Aquaculture: A little salt healthy, but not too much

The addition of saltwater should be limited to a salt content of 15 parts per thousand.

Earlier studies indicate that more than half of all fry producers add saltwater.

However, a scientific basis for this practice has been lacking. To the contrary, earlier research has implied potential harmful effects of using too much saltwater.

However, fresh research from a project carried out by Nofima Marin for the Fisheries and Aquaculture Industry Research Fund (FHF) and the Research Council of Norway shows that under certain conditions the use of saltwater is favourable.

One of these is moderate salt content. The correct sterilisation of the saltwater and good hygiene is also important, including in the pipe trenches which carry the saltwater into the fry hatchery.

"The use of saltwater in fry production has several advantages," says Head of Project Hilde Toften at Nofima Marin.

"By getting water from deeper parts of the sea with a relatively stable temperature, you can heat the water in the fry hatchery in winter and cool it down in the summer," says Toften.

Toften says that a small amount of seawater (up to one part per thousand) can also help to detoxify fresh water with high aluminium content.

Some scientists have suggested that a little seawater in the fry phase can make the fish's later adaptation to saltwater easier, but there are some conflicting results here.

What is known for certain is that the addition of seawater can increase the production capacity for a coastal fry producer with limited access to fresh water.

However, seawater can also have negative effects. Several organisms in the sea can result in diseases for the fry, including the winter ulcer bacteria Moritella viscosa.

Besides, the salt can create unfavourable reactions with aluminium fixed to particles in the freshwater to the detriment of the fish. Too high salt content too early in the salmon's life is detrimental.

Earlier research at Nofima Marin shows that salt content of 20 parts per thousand can produce more winter ulcers and several other welfare problems for the fry, such as poorer appetite and lower growth rates.

"We don't recommend a rate of 20 parts per thousands before the fish is smoltified or in other words physiologically mature to be transferred to saltwater," says Toften.

The project is now progressing with other types of analysis of the effect mixing in a moderate amount of saltwater has on the fry's disease resistance, welfare and quality.

Nofima Marin is collaborating on this research with the Norwegian Institute for Water Research (NIVA), the University of Bergen and the NorwegianUniversity of Life Sciences.

The project is being carried out for the Research Council of Norway and the Fisheries and Aquaculture Industry Research Fund (FHF).