Introduction
Due to new operation models ( e.g. offshore farming) demand for larger fish for sea grow-out is increasing (Bergheim et al., 2009, Ytrestøyl et al., 2022). RAS offers possibl e way of producing larger fish with control on nutrient output and stable environmental conditions for fish. S ea cage grow-out producers in seasonal environment would preferably have all the fish at the same time in spring after the ice cover has left, while RAS is most cost-effective when it does not have to produce fish with all-in-all- out -fashion. However, it is uncertain how the fish grown to larger sizes in stable RAS conditions will adapt to seasonal sea grow-out environment (Lai et al. 2024 , Ytrestøyl et al., 2022) . In this study we tested how RAS grown 500 g rainbow trout perform in sea cages when they arrive to sea in different times of the year.
Materials and methods
At early summer of first grow-out period < 50 g f ish from early hatched egg batches were transported from both freshwater flow-through (Control) and RAS (RAS1) farm to sea cages .
Fish from RAS1-group were also kept in RAS and grown to size > 500 g after which they transferred to sea cages at late autumn of first grow-out period (RAS2). Fish from late hatch egg batch were grown in RAS until size > 500 g, and then transported to sea cages in late winter/ early spring of second grow-out period (RAS3). From RAS3-group 5 g fish were also taken to colder flow-through water for the summer of first grow-out period. These fish were returned to RAS at size of approx. 50 g at autumn and subsequently raised in RAS until > 500 g when they were transferred to sea cage at late spring at the onset of the second grow-out period (RAS4).
All the fish groups were grown with automated feeders according to feed manufacturer’s table adjusted to correspond optimal feeding by visual inspection of appetite (Optimised). In addition, Control and RAS4 -groups were fed with feeders that are adapt (Adaptive) to the feeding amount based on fish appetite.
Results
Control and RAS1 -groups had a good growth in optimised feeding. However, fish transferred as > 500g from RAS showed difficulties to adapt for the grow-out in sea cages (Fig.1)
In adaptive feeding the differences in performance between Control and RAS4 were even more dramatic (Table 1.) even though fish had opportunity to adjust its feeding.
Discussion
Dramatic reluctance of RAS-raised larger rainbow trouts to feed and grow in sea cages makes combination of RAS and sea cage farming challenging, and more causal and physiological investigations to solve this puzzle are needed (Hänninen et al. in prep.) .
References
Bergheim, A. , Drengstig, A., Ulgenes , Y. & Fivelstad , S. 2009. Production of Atlantic salmon smolts in Europe—Current characteristics and future trends. Aquacultural Engineering 41(2): 46–52.
Lai, F., Rønnestad , I., Budaev , S., Balseiro , P., Gelebart , V., Pedrosa, C., Stevnebø , A., Haugarvoll , E., Korsøen , Ø. J., Tangen , K. L., Folkedal , O. & Handeland, S. 2024. Freshwater history influences farmed Atlantic salmon ( Salmo salar) performance in seawater. Aquaculture 586: 740750.
Ytrestøyl , T., Hjelle , E., Kolarevic , J., Takle , H., Rebl , A., Afanasyev , S., Krasnov, A., Brunsvik , P. & Terjesen , B. F. 2022. Photoperiod in recirculation aquaculture systems and timing of seawater transfer affect seawater growth performance of Atlantic salmon ( Salmo salar). Journal of the World Aquaculture Society 54: 73-95