Dietary protein affects production of hybrid tilapia and water quality in an outdoor biofloc technology production system

Authors: Green, Bartholomew - Bart; Rawles, Steven - Steve; Schrader, Kevin; GAYLORD, T. GIBSON - Us Fish And Wildlife Service; McEntire, Matthew - Matt

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 3/8/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Tilapia are omnivores that can consume the microbial floc (biofloc) in biofloc technology (BFT) production systems. The degree to which biofloc can substitute for formulated feed protein appears to vary with tilapia life stage but has not been investigated in fish grown to market size. Since ideal protein (IP) theory has not been used to formulate diets for fast-growing tilapia to market size in a BFT production system, we tested the hypothesis that significant reduction in protein could be achieved in hybrid tilapia (Oreochromis aureus × O. niloticus) reared to market size (454 g/fish) in an outdoor, photoautotrophic/chemoautotrophic BFT production system without constant organic carbon additions by supplementing the first four limiting amino acids (Lys, Met, Thr, Ile) and formulating diets (6% lipid) to the IP profile (muscle) at 22.5%, 27.7%, and 32.3% digestible protein (DP). Fingerlings (32.2 +/- 10.1 g/fish) were stocked in 3 tanks/diet (18.6 m2; 16.6 m3) at 25/m2 (29/m3) and grown for 5 months to market size. Fish were fed to apparent satiation twice daily during the week and once daily on weekends. At harvest, fish fed the 22.5% DP diet were significantly smaller (518 g/fish) and had significantly higher feed conversion (1.5) than fish fed the higher DP diets (553-564 g/fish and 1.4, respectively). Gross fish yield for the 27.7% DP diet (15.6 kg/m3) was significantly greater than for the 22.5% DP diet (14.0 kg/m3), and intermediate for the 32.3% DP diet (14.9 kg/m3). Survival averaged 99.0% and did not differ significantly among diets. Fish market size-class distributions were associated strongly with dietary DP level, with fish in the 22.5% DP treatment significantly skewed towards smaller size classes compared to fish fed the two higher DP diets. Apparent grazing of biofloc by hybrid tilapia appeared to allow diet digestible protein to be reduced to 27.7% from 32.3% with no adverse effects on fish production. However, grazing the biofloc did not compensate for further reducing diet DP to 22.5%, which decreased tilapia production significantly and shifted fish to smaller size classes. Fish fed the 22.5% DP diet converted feed less efficiently but retained protein (PRE) as efficiently as those fed the 27.7% DP diet, whereas the lower PRE for fish fed the 32.2% DP diet suggests excessive dietary protein content. Tilapia grazed on the water line biofilm/crust, which contained high concentrations of MIB and geosmin, and was the suspected main source of these off-flavor compounds in the fish flesh. Accumulation of nitrogen and phosphorus in tank water was related to feed protein and phosphorus content and high feeding rate.