Effects of protease supplementation in low fish meal pelleted and extruded diets on growth, nutrient retention and digestibility of gibel carp, Carassius auratus gibelio

• Adding protease in low fish meal pelleted diet improved the growth and nutrient utilization of gibel carp.
• Adding protease in low fish meal extruded diet showed no positive effects on growth and nutrient utilization of gibel carp.
• Gibel carp fed extruded diet had significantly higher body lipid level and lipid retention than the fish fed pelleted diet.

The present study was conducted to evaluate the effects of exogenous protease supplementation in low fish meal pelleted and extruded diets on growth, nutrient retention and digestibility, and serum biochemical indices of gibel carp, Carassius auratus gibelio. A high-fish meal (HF) diet containing 90 g/kg fish meal and a low-fish meal (LF) diet containing 30 g/kg fish meal (60 g/kg fish meal was isonitrogenously replaced by soybean meal) were designed as the positive and negative control diet, and 125, 150 and 175 mg/kg exogenous protease were supplemented in the LF diet, respectively. The five formulas were then processed with pelleting and extruding, respectively, to obtain ten diets. After the pelleting and extruding, the retention rate of the protease activity was 77.98% and 37.65%, respectively. The diets were fed to gibel carp with an initial body weight of 35.0 ± 0.2 g for 12 weeks. In pelleted diets, the supplementation of 150 and 175 mg/kg protease in LF diet significantly improved weight gain, nutrient apparent digestibility coefficients (dry matter and crude protein), nutrient retention (crude protein and crude lipid) and decreased feed conversion ratio (FCR) when compared to negative control (P < 0.05), and showed the similar values as the positive control (HF group). In extruded diets, despite no significant differences in growth, nutrient digestibility and retention among treatments, fish fed diet supplemented with 150 mg/kg protease showed numerically higher values than fish fed the negative control diet (P < 0.10). When comparing the extruded diets and the pelleted diets, fish fed extruded diets had significantly higher body lipid level and lipid retention than fish fed pelleted diets, and the fish fed extruded diets except the HF diet showed the significantly higher protein retention than fish fed pelleted diets with the same formulation, respectively. There were no significant differences in serum triglycerides, total cholesterol and glucose levels among all groups, and no significant differences in serum total protein and albumin levels among groups except the extruded HF group. The above results showed that the supplementation of 150–175 mg/kg protease in pelleted LF diet, but not in extruded LF diet, could improve the growth performance, nutrient digestibility and nutrient retention of gibel carp when compared to the LF diet without protease.Statement of relevanceThe present study showed that supplementing exogenous protease to pelleted diet containing 30 g/kg fish meal, but not to the extruded diet, could improve the growth, apparent digestibilities of dry matter and crude protein, and retention of protein and lipid for gibel carp. The above results will guide the proper application of protease in fish diet.