Stress increases
the level of cortisol
Is the fish in good shape?
It is relatively easy to see when we humans feel tired and have reduced general condition. A fish, on the other hand, does not change its facial expression, and it can be challenging to assess how it is really feeling.
On a physiological level, however, fish react in exactly the same way as we do. When the body feels threatened, a wide range of signaling molecules are activated putting the body on alert. The most important signaling molecule in all vertebrates is the stress hormone cortisol.
Increased levels of cortisol have a negative effect on fish health
Short -term stress creates an appropriate alarm response that helps us survive an acute challenge, but prolonged or repeated stress has negative physiological effects on both animals and humans. Stress suppresses the immune system and increases susceptibility to disease. Elevated levels of cortisol also lead to slower wound healing and reduced appetite and growth.
Cortisol measured in plasma or in feces?
If cortisol is to be measured in blood plasma, high demands are placed on a standardized sampling procedure, as cortisol is expressed very quickly in the blood. If, on the other hand, we measure cortisol metabolites in feces, we "look back in time" (the feces have already passed through the body), and we measure the stress level that the fish had a day earlier. The measurement result is therefore only slightly affected by the sampling procedure itself. In addition, measurements on feces have the great advantage that the fish do not need to be euthanized. It is easy to strip a little feces from the fish - only around 0.2 g of feces is needed. Feces from 10 fish are sufficient to get a good picture of the stress level in a cage or tank.
Cortisol measured in feces is simpler and offers new possibilities
Measurements of cortisol in feces provide new opportunities to understand causal relationships and work on continuous improvement of operational activities. If the fish have elevated cortisol levels, they will tolerate less handling. By monitoring the stress level of the fish until slaughter, the risk of mortality can therefore be reduced by harvesting first the cages where the fish are weakest. Measuring stress can also be a tool to assess whether low feeding is due to reduced fish health or whether there is a risk of underfeeding.
Pre-slaughter stress monitoring
Both illness and physical handling will lead to increased stress. The cortisol level of the fish is therefore a broad-spectrum marker that is reflected in most situations where the fish's health is compromised.
The figure shows results from stress measurements in two cages that were monitored for three months leading up to slaughter. In cage no. 2, the fish began to experience gill problems in September. By October, the stress level in cage 2 had become significantly higher both in relation to cage 1 and in relation to previous measurements in cage 2.
As the figure shows, six out of ten fish had significantly elevated stress levels in cage 2 in October. Such information can be actively used to reduce risk and optimize harvesting plans.
