Comparison of unused water and year-old used water for production of channel catfish in the biofloc technology system

Authors: Green, Bartholomew - Bart; Schrader, Kevin; Rawles, Steven - Steve; Webster, Carl; McEntire, Matthew - Matt

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2019
Publication Date: 2/7/2020
Citation: Green, B.W., Schrader, K.K., Rawles, S.D., Webster, C.D., McEntire, M.E. 2019. Comparison of unused water and year-old used water for production of channel catfish in the biofloc technology system. Aquaculture. https://doi.org/10.1016/j.aquaculture.2019.734739.
DOI: https://doi.org/10.1016/j.aquaculture.2019.734739

Interpretive Summary: During start-up of a new biofloc technology production cycle for finfish, algae and other microbes consume ammonia excreted by fish as a by-product of feed protein metabolism. However, four to six weeks typically are needed before bio-transformation of the excreted ammonia to nitrite and then to nitrate (a non-toxic nitrogen compound) proceeds efficiently. Ammonia and/nitrite can accumulate to acute or chronic concentrations as the microbial populations responsible for bio-transformation become established. Once the bio-tranformation process is established fully in the biofloc technology production system, ammonia and nitrite concentrations remain very low. Thus, re-using this biofloc water over multiple production cycles should be beneficial. This experiment evaluated impacts on fish production characteristics and mineral status, common microbial off-flavors, and water quality dynamics for channel catfish reared in one-year-old waters with low or high total suspended solids used previously for two consecutive catfish biofloc studies or in unused (new) water. At harvest, six months after stocking, no statistical differences were detected among the tested waters for catfish production characteristics or macro- and trace-minerals concentrations in fillet meat. Whole-body and fillet mineral concentrations did not differ when channel catfish were grown in one-year old or new biofloc system water. One advantage to re-use of biofloc system water is the absence of ammonia and nitrite concentration spikes typical of BFT start-up. Results suggest that removal of total suspended solids may reduce ammonia bio-transformation because discharging solids washes the microbes out of the system. Macro and trace minerals accumulate water during the culture cycle, and treatment differences were related more to solids removal than to new versus old biofloc water. In summary, one-year old water from a channel catfish biofloc production system can be used for a second year of biofloc production with no adverse effect on channel catfish production responses.

Technical Abstract: Since excreted feed nitrogen is bio-transformed efficiently in a fully functional mixotrophic biofloc technology production system, re-using this biofloc water over multiple production cycles should be beneficial. The present study, conducted in an outdoor biofloc technology production system, evaluated impacts on fish production characteristics and mineral status, common microbial off-flavors, and water quality dynamics for channel catfish (Ictalurus punctatus) reared in one-year-old waters with low or high total suspended solids used previously for two consecutive catfish biofloc studies or in unused (new) water. Total suspended solids were maintained at 300 to 400 mg/L in the unused and low total suspended solids used water treatments and allowed to accumulate in the high total suspended solids used water treatment. Tanks (18.6 m2, 15.7 m3) were stocked with fingerlings (47.5 ± 0.8 g/fish) at 13.5 fish/m2 (16 fish/m3) and grown for 181 days. Channel catfish production characteristics did not differ significantly among treatments. Overall, gross fish yield averaged 10.2 kg/m3 and fish averaged 642 g/fish. Nitrate accumulation rate was affected by total suspended solids concentration with a significant reduction observed at the highest discharge of solids from the system, suggesting wash-out of nitrifiers. Treatment effects on water quality dynamics, macro- and trace-mineral status of water, feed, and fish, 2-methylisoborneol and geosmin off-flavors and associated phytoplankton populations also are discussed. Results of this study suggest that one-year-old biofloc water can be used without adverse impact for a second year of channel catfish production in the biofloc system.