Mealworm larvae meal in diets for meagre juveniles: Growth, nutrient digestibility and digestive enzymes activity

This study aimed to evaluate the effect of including 10%, 20% and 30% of mealworm (Tenebrio molitor, TM) larvae meal (diets TM10, TM20 and TM30, respectively), replacing 25%, 50% and 75% of fishmeal (FM) in a isoproteic (50%) and isolipidic (19%) FM-based diet (CTR) for meagre (Argyrosomus regius). The diets were fed for 9 weeks to triplicate groups of fish with 18.0 g initial body weight and results were evaluated for growth performance; whole-body composition; amino acid (AA) catabolism and digestive enzymes activity; nutrients and chitin digestibility. A digestibility trial was also simultaneously conducted for 48 days. Growth and feed utilization parameters decreased linearly as dietary TM inclusion level increased, while mortality, hepatosomatic and visceral indexes, and AA catabolism enzymes activity were unaffected by the dietary treatments. Whole-body protein was reduced in fish fed TM20 and TM30 diets, with the latter group also having lower energy content than fish fed the CTR diet. The apparent digestibility coefficients (ADC) of dry matter, energy, protein, and AA (except leucine, valine and phenylalanine), decreased as dietary TM inclusion level increased. The lower ADC of protein was related to a linear decrease in trypsin activity. Lipase and total alkaline protease activities were also lower in fish fed the TM30 diet than in the other groups. A quadratic response of total alkaline protease with the increase of dietary TM meal was similar to that observed in the intensity of zymograms bands corresponding to proteases other than trypsin or chymotrypsin-like. Chitin ADC was null and no chitinolytic activity was found in the intestine. Overall, meagre presented a limited capacity to utilize TM, with a 10% TM dietary inclusion already resulting in significant impairment of fish digestive capacity and growth performance, while higher TM dietary levels further negatively affecting fish whole-body composition.