Higher mortality when saltwater used

Too much saltwater in fry production can lead to reduced growth and higher risk of mortality among farmed salmon, concludes a fresh Nofima study.

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Portrettbilde av Hilde Toften
Hilde Toften

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While testing the winter ulcer bacteria Moritella viscosa, scientists recorded three to four times higher mortality rates among salmon farmed in freshwater with added seawater (20 per thousand) than salmon farmed in pure freshwater.

The test was carried out straight after the vulnerable salmon fry underwent a physiological adaptation to tolerating saltwater – smoltification.

Poor appetite
Poorer appetite, lower growth rates and more fin damage was also discovered in the period the fish were exposed to saltwater treatment, something which may have contributed to more fish becoming infected.

The research is of major significance for the aquaculture industry.

"The scientists recommend fry producers to be careful about using too much saltwater and too intensive operation during the vulnerable period when the salmon adapts to the saltwater," says Head of Project Hilde Toften at Nofima Marine in Tromsø.

Widespread problem
Winter ulcer has long been a widespread problem in the Norwegian salmon farming industry.

Previously the disease affected only salmon in salt water, but in recent years winter ulcer has been a larger problem for fry production.

This is a financial burden for the salmon farmers because of the increased mortality rate. Ulcers and scars on surviving fish also means lower prices, while the virus can lead to reduced growth as fish with ulcers often stop eating.

It is, therefore, important to prevent any outbreaks.

More using seawater
In recent years, the use of seawater in fry production has increased.

Around 60 percent of fry producers in Northern Norway use seawater compared to around 30 percent in Southern Norway.

There are several reasons for using seawater. One important reason is increasing the production capacity, especially when there is insufficient freshwater. In certain conditions, the water quality can be improved by using saltwater and a favourable water temperature can be achieved.

But not everything to do with seawater is positive. The seawater can contain disease-producing organisms, such as the winter ulcer bacteria.

Parts of the research will be continued in a new project funded by the Research Council of Norway and the Fisheries and Aquaculture Industry Research Fund.

This project will study among other things if better results are achieved by adding smaller quantities of saltwater during the fry production.

Fish health  

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