Smolts kept in salt water ‘less susceptible to tenacibaculosis’

The period immediately after exposure in the sea is a risky one for smolts when they must adapt to a life in the sea. During this period, the fish are exposed to infection with the bacterium Tenacibaculum Finnmarkense, which can cause tenacibaculosis

Tenacibaculosis is a significant fish welfare issue in many economically important species all over the world and has been reported in all Atlantic salmon farming regions.

High acute mortality

Outbreaks are often associated with high acute mortality and are characterised by skin lesions, mouth erosion, frayed fins and tail rot, often with a yellow ulcer.

Salmon farmer Cermaq has over several years worked to develop tools and strategies to reduce the problems associated with T. finnmarkense. In this connection, it was discovered that smolt, which had lived in salt water (26 parts per thousand) for a period before infection with T. finnmarkense, became less ill.

This observation then became the basis for a master’s thesis in fish health by Kristense Solheim at the fish disease group at the University of Bergen, with Are Nylund as the main supervisor.

The experiments were led by Cermaq R&D by researcher Sverre Småge.

Stronger smolts

In her thesis, Solheim has looked at different strategies for smolt production and what effect they have on susceptibility to tenacibaculosis in smolts.

“In the experiments, the smolts were divided into two groups. One group was kept in purified fresh water and one group was kept in purified 26‰ sea water after smoltification,” said Solheim in an article on Cermaq’s website.

“Both groups were infected in sea water to simulate infection after smolt exposure. Smoltvision tests were taken in advance of the experiments to ensure that the fish had not desmoltified.”

The tests showed, among other things, that the smolts exposed to 26‰ salt water for four weeks before transfer to sea water and infection with T. finnmarkense were less susceptible to tenacibaculosis.

Earlier feeding

It was also found that keeping the smolts in 26‰ salt water had other positive effects, such as a better welfare score and that they started eating earlier than was the case with the smolts that had only been in fresh water after smoltification.

“The result of this experiment adds further strength to the hypothesis that extended time in purified sea water before release provides a number of benefits both during the land phase, but also later on the fish’s performance, health and welfare,” said Sverre Småge, who previously identified the strains of Tenacibaulum Finnmarkense that are the causative agents of tenacibaculosis as it presents in northern Norway.

There are still some challenges that need to be addressed, such as brackish water requiring a more demanding water treatment than fresh water during the land phase. But work is under way to solve these challenges.

Solheim’s master’s thesis is part of the FHF-funded project “Limiting the effect of tenacibaculose in Norwegian salmon farming (LimiT)” (UiB), Nofima, Marine Health and Cermaq.