Background
Production of a large salmon smolt is a relatively new strategy in Norwegian salmon farming, and several production protocols are currently used in the land-based phase (Ytrest��yl et al., 2023). Most farmers apply a winter signal with short days (12 hours light and 12 hours dark; 12L:12D) to smoltify the fish, while others apply continues light (24 h) throughout the production period. Salinity regimes also vary considerable , some facilities use freshwater while others use brackish water. Farmers report variable performance of larger smolts after transfer to sea, but the wide diversity in production strategies combined with year-round sea transfer, makes it difficult to draw conclusions about the factors underlying reduced performance in sea. Thus, documentation is still lacking on which production protocolest supports growth and survival, while at the same time reduce the risk of early sexual maturation in males (Good and Davidson, 2016)
Through several projects, Nofima and partners have tested and evaluated the effect of salinity (brackish water versus freshwater), photoperiod (continues light (24 h) versus a winter signal (12L;12D) to induce smoltification) and fish size at sea transfer (120, 320 and 850g) on the seawater performance in Atlantic salmon. Additionally, fish size at the initiation of a winter signal (50 versus 120 g) and winter signal duration (6 weeks and 18 weeks) was examined with respect to growth and performance in seawater cages until slaughter and in seawater tanks for 3 months. In seawater tank trials, the fish were exposed to environmental conditions simulating either spring (6°C and increasing day length) or autumn (12°C and decreasing day length).
Results
The results indicate that exposure to a winter signal protocol generally imporved growth in the seawater phase. However, final slaughter weight could still be higher in fish exposed to 24 h light in RAS, largely because they were bigger at the time of sea transfer. Survival and growth in seawater were influenced more strongly by the size and transfer time than photoperiod and/or salinity. Fish transferred later in autumn at a larger size showed poorer growth and higher mortality in the seawater phase than fish transferred at a smaller size earlier in the autumn. However, both season and size could have affected the outcome.
Fish exposed to a winter signal combined with brackish water RAS showed improved growth after transfer to seawater, particularly when transferred at a larger size at. However, a winter signal also increased the risk of sexual maturation in males, especially in fresh water. Exposing the fish to low temperature and increasing day length that simulated spring gave less sexual maturation than high temperature and decreasing day length that simulated autumn. Thus, which protocol to use should be selected based on the timing of sea transfer for the larger smolts/post-smolt.
Welfare, quality, seawater tolerance and the incidence of nephrocalcinosis was also assessed in the different production protocols. There were no large effects of photoperiod and salinity in RAS, but slightly higher score for nephrocalcinosis in fish on 24 h light in RAS than in fish given a winter signal. However, smolt development and osmoregulatory capacity, assessed using NKA enzyme activity, in gill, intestine and kidney where differentially affected by photoperiod and salinity protocols. Welfare and quality were not affected by protocol in RAS, but there were some minor effects of experimental conditions in the tanks with seawater, with reduced welfare scores in fish at higher temperatures.
Acknowledgment
The research was funded by the Norwegian Seafood Research Fund (FHF) and the centre for research-based innovation (SFI) CtrlAQUA
References
Good, C. & Davidson, J. 2016. A review of factors influencing maturation of Atlantic Salmon, Salmo salar, with focus on water recirculation aquaculture system environments. Journal of the World Aquaculture Society, 47(5), 605–632.
Ytrest��yl, T., Jenssen, I., Wetterwald, V.E., Striberny, A., Alvestad, R., Dam, R., Mortensen, H., Johansen, E., Espmark, ��., Johansen, L-H., Kolarevic, J., Larsen, J., Sandberg, M.G., Nilsen T.O. Kunnskapskartlegging-produksjon av stor laksesmolt. Report 12/2023 Nofima (In Norwegian). ISBN 978-82-8296-746-4