and the 48:16 (1 ± 0.03) treatment resulted in no significant differences compared to the 12% lipid dietary treatments. Similar trends were observed for protein efficiency ratio (PER), with comparable values for the 40:12 (2.0 ± 0.1), 44:12 (2.2 ± 0.1), 48:12 (2.0 ± 0.1), and 48:16 (2.1 ± 0.1) treatments. A strong relationship between dietary P:E ratios and SGR, PER, and FCR was observed among treatments, which helped explain the interactions between protein and lipid obtained. An estimated dietary P:E ratio between 11 and 12 is suggested for efficient protein and feed utilization for this species under our culture conditions. The apparent digestibility coefficient (ADC %) was significantly higher in the 12% lipid diets (65.01 ± 3.06%), influenced solely by lipid content. Enzyme activity for trypsin and chymotrypsin in the pyloric caeca (PC) and intestines (I) were higher in the 48:12 treatment (893.5 ± 18.1, 1087.3 ± 55.5, 12.2 ± 0.3, and 8.3 ± 0.2 Units/g organ, respectively). L-aminopeptidase activity (PC) was higher in the 48:12 treatment (1007.7 ± 101.8 Units/g organ), while L-aminopeptidase (I) had higher values in the 48:16 treatment (2217.2 ± 161.3 Units/g organ). In the case of lipase (PC), activity was higher in the 12% lipid treatments, while in the intestine, the highest activity was recorded in the 44:12 treatment (1084.4 ± 87.0). The activity of proteases, except for L-aminopeptidase (PC), directly correlated with improvements in SGR, PER, feed intake (FI), and FCR which helps explain the observed results. The Hepatosomatic index (HSI) was significantly higher in the 40:12 treatment while the liposomatic index (LSI) and viscerosomatic index (VSI) indices showed no significant differences between the 40:12, 44:12, 48:12, and 48:16 treatments, respectively. Importantly, the lipid content in the fillet did not significantly decrease with reduced lipid content in the diet, an important issue for market acceptance. Results from the present study suggest an optimal P:L ratio of 48:12 for juvenile striped bass raised in seawater under local summer conditions (21 °C).