Growth and feed utilisation of Nile tilapia (Oreochromis niloticus) fed different protein levels in a clear-water or biofloc-RAS system

A 9-week feeding experiment was conducted to evaluate different dietary protein levels in juvenile Nile tilapia reared in either a biofloc (Bio-RAS) or clear water recirculating aquaculture system (CW-RAS). The fish were fed four isoenergetic (19 MJ kg(-1)) diets with graded levels of a fixed mixture of three protein sources (animal, plant, and microbial origin) and containing 23, 27, 31 or 35% crude protein. Triplicate groups of 20 juvenile tilapia (initial weight: 39.1 +/- 2.5 g) were randomly assigned to 24 identical conical 500-L tanks equipped with feed waste traps. Twelve of the tanks were supplied with clear water purified by a mechanical and biological filter, denoted CW-RAS, and on average 15% daily replacement of water. The other 12 tanks were supplied with a biofloc suspension, maintained by a bioreactor system, denoted Bio-RAS, where the mechanical and biological filters were replaced with four serial 10 m(3) open bioreactor tanks. In this later system only evaporated water was replaced. The bioreactor tanks were initially inoculated with Bacillus subtilis, and nutrients were supplied to achieve a C: N ratio of 10, supporting heterotrophic bacterial growth. Feed and protein intake, weight gain, specific growth rate, protein efficiency of fish in all treatments, and biomass of micro components in the bioreactor tanks, as well as apparent digestibility of dietary components, were determined at end of the experiment. Growth, feed intake and protein efficiency, especially if retention of the floc was included, was overall higher in the Bio-RAS than in the CW-RAS system. Feed conversion was influenced both by protein level and by the availability of biofloc, with a general lower value in the Bio-RAS and fish given higher protein containing diets. Apparent digestibility of total protein did not vary with the rearing system but was higher with higher dietary levels of crude protein. No significant effect was found for mortality or body index, except for hepato-somatic index and intestinal quotient index, with the latter being significantly higher in the fish given access to biofloc. In all fish given access to biofloc, given all other factors being equal, displayed higher and more feed efficient growth than fish kept in clear water with only access to fabricated diets. Our results tally with other studies using biofloc tank technology and reports from more commercial settings indicating that our data is valid also at a more general level.

Automated summary

The results of the experiment indicate that Nile tilapia reared in a biofloc system exhibit better growth, feed intake, and protein efficiency, with higher feed conversion rates. The feed conversion rate is influenced by protein levels and the availability of biofloc, while the apparent digestibility of total protein increases with higher levels of dietary crude protein.