Long-lasting stress in farmed salmon makes them more susceptible to diseases such as pancreas disease (PD), one of the most serious salmon diseases in the fish farming industry today.
PD cannot be treated with drugs, and although an approved vaccine exists against the disease, vaccination has only limited effectiveness.
The PD virus occurs naturally in the sea and in fish, without the fish necessarily getting sick. But when environmental factors—such as a rise in temperature, pollution and fish management—create stress over time, the immune system is weakened and disease can break out.
“It’s difficult for us to interpret how the fish are actually doing. A lot of research is going on to find operational welfare indicators for farmed fish, and now we’ve developed tools to test and calculate stressors,” says marine biologist and associate professor Anne Stene. She heads a research group at NTNU in Ålesund that is studying fish welfare, disease and the spread of viruses and sea lice in fish farming.
The researchers have developed a method that measures stress levels in salmon in a reliable and simple manner. The stress hormone cortisol can be identified in the blood of most vertebrates that are experiencing stress.
The problem is that because cortisol is released into the bloodstream so rapidly, the stress resulting from taking a measurement impacts the cortisol level. This method therefore does not give a clear picture of the resting levels of cortisol.
Cortisol is also secreted in fish faeces, and the stress from sampling them affects cortisol levels only minimally. Experiments have shown that this method of cortisol measurement provides a consistent and reproducible measure of environmental stress in fish. Understanding what stresses salmon can help improve aquaculture operations and management.
The research group is also working to identify and understand how the fish become infected and how infected fish get sick. They are also looking for the pathways that spread viruses and parasites, and are exchanging resistant lice between farmed and wild salmon.
“The goal is to outline operational and management strategies to reduce and combat disease,” Ms Stene says.