Wounded fish are 21 times more vulnerable during handling
Poor gill health and stress can also play their part, Norwegian researchers say.
Winter wounds in salmon can lead to reduced fish welfare, increased mortality, and downgrading of harvested fish. Handling the fish at low sea temperatures is considered an important risk factor. In the ReduSår (ReduceWounds), Norwegian research organisation SINTEF Ocean and partners have investigated how wound risk can be assessed before handling, and how the fish can be handled as gently as possible.
The results particularly point to the condition of the fish and its stress history. Production data from cages in Norwegian production areas 8 and 9 show that groups with high wound incidence more often had reduced health status, poorer gill health or wounds even before the last treatment was carried out. Production area 8, from Helgeland to Bodø, is the core mid-Norwegian aquaculture corridor, and production area 9, from Vestfjorden to Vesterålen, bridges central and northern coastal farming sites.
Researchers found that it was not the number of treatments that separated the groups, but rather the length of time between treatments. In the groups with high ulcer incidence, an average of four weeks passed between the last two treatments, while there were 10 weeks between the groups with low ulcer incidence. The total number of treatments was the same.
In the control experiment, existing wounds were also clearly evident. Fish that already had wounds at the time of entry had around a 21-fold higher risk of developing wounds at the end of the experiment. The experiment only covered one season, and the results must be read as indications rather than definitive answers.
“The results show how important it is to know the condition and stress history of the fish in advance. A good risk assessment can provide a better basis for adjusting the timing and implementation of a handling operation,” said Morten Lund, head of fish health and welfare at Fisheries and Aquaculture Industry Research Funding (FHF), which paid for the study.
More indicators give a better picture
The project has also investigated new methods for monitoring wound risk. No method alone could predict wound development. The researchers therefore point out that health status, temperature, previous treatments, wound development, and information about wound bacteria should be considered together.
Methods for more gentle handling also had no measurable effect on wound development.
Although bacterial detection itself did not predict ulcers, there was a correlation between increases in Moritella viscosa and increased camera-measured ulcer incidence for a limited period.
Camera-based monitoring also showed potential as a decision support tool. In the experiment, the number of active wounds and wound size increased two to three weeks after seawater temperatures dropped below 8 degrees, before mortality increased. The results suggest that cameras can contribute to earlier warning, but it is specified that the method needs further development and validation.
Better basis before handling
ReduceWounds has compiled the knowledge into recommendations for best practice in wound risk assessment and management. The recommendations are divided into four phases: prevention, risk assessment before handling, gentle implementation, and systematic follow-up afterwards.
The results show that no single indicator or measure alone can solve the wound challenge. The condition of the fish, stress history, and several indicators should therefore be assessed together before new handling, and the development should be systematically monitored afterwards.
