Stable isotopes of C and N as dietary indicators of Nile tilapia (Oreochromis niloticus) cultivated in net cages in a tropical reservoir

Abstract

The relative contribution of natural feed sources and commercial diets to the nutrition of Nile tilapia (Oreochromis nilloticus) was evaluated in a net cage production carried out in a reservoir in the semi-arid northeast region of Brazil. The present study tested the hypothesis that tilapia uses natural sources of feed for growth, even when confined in cages and artificial feed resources are available. The fish and various feed sources (commercial feed, seston and periphyton) were sampled and the water quality was monitored during the cultivation, which was carried out for 120 days. The fish were fed three times daily with two commercial feeds of 40 and 32 % crude protein, both of which used fish meal as a protein source. Stable isotopes analyses were carried out by measuring the 13C and 15N of fish muscle tissue and the feed items using the MixSIAR mixing model to determine the relative contribution of the nutrient sources to fish growth. The tilapia showed adequate weight gains with the total contributions of the feed items: periphyton (14.4 %), seston (51.2 %) and commercial feed (34.4 %) throughout the 120 days of culture. It can be concluded that the natural feed sources of seston and periphyton contributed more to the growth of the Nile tilapia than the commercial feed when cultivated in net cages, changing the notion that the commercial feed contributes predominantly to the intensive farming of this species in cages. The results of the present study suggest a more sustainable feed management in the farming of Nile tilapia in net cages.