Is growth retardation present in Nile tilapia Oreochromis niloticus cultured in low water exchange recirculating aquaculture systems?

Abstract

It has been suggested that fish cultured in recirculating aquaculture systems (RAS) grow less as compared with fish cultured in flow-through systems due to the accumulation of substances. In the Netherlands, the commercial culture of Nile tilapia Oreochromis niloticus in 300 and 600 MT's systems is done exclusively in RAS operated at water exchange rates as low as 30 L/kg feed/day due to nitrate control by single-sludge denitrification reactors. The use of such nearly closed RAS raises the question whether growth retardation (GR) is present in Nile tilapia. This study is the first to investigate the existence of growth retardation in Nile tilapia by comparing the growth, feeding behaviour and stress response of Nile tilapia cultured in RAS with different levels of substances accumulated. Three RAS, operated at 30 L/kg feed/day (HIGH accumulation), 70 L/kg feed/day (MIDDLE accumulation) and 1500 L/kg feed/day (LOW accumulation) were used. Each RAS contained 24 glass aquaria with individually housed fish. To determine whether GR is size-dependent, per RAS 3 fish size categories were tested in the 57 day experimental period: large (288.7 ± 34.2 g; N = 8), medium (162.4 ± 23.4 g; N = 8) and small (81.4 ± 21.0 g; N = 8). Experimental fish were fed ad libitum, twice per day. Feeding behaviour was determined once per week and was measured as the time taken by each fish to eat the first pellet (latency, LAT) and the total time spent feeding (total feeding time, TFT). Temperature, pH, conductivity, alkalinity, dissolved oxygen, dissolved CO 2, nitrogen compounds (TAN, NO 2-N and NO 3-N), chemical oxygen demand and orthophosphate-P were measured over time. At day 57 fish were weighed, blood sampled and returned to their tanks for an extra experimental period of 15 days, and subjected at day 72 to an acute stress followed by blood sampling. Blood was analysed for glucose and cortisol. Results showed that the water quality parameters measured in the 3 RAS (with the exception of alkalinity) were still within the optimum range for growth of Nile tilapia. Large individuals showed a tendency to grow more in the LOW treatment (2.66 ± 1.35 g/kg 0.8/day) as compared with the MIDDLE treatment (0.93 ± 1.63 g/kg 0.8/day). On the contrary, small individuals grew significantly less in the LOW treatment (3.60 ± 1.74 g/kg 0.8/day) as compared with the HIGH (7.22 ± 1.58 g/kg 0.8/day) and MIDDLE (6.82 ± 4.54 g/kg 0.8/day) treatments. Small fish were more motivated to eat (lower latency) in the MIDDLE (4.63 ± 5.24 min) as compared with the LOW treatment (8.94 ± 6.41 min). In the HIGH accumulation treatment higher glucose levels were observed in the small fish, before and after acute stress, as compared with the LOW accumulation treatment. In conclusion, this study showed that the extent to which the accumulation of substances in RAS affects growth depends on fish size: large individuals show a trend towards growth retardation in the highest accumulation RAS while small individuals, on the contrary, seem to grow better in such systems.