Aleksei Krasnov, a senior scientist at Nofima.
Aleksei Krasnov, a senior scientist at Nofima.

Immune system health check developed for smolts

Salmon need a robust immune system to make the transition from fresh water to seawater. Scientists at research institute Nofima have created a test that measures immune status.


The test can provide information that fish farmers will benefit from when transferring salmon to the sea. The results can help increase the smolts’ chances of survival. Every year, approximately 15% of farmed salmon die after being transferred from land-based freshwater tanks to net pens at sea. Most of these fish die shortly after being transferred. This test is therefore very relevant.

What is the test?

It is a diagnostic test that measures the activity of 44 genes that are important for the immune system. A sample is taken from the gills or the dorsal fin without harming the fish.

“A few years ago, we found that the immune system was weakened during smoltification. As a result, we decided to develop a test to measure the immune status of salmon during smoltification,” said Aleksei Krasnov, senior scientist at Nofima.

The immune status of smolts can now be checked by taking a sample from the gills or dorsal fin without harming the fish.
The immune status of smolts can now be checked by taking a sample from the gills or dorsal fin without harming the fish.

To do that, Krasnov and his colleagues evaluated many genes and selected those that provided the most information about immune status. The scientists compiled large amounts of data from many different experiments performed in Nofima. Krasnov is not aware that anyone else has such a large and well-organised database of gene expression for salmon.

The activity of the selected genes was analysed on salmon from various fish farming sites, different groups of fish, and different environmental conditions. The scientists used this data to develop a database that provided the basis for the diagnostics of the immune status.

Gene activity reveals the body’s plan

The genes give ‘orders’ to the body’s cells to produce proteins that have a given function.

Genes may increase or decrease activity long before we can see changes in fish condition. Scientists measure activity of carefully selected genes for assessment of salmon immune status and health.

The scientists have found the normal activity level for each of the 44 genes in a fish of good condition. If the activity level is too high or too low, it may indicate poor immune status. This means that they can now offer tests for immune status in salmon smolts.

“We are able to see which fish are in good condition and which are not. In addition, we know which fish seem healthy, but in reality are not,” Krasnov said.

Industry representative, Dr Gordon Ritchie, who is group manager fish health and welfare for the world’s biggest salmon farmer, Mowi ASA, said: “The successful development of this novel technique provides companies with a new tool in the smolt quality assessment box, for verifying the immune competency and robustness of smolt before transfer to sea.

“This immune status diagnostic tool can also be utilised in future research projects on smolt performance and survival.”

Useful in other projects

Krasnov also sees the benefits of using the test in other research projects.

“We used the test in the CtrlAQUA research centre to test whether different ways of producing large smolt in closed recirculation facilities affect immune status before transfer to sea takes place,” said Krasnov.

“The test showed that immune status was the same for the different production methods. We also use the test in projects where the nutrition scientists investigate whether feed ingredients affect fish health.”

CtrlAQUA is the Centre for Research-Based Innovation in Closed-Containment Aquaculture, hosted by Nofima at Sunndalsøra in Norway. Its main goal is to develop technological and biological innovations that will make closed systems a reliable and economically viable technology.