Winter ulcer - now also in fry

Adding seawater contributes to bacterial disease in salmon hatcheries

October 11, 2006

Winter ulcer – now also in fry

Winter ulcer is a bacterial disease that has caused the salmon industry rather large losses. Now there are reports about an increasing tendency in fry hatcheries. Results indicate that adding of seawater to the hatcheries is one of the causes.

Winter ulcers bacteria thrives in cold seawater

The winter ulcer bacteria Moritella viscosa is well adapted to low temperatures in seawater, and in brackish water with more than 30 % seawater. If the water is cold, i.e. below 10 °C, there is an increased probability of a winter ulcer outbreak.


Tests show that juvenile salmon (smolts) that are introduced to brackish water before they are ready for it will become weakened and have bad health. They also have lower appetite and no increase in weight gain.

Salmon hatcheries with intensive production that have low water exchange and add too much seawater will have favourable environmental conditions for the bacteria. However, these conditions are unfavourable for the smolts, and can reduce their resistance against diseases.

Increased susceptibility to winter ulcers

- We examined what effect the water quality in the tanks has for the susceptibility of the smolt to infection with the winter ulcers bacteria after it is introduced into the sea. We tested different water qualities such as high and low water exchange and fresh versus brackish water with 2 % salinity, says Scientist Helene Mikkelsen at Fiskeriforskning.

- Our results show that mortality after infection was highest in smolts that had been exposed to brackish water with 2 % salinity. In contrast, the fish that had received optimal water quality had the lowest mortality.

- Unfortunately, there is no obligation to report this disease, and thus it isn't possible to know the extent of the problem, says the scientist.

All seawater that is brought into salmon hatcheries must be UV treated to reduce the level of bacteria in the water, but the treatment is probably not effective enough. The bacteria that cause winter ulcers most likely have a greater ability to survive UV radiation than other bacteria that are naturally found in seawater.

Reduced use of antibiotics

Fish with winter ulcers are treated by giving them antibiotics mixed in their feed. A large part of today's use of antibiotics is due to winter ulcers. (Ref. report from the National Veterinary Institute)

- The efficacy of the antibiotic treatment varies because sick fish have lower appetite and don't eat enough of the antibiotic feed. Better control of the incidence of the disease will thereby also result in less use of antibiotics, says Mikkelsen.

The tests are part of a larger joint project amongst Fiskeriforskning, NIVA, Norwegian University of Life Sciences at Ås, University of Bergen, Marine Harvest and AquaNet.

The project is financed by the Research Council of Norway, Marine Harvest, AquaNet and Fiskeriforskning.

More information:

Contact Scientist Helene Mikkelsen or by phone +47 77 62 90 55

[Source: Fiskeriforskning]