Study links later smolt transfer to stronger post-smolt growth.
Atlantic salmon transferred to seawater at a later stage of smolt development showed stronger growth and performance than fish moved earlier, according to a study from the University of Gothenburg.
The research examined the effects of transfer timing, pre-transfer fasting, and freshwater diet composition on post-smolt performance.
The researchers measured smolt development using degree-days, a standard aquaculture metric that combines water temperature and time to track biological progress. One degree-day represents one day at 1°C, meaning fish held at 10°C accumulate 10 degree-days per day. Because salmon physiology develops in response to temperature rather than the calendar, the measure provides a more accurate indicator of smolt readiness across different farming conditions.
Fish transferred to seawater at 480 degree-days outperformed those moved earlier at 144 or 300 degree-days, recording higher body weights during the post-smolt phase.
Researchers linked the difference to the activity of Na⁺,K⁺-ATPase (NKA), the enzyme responsible for saltwater osmoregulation. The study found that NKA activity increases sequentially across organs during smoltification, with the gills responding first, followed by the kidney and intestine. Full synchronisation of NKA activity across the three organs occurred at around 480 degree-days, the point at which fish showed the strongest seawater performance.
The study also found that pre-transfer fasting reduced growth, particularly in fish transferred early at 144 degree-days. The negative effect was smaller in fish moved later in the smoltification process.
A high-lipid freshwater diet, containing around 29 percent fat compared with 24 percent in the control diet, accelerated smoltification and increased NKA activity across osmoregulatory organs. The higher-fat diet also reduced the negative impact of fasting on post-smolt growth.
The findings suggest that transferring smolts before NKA activity is synchronised across gills, kidney, and intestine may reduce growth and increase vulnerability during the early seawater phase.
The research was published under the title “Synchronisation of Na+,K+-ATPase activity in intestine, gills, and kidney at the time of seawater transfer improves post-smolt performance in Atlantic salmon.”
