Effects of water flow rates on growth and welfare of Nile tilapia (Oreochromis niloticus) reared in a recirculating aquaculture system

The effects of water exchange on growth and welfare of Nile tilapia (Oreochromis niloticus) were investigated in a recirculating aquaculture system during an 8-week trial. Fish of initial body mass of ~ 27 g (n = 8 per tank) were reared in 60-L tanks with water exchange rates, corresponding to 1.5 (LE), 3 (ME) and 6 (HE) tank volumes/h. Treatments were triplicated and fish were fed at 3% of their biomass each day. The LE treatment resulted in significantly higher (p < 0.05) levels of ammonia nitrogen and phosphate in the culture water relative to the higher water exchange treatments. The specific growth rate (SGR) of fish cultured under the HE treatment (2.74% day−1) was significantly higher than the SGR (2.21% day−1) of the LE fish. Mean final body weights of the LE, ME and HE fish were 97.67 ± 8.13, 110.50 ± 22.45 and 123.92 ± 10.00 g, respectively. Higher prevalence of dermal ulcerations, oral lesions and poor fin conditions were associated with the LE and ME fish. After 4 weeks, 34 and 24% of the LE and ME fish, respectively, had advanced mouth lesions compared to 0% for the HE fish. Haematological indicators of long-term oxygen stress and disease conditions, as evidenced by erythrocyte and platelet indices, were generally higher in fish under the LE and ME treatments. This study has shown that although the Nile tilapia is a hardy species, long-term exposure to poor water conditions can result in reduced growth and compromised welfare