It is estimated that between 10 and 20% of all farmed salmon in Norway die between transfer to the sea and harvest, of which a significant proportion are lost in the post-smolt phase immediately after transfer.
One reason for this is that salmon’s barrier functions are reduced in the period after transfer to the sea and take a long time to recover.
“The first period in the sea is a vulnerable phase, as the fish’s defence systems are weaker in the first three months,” says Christian Karlsen, a scientist with research institute Nofima.
“During this period, the fish are especially prone to injuries and infectious diseases. It is therefore important to understand why and how salmon’s barrier functions change and what the industry can do to adapt.”
Together with his Nofima colleague Elisabeth Ytteborg, and other scientists in Norway’s Centre for Research-Based Innovation, CtrlAQUA, he has studied how factors such as stress, temperature and treatment during transfer to the sea affect the fish’s external barriers.
Handled more gently
“The results of the research show that the skin becomes thinner and weaker in the period after transfer, but as the fish adapts to the new environment, the skin stabilises again,” says Ytteborg.
It is too early to conclude whether it is a single factor or combinations of environmental changes that cause the weakening of salmon’s external barriers, the scientists say in an article in the Nofima website.
“By increasing knowledge about the sensitive period in the sea, we will be able to suggest how the fish can be protected and handled more gently. Safer transfer of fish will ensure better fish health and fish welfare and will help reduce post-smolt mortality,” says Karlsen.