September 30, 2008
Too much saltwater in fry production can lead to reduced growth and higher risk of mortality among farmed salmon, according to a new study by Nofima, the renamed agency from Norwegian aquaculture research institute Fiskeriforskning.
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.
During the time when the fishes were exposed to saltwater treatment, factors like winter sores on salmon, poor appetite, low growth rate and fin damages were discovered and these may have contributed to more fish becoming infected.
The research is of major significance for the aquaculture industry.
Head of Project Hilde Toften at Nofima Marine in Troms said scientists recommend producers to be cautious about the usage of saltwater and too intensive operation during vulnerable periods when the salmon adapts to the saltwater.
Previously the disease affected only salmon in salt water, but in recent years winter ulcer has been a larger problem for fry production.
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.
Around 60 percent of fry producers in northern Norway use seawater compared to around 30 percent in southern Norway.
Farmers often use seawater to increase production capacity, especially when there is insufficient freshwater. In certain conditions, the water quality can be improved by using saltwater to achieve favourable water temperature.
However, seawater can contain disease-producing organisms, such as the winter ulcer bacteria.
A new project funded by the Research Council of Norway and the Fisheries and Aquaculture Industry Research Fund will study among other things if better results can be achieved by adding smaller quantities of saltwater during fry production.