Introduction
In combined RAS (recirculating aquaculture system) and sea cage farming, fish are first farmed in RAS and then transferred to sea cages for the rest of their lives. By increasing the size of fish in RAS, the production cycle of farming can be accelerated, and the sea cage period can be shortened (Bjørndal and Tusvik, 2017). A shorter sea cage period provides means to capture nutrients more efficiently and reduces biological risks (Bjørndal and Tusvik, 2017). Shorter sea cage period would also make it possible to avoid risks associated with overwintering of fish (Donaldson et al., 2008). However, RAS and sea cage farming are very different production methods and the effects of combining them on fish growth and welfare are not well understood. In this study, we investigated 1) Does growth of rainbow trout (Oncorhynchus mykiss ) farmed in RAS and PRAS (partial RAS) differ after transfer to freshwater and brackish water conditions? 2) Does the size of rainbow trout and/or the timing of transfers affect the growth of rainbow trout? and 3) Does the water quality of initial farming influences the success of transfers?
Materials and methods
Rainbow trout were first f armed in RAS and PRAS. In summer, some fish were transferred to fresh and brackish water, while the rest remained in the original RAS and PRAS tanks. The fish were farmed in the fresh and brackish water for five weeks (the summer period) . In autumn , another transfer took place, and the fish were again transferred to the fresh and brackish water. The autumn period took also five weeks. The f ish were sampled after the summer and autumn periods. The sample fish were killed by an overdose of anaesthetic. The total length, weight and gutted weight were measured. Based on the fish growth data, feed conversion ratio, condition factor, specific growth rate, and thermal-unit growth rate were calculated. A blood samples were taken for haematocrit, plasma osmolality, plasma chloride content, and plasma cortisol content determination.
Results
During the summer period, t he RAS-farmed fish grew better in the freshwater than in the RAS environment (SGR 34%, TGC 37%, p < 0.001) or in the brackish water (SGR 27%, p < 0.001). The RAS-farmed fish grew better in the RAS environment than in the brackish water (TGC 30%, p < 0.001) . T he PRAS-farmed fish grew better in the freshwater than in the PRAS environment (SGR 8%, p < 0.05) or in the brackish water (SGR 8%, TGC 35%, p < 0.001) . The PRAS-farmed fish grew better in the PRAS environment than in the brackish water (TGC 33%, p < 0.001) . In the RAS environments, the PRAS-farmed fish grew better than the RAS-farmed fish (SGR 22%, TGC 37%, p < 0.001). In the freshwater, there was no significant difference between the RAS- and PRAS-farmed fish. In the brackish water, the RAS-farmed fish grew better than the PRAS-farmed fish (9%, p<0.05).
During the autumn period, the RAS-farmed fish grew better in the RAS environment (SGR 295%, TGC 283%, p < 0.001) and in the freshwater (SGR 270%, TGC 333%, p < 0.001) than in the brackish water. There was no difference in the RAS-farmed fish between the RAS environment and the freshwater. The PRAS-farmed fish grew better in the PRAS environment than in the freshwater (32%, p < 0 .01). The PRAS-farmed fish grew better in the PRAS environment than in the brackish water. In the PRAS environment the fish continued to grow normally (SGR 1.39, TGC 0.23), while in brackish water fish did not eat and lost weight (SGR -0.07, TGC -0.01). PRAS-farmed fish grew also better in the fresh than in the brackish water. In the freshwater, the fish continued to grow (SGR 1.05, TGC 0.23), while in the brackish water the fish did not eat and lost weight (SGR -0.07, TGC -0.01). In the RAS environments, there were no difference between the RAS- and PRAS-farmed fish. In the freshwater, the RAS-farmed fish grew better than the PRAS-farmed fish (29%, p < 0.01). In the brackish water, the RAS-farmed fish grew better than the PRAS-farmed fish. The RAS-farmed fish continued to grow (SGR 0.37, TGC 0.06), while the PRAS-farmed fish did not eat and lost weight (SGR -0.07, TGC -0.01).
Discussion
Seawater transfer plays an important role in salmonid production, where juveniles are produced on land in freshwater and then trans ferred to the sea for further farming (Thorstad et al., 2012; van Rijn et al., 2021). This study investigated the transfer of two different sizes of rainbow trout from two different RAS environments to fresh and brackish water. We observed differences in the g rowth of RAS- and PRAS-farmed fish after transfers to the fresh and brackish water. The most striking exception was the results for the PRAS-farmed fish in the brackish water during the autumn period. The f ish did not eat despite continuous offering of the feed and therefore lost weight. Stress does not seem to explain the differences in appetite and growth of fish , as we did not observe significant differences with a clear pattern in the physiological pa rameters. Ou r results also show that the size of fish and the t iming of transfer affect growth of fish . In this study, the s maller fish transferred in the s ummer grew better than the larger fish transferred in the autumn. Based on our study, t he environment and water quality in the initial farming did not seem to affect growth of fish after transfer in themselves. Continuous lighting and feeding, as well as the fish growth in the RAS and PRAS environments, may influence the growth success in the further farming. Ultimate reasons for these phenomena remain unclear and need clarification to develop the farming method for wider use in salmonid aquaculture.
References
Bjørndal, T., Tusvik, A., 2017. Land based farming of salmon: economic analysis. Norwegian University of Science and Technology, Department of International Business, Working Paper Series 1/2017, 1–157.
Donaldson, M. R., Cooke, S. J., Patterson, D. A., Macdonald, J. S., 2008. Cold shock and fish. Journal of Fish Biology 73, 1491-1530.
Thorstad, E.B., Whoriskey, F., Uglem, I., Moore, A., Rikardsen, A.H., Finstad, B., 2012. A critical life stage of the Atlantic salmon Salmo salar: behaviour and survival during the smolt and initial post-smolt migration. Journal of Fish Biology 81, 500–542.
van Rijn, C.A., Jones, P.L., Evans, B.S., Afonso, L.O.B., 2021. Physiological and growth responses of juvenile Atlantic salmon (Salmo salar) transferred to seawater during different stages of smolt development. Aquaculture 538, 736527.