FEEDING AND NUTRITION OF JUVENILE COBIA Rachycentron canadum: EVALUATION OF PRACTICAL FEEDS IN FLOATING NET CAGES, COMPARISON OF DIFFERENT DIETARY FISHMEAL REPLACERS WITHIN INDOOR TANKS, AND ESTIMATION OF ESSENTIAL AMINO ACID REQUIREMENTS

Two feeding trials were conducted with juvenile cobia Rachycentron canadum at the Oceanographic Institute, University of São Paulo, Brazil: (1) a 10-week feeding trial conducted within floating net cages to test the nutritional efficacy of different dietary feeding regimes ranging from the use of trash-fish, in-house formulated feeds, to dry commercial extruded marine fish feed; and (2) a 10-week feeding trial conducted within indoor water-recirculated tanks to test the nutritional efficiency of different potential dietary fishmeal replacers within dry in-house prepared diets.

Different commercially-based diets were tested within floating net cages, including a trash-fish control diet, a semi-moist diet composed of trash-fish and a binder premix, an in-house dry formulated diet, and one commercially available extruded marine fish feed. In the case of the indoor tank-based feeding trial, different practical-type diets were formulated and manufactured containing 50% crude protein from different animal and plant protein sources, including a national fishmeal diet, a salmon fishmeal diet, a poultry-by-product meal diet, a soy protein concentrate meal diet, and a combination of poultry-by-product meal and feather meal diet. In addition to the above different protein sources, a diet was also tested without supplemental taurine together with a dry commercial fish feed. Juveniles cobia were randomly stocked in both systems and hand-fed ad libitum two times a day. Feces were recovered by the fecal stripping technique in anesthetized individuals, in order to assess dry matter and crude protein apparent digestibility coefficient.

Generally, fish performance was superior in the net-cage feeding trial compared with the indoor water-recirculated tank trial. Fish weight gain in both feeding trials were not significantly different between the experimental diets, with the exception of fish fed the trash-fish control diet which displayed the best growth response and feed performance. Overall, the fish growth and performance of the experimental diets were very similar, showing that the alternative ingredients could be included and replace part (50%) of the fishmeal component. Apparent crude protein digestibility coefficients of over 80% were obtained for all diets from both trials. The results from both trials concluded that the cobia requires practical diets with high levels of crude protein and lipid, and the inclusion of alternative plant-based and terrestrial animal protein sources was possible. Comparing the estimated essential amino acid (EAA) requirements of juvenile cobia using two different approaches (protein accretion and E/A ratio), an underestimation of the EAA requirement values was noticed when lysine was used as the reference EAA. The quantitative EAA requirement values estimated by the protein accretion method was highly correlated to the average of each of the EAA requirement for the species of carnivorous fish reported in the literature, and could be recommended for formulation of commercial feed for cobia. Using amino acids profile to set basis for dietary amino acids profile and requirements is a viable technique and may bring additional benefits of formulating diets eliciting higher feeding efficiency and reduced nutrient loss and waste and metabolites excretion.