Farmed salmon have never been handled as much as they are today. Handling involves stress and is a challenge for fish welfare, fish health, and ultimately the survival of the salmon. Stress is also a substantial economic challenge as it can lead to reduced growth and increased mortality. It is therefore extremely important for salmon and fish farmers to both reduce stress on salmon and increase their stress tolerance.
How fish health is affected by stress
When the fish encounter challenges in an environment that threatens their physiological balance, this will trigger a variety of changes in the bodily functions of the fish.
This is called a "fight or flight" response.
It comprises an immediate adrenaline-based response, and a more long-term cortisol-based response that prepares the fish to either escape from or deal with the threat. This response is completely crucial to the salmon being able to deal with its surroundings. But if the stress becomes long term, the changes can lead to the fish using its available energy to get through the challenging situation, at the cost of functions such as growth and the immune system.
Research points to the fact that salmon have good ability to recover from acute stress. This is because the fish often manage to quickly restore the balance and the consequences are not necessarily severe. However, one example of where acute stress can have fatal consequences is if the stress triggers a panic reaction.
Farmed fish operate as a shoal, and in individual cases the flight reaction in the fish during delousing can lead to the fish in the cages seeking out the bottom of the net. The risk here is insuficient oxygen levels for the nubmer of fish crowded into a small area.
If stress becomes long term, this can affect growth and resistance to disease, and lead to increased mortality.
Operational measures cause stress:
Farmed salmon have to endure a variety of necessary operational measures, such as grading, vaccination, and transportation and transfer to the sea. But to deal with the current challenges from lice, farmed salmon today have to be handled more than ever before. Both chemical and physical treatment methods are used, and treatment often involves the fish having to be handled and pumped out into the well-boat for treatment. This will lead to the fish being exposed to physical stress, and probably also psychological stress.
How stress can be reduced through strategic feeding:
- If we can prevent lice outbreaks by means of a thicker and more effective mucous layer, this can reduce the number of delousing procedures.
- Antioxidants in the feed can reduce the oxidative stress on tissue in situations with reduced access to oxygen.
- The fatty acid profile of the feed can affect the fatty acid composition in the cell membranes. These fatty acids are precursors to neurotransmitters that can contribute to controlling the stress reaction in a beneficial and non-beneficial way.
- Components in krill can control circulatory health. In stressful situations, the fish use more oxygen, and in addition the access to oxygen will often be reduced. A well-functioning circulatory system can be critical in periods where the fish have to compensate for the increased pressures associated with stress.
- Krill can maintain appetite during periods of stress. This ensures that the fish take in nutrients that can be critical for coping with stress.