Introduction
Atlantic cod (Gadus morhua) is a commercially important species in Norway, but its production has traditionally relied on flow-through and sea-based systems (Puvanendran et al., 2022; Nardi et al., 2021). Recirculating aquaculture systems (RAS) offer improved control of environmental conditions, biosecurity and water quality, providing opportunities for more predictable land-based cod production (Wold et al., 2014; Badiola et al., 2017). However, water exchange is an important operational parameter because increasing exchange rates can improve the dilution of metabolites and dissolved compounds while simultaneously increasing water demand, pumping and treatment requirements. Identifying the lowest effective water exchange rate is therefore important for developing water- and energy-efficient RAS production protocols for Atlantic cod juveniles. This study evaluated the effects of reduced water exchange on growth performance, water quality and welfare of Atlantic cod juveniles reared in RAS.
Materials and Methods
Atlantic cod juveniles with an initial body weight of 137–138 g were stocked at 75 fish per 500 L SingleRAS tank in triplicate. Two water exchange regimes were tested: low water exchange (LWE; 500 L kg-1 feed day-1) and high water exchange (HWE; 1000 L kg-1 feed day-1). Fish were reared for seven weeks at 12 °C, salinity 34���, pH 7.9, dissolved oxygen 8.6 mg L-1 (>95% saturation), total gas pressure (TGP) 99.5% and, 24-hour photoperiod. Fish were fed a commercial diet (Skretting Amber Fortuna) using automatic belt feeders with hourly feeding. Growth performance, feed conversion ratio, survival, water quality and external welfare indicators were monitored throughout the experiment.
Results
Reducing water exchange from 1000 to 500 L kg-1 feed day-1 did not significantly affect growth performance, feed conversion or survival. Final body weight was 187.7 ± 6.4 g in LWE and 194 ± 2.3 g in HWE, while specific growth rate was 0.61 ± 0.07 and 0.68 ± 0.02% day-1, respectively. FCR was 1.54 ± 0.12 in LWE and 1.37 ± 0.09 in HWE, and survival remained high at 96.4 ± 0.8% and 97.3 ± 1.3%, respectively.
Core water quality parameters remained stable in both treatments. TAN, nitrite and CO2 did not differ significantly between treatments. However, nitrate increased significantly under LWE, reaching 76.1 ± 1.9 mg L-1 compared with 40.8 ± 6.4 mg L-1 in HWE. Total organic carbon and dissolved organic carbon were also significantly higher under LWE, at 21.07 ± 0.29 and 18.63 ± 1.34 mg L-1, respectively, compared with 15.5 ± 0.53 and 12.3 ± 0.17 mg L-1 in HWE. Despite the increased dissolved organic load, suspended solids and turbidity did not differ significantly between treatments. UV transmittance decreased to 53.2% under LWE compared with 66.1% under HWE, indicating a potential reduction in UV disinfection efficiency at lower water exchange.
External welfare scoring indicated generally low levels of welfare-related abnormalities in both treatments, with few eye, opercular, jaw, and skeletal deformities. Mandibular deformities were observed more frequently than maxillary deformities. Lower jawbone protrusion and caudal haemorrhaging were observed in some fish, while nephrocalcinosis was also observed in some individuals, although its prevalence was significantly lower.
Conclusion
This study demonstrates that Atlantic cod juveniles can be successfully reared at 500 L kg-1 feed day-1 without compromising growth, survival or welfare. This supports the potential of RAS for efficient land-based production of Atlantic cod (Badiola et al., 2017; Puvanendran et al., 2022). However, reduced water exchange increased nitrate and dissolved organic matter accumulation and reduced UV transmittance, highlighting the importance of effective water treatment and organic matter management in low-exchange RAS (Wold et al., 2014).
Acknowledgment
This study was conducted as part of the TorskRAS project (GB project 14475) at Nofima, Sunndals��ra, Norway.
References
Badiola, M., Albaum, B., Curtin, R., Gartzia, I. and Mendiola, D., 2017. Land based on-growing of Atlantic cod (Gadus morhua) using recirculating aquaculture systems; a case study from the Basque region (Northern Spain). Aquaculture, 468: 428–441. https://doi.org/10.1016/j.aquaculture.2016.11.006
Nardi, G., Prickett, R., van der Meeren, T., Boyce, D. and Moir, J., 2021. Atlantic cod aquaculture: Boom, bust, and rebirth? Journal of the World Aquaculture Society, 52(3): 672–690. https://doi.org/10.1111/jwas.12811
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