Introduction
In a combined RAS (recirculating aquaculture system) and sea cage farming, the fish are first farmed in a RAS and then transferred to sea cages. Using continuous light and feeding is a common practice in RAS. This helps maintain stable water quality, which is crucial for the fish health and the good performance. In contrast, the light conditions in sea cage farming follow natural diurnal and seasonal cycle, affecting fish physiology. For example, although continuous light can result in a higher growth rate in RAS, performance of salmonids farmed under continuous light has been unoptimal after they are transferred to sea (H��nninen et al., 2026; Ytrest��yl et al., 2023). Yet, there is lack of information if various light rhythms in RAS could prepare the rainbow trout (Oncorhynchus mykiss) for natural light in the open sea, and how this could affect the hormonal control of fish growth. Therefore, here we investigated 1) do growth and/or hormonal parameters of rainbow trout differ between different light and feeding rhythms in RAS, 2) do different light and feeding rhythms in RAS affect growth and/or hormonal parameters of rainbow trout in brackish water cages, and 3) do growth and/or hormonal parameters of rainbow trout differ between different light rhythms in brackish water cages.
Materials and Methods
Juvenile rainbow trout (7 g) were farmed in RAS at 15 °C under continuous light (LD24:0) and a light rhythm (LD18:6) for six months. The fish were then transferred to brackish water cages located in the Baltic Sea. An underwater light was installed in half of the cages to create light groups with continuous (CL) and natural light (NL). Thus, at the cages we had four groups: 1) LD24:0/CL, 2) LD24:0/NL, 3) LD18:6/CL, and 4) LD18:6/NL. The fish were farmed in the cages for one month to size of approximately 630 g. Sampling was carried out the day before the transfer and after one month in the cages. Length and weight were measured. Based on the fish growth data, the specific growth rate (SGR), the feed conversion ratio, and the condition factor were calculated. Blood samples were taken for haematocrit, haemoglobin, plasma vasotocin (VT), plasma growth hormone, and plasma insulin-like growth factor 1 analyses.
Results
In the first sampling (before transfer), shortening the photoperiod did not have a negative effect on fish growth or hormonal parameters in the RAS. In the second sampling (after transfer), the light rhythm (CL or NL) during the brackish water cage period significantly affected the SGR (p < 0.001). However, the light and feeding rhythm during the RAS period, or the interaction between the light rhythms during the RAS and brackish water cage periods, had no significant effect (p = 0.626 for both). The SGR was 1.13 ± 0.02 in the LD24:0/CL treatment, 1.02 ± 0.02 in the LD24:0/NL treatment, 1.13 ± 0.02 in the LD18:6/CL treatment, and 1.04 ± 0.02 in the LD18:6/NL treatment. Additionally, we observed that the light and feeding rhythm (LD24:0 or LD18:6) during the RAS period significantly affected the plasma VT levels (p = 0.050). Conversely, light rhythm during the brackish water cage period, or the interaction between the light rhythms during the RAS and brackish water cage periods, had no significant effect (p = 0.108, p = 0.368, respectively). Plasma VT levels were 32.0 ± 8.3 ng L-1 in the LD24:0/CL treatment, 40.5 ± 10.2 ng L-1 in the LD24:0/NL treatment, 47.0 ± 8.3 ng L-1 in the LD18:6/CL treatment, and 73.6 ± 10.2 ng L-1 in the LD18:6/NL treatment.
Discussion
The fish grew equally well in both treatments (LD24:0 and LD18:6) in the RAS, despite previous studies have shown better growth under continuous light (Ytrest��yl et al., 2023). The discrepancy may be due to differences in the duration of the dark period used in the experiments. Light and feeding rhythms applied in the RAS did not affect the growth of rainbow trout in brackish water cage farming, and all groups were able to start growing in brackish water as compared to some previous studies where even negative growth have been detected (H��nninen et al., 2026). However, continuous light and feeding (LD24:0) during the RAS period seem to decrease plasma VT level compared to the light and feeding rhythm (LD18:6) in brackish water cage farming. VT regulates osmoregulation, as well as circadian and annual rhythms (Kulczykowska and Stolarski, 1996). VT levels were expected to behave in the opposite way, since they rise in response to stressors (Gilchriest et al., 2000). Continuous light during the brackish water period seems to increase the growth of rainbow trout, but it does not seem to affect their hormonal parameters. In conclusion, since the fish are transferred in spring when the days are naturally long, there is no need to acclimatise them to the natural light rhythm by adjusting light rhythm in the RAS. However, if the aim is to enhance fish growth in brackish water, providing a more consistent supply of light through underwater light in cages or delaying the transfer could be viable solutions.
Acknowledgment
The experiment was partially funded by the European Union through the European Maritime, Fisheries and Aquaculture Fund (EMFAF).
References
Gilchriest, B.J., Tipping, D.R., Hake, L., Levy, A., Baker, B.I., 2000. The effects of acute and chronic stresses on vasotocin gene transcripts in the brain of the rainbow trout (Oncorhynchus mykiss). J Neuroendocrinol 12, 795-801
H��nninen, J., Pulkkinen, J., Vehvil��inen, H., 2026. Shifting production cycle from sea to RAS - what does the fish say? Aquaculture 612, 743175.
Kulczykowska E., Stolarski, J., 1996. Diurnal changes in plasma arginine vasotocin and isotocin in rainbow trout adapted to fresh water and brackish water. Gen Comp Endocrinol 104, 197-202.
Ytrest��yl, T., Hjelle, E., Kolarevic, J., Takle, H., Rebl, A., Afanasyev, S., Krasnov, A., Brunsvik, P., Terjesen, B. F., 2023. 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 Soc 54, 73-95.