Traditionally, farmed Atlantic salmon spend their early years in freshwater tanks before being transferred to open-sea cages for further growth. However, this sea-based approach comes with challenges such as unpredictable temperatures, disease outbreaks, and escapes.
A promising alternative is to keep salmon on land for a longer period. However, land-based farming often relies on continuous light and warmer water for optimal growth, potentially triggering an unwanted issue: early sexual maturation.
Scientists from Nofima, the Norwegian University of Life Sciences, Havsbrún P/F, the University Hospital of North Norway, and UiT-The Arctic University of Norway investigated how manipulating the photoperiod, the light-dark cycle, during the post-smolt phase can influence and potentially delay unwanted maturation.
The issue of early maturation
Early maturation of male land-farmed Atlantic salmon can be a headache for fish farmers.
But why is early maturation undesirable? Firstly, mature salmon stop eating, their meat quality is affected, and their ability to regulate salt levels weakens. This translates to slower growth, reduced well-being, and ultimately, loss of profits for salmon farmers.
The culprit? The photoperiod.
Light, or the lack thereof, acts as a powerful signal for salmon, influencing when they reach sexual maturity. Studies have shown that continuous light and “winter signaling” photoperiods (mimicking short winter days) can trigger early maturation, especially in males.
So, how can we keep our salmon growing and postpone puberty?
This is where the new study comes in. Scientists studied the effects of three different photoperiod regimes on post-smolt Atlantic salmon in brackish water (a transition phase between freshwater and seawater):
- Continuous light: The traditional CCS approach, is known to promote early maturation.
- The gradual decrease in day length: Mimicking the natural shortening of days as winter approaches.
- Short to long day length: Briefly expose fish to shorter days before increasing the hours of light.
The fish were monitored for 29 weeks, with 18 weeks in brackish water and 11 weeks in seawater. Throughout the study, scientists assessed growth, body condition, gonad development, and hormone levels.
Key findings
The study’s results were clear:
- Continuous light produced the highest rate of early maturation in male salmon.
- The gradual decrease in day length significantly reduced maturation compared to continuous light.
- The short to long-day length group showed intermediate maturation rates.
It is important to note that the decrease in day length did not negatively impact growth, suggesting it can effectively control maturation without sacrificing performance.
Application in the salmon industry
The research provides valuable insights for land-based salmon producers:
- The gradual decrease in “day length” (light hours) is a promising tool to prevent early maturation in male salmon.
- Further research can refine photoperiod manipulation strategies for different salmon strains and farming conditions.
- Understanding the hormonal and physiological mechanisms behind photoperiod-regulated maturation can lead to even more effective control methods.
Conclusion
“These results show that the photoperiod can be used to regulate maturation in male Atlantic salmon. Additionally, decreasing the photoperiod after smoltification appears to limit early sexual maturation without compromising body size development,” conclude the scientists.
The study’s findings shed light on how the photoperiod can be utilized to manage early maturation in Atlantic salmon, paving the way for a new era of efficient and sustainable salmon farming. By harnessing the power of natural light cycles, salmon farmers can achieve healthier fish and more robust profits.
The study was partially funded by the Research Council of Norway, Strategic Institute Research Nofima AS, project nr 12878 ‘Atlantic salmon biology in focus: for a robust fish in a shifting aquaculture industry’.
Reference (open access)
Skjold, V., Rørvik, K. A., Sveen, L., Mota, V., Weihe, R., Ytrestøyl, T., … & Dessen, J. E. (2024). Gradually decreasing daylength after smoltification induced by¨ winter signal¨ reduced sexual maturity in male Atlantic salmon. Frontiers in Aquaculture, 2, 1235584.
