However, incorrect use of seawater can weaken the fish's health and might introduce pathogenic bacteria. Recent studies at Fiskeriforskning show that too high doses and too early exposure of seawater during smolt production can result in reduced growth and additional problems with winter ulcers on the fish utbreaks that could reduce the fish welfare and have negative influence on the company's economy.

Life begins in fresh water

The salmon fry's life begins in fresh water. When it's big enough, it can develop into a smolt, i.e. a salmon that's capable of growing and living in seawater. In nature, the juvenile salmon's development is controlled by natural changes in light and temperature, and when it's ready, it migrates out to sea.

In smolt production, under controlled conditions, the time for transfer to the sea is determined by the producer. As in nature, light and temperature are used to control smoltification, and the fish farmer conducts tests on the fish to determine when it's ready for seawater.

Seawater - not just "a piece of cake"

It has been asserted that a gradual adaptation to saline water before the salmon is fully smoltified is favourable, but the research in this area has shown contradictory results.

- In this concrete experiment, we chose to use 60 % seawater (i.e. 2 % salinity) before the fish was fully smoltified, which is not uncommon with smolt producers. We have now evidence that such a high intermixture isn't particularly favourable for the fish, says Senior Scientist Hilde Toften at Fiskeriforskning.

- The fish ate and grew more poorly, and became clearly more susceptible to winter ulcers. Our results also show that these problems are aggravated when we combine them with a very intensive production, which has become relatively widespread in the industry in recent years. So far, we haven't known enough about this, but we're hoping that several factors will be clarified when the project is completed, says Toften.

The fish should thrive

The fish's welfare is also very important for the industry, and the scientists are interested in studying whether the practice of intermixing seawater affects the salmon's well-being.

- Of course, we can't say anything with complete certainty about how the fish experiences these problems, but we can find the optimal conditions that ensure that the farmed fish thrives, and which prevent the fish from becoming stressed and sick.

- Winter ulcers are a serious disease which causes large and deep wounds on the fish. If we can manage to prevent outbreaks, we can help improve the welfare of the smolt, says Hilde Toften in closing.

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.

Need more information? Contact Senior Scientist Hilde Toften