Dr. Paddy Gargan:

It is slightly complicated. We have done many studies over the years to show that sea trout will have an average of three to five sea lice when there is no salmon farming nearly. In salmon farming areas, they will have 30 or 40 sea lice, or perhaps ten times more. Those levels are not comparable to the level one wants to keep on the salmon farm because there are hundreds of thousands of fish on the farm. At present, ensuring that each fish on the farm has only half of an adult female lice cannot be compared to a level that is normal on a sea trout because of the number of sea trout eggs that are produced from the farm that have the potential to get on to wild salmon and sea trout in the locality.

This is back to the risk based approach that the Norwegians use. They monitor their salmon and sea trout.

If the number of lice per gram is above a certain threshold, there is a high risk of mortality among wild fish. They advise that if the lice level on the sea trout and salmon they monitor is at a certain level they will be happy. If it is at a certain low threshold there is no impact, but the higher it gets, the more potential there is for mortality among wild fish. That is the risk-based approach they are taking in Norway.

Rather than set a trigger level regardless of the size of the fish farm, it would be better to ensure the level set is one at which there is no impact on the local wild stock.