Researcher Anne Sandvik of the Norwegian Institute of Marine Research reports an explosion in salmon lice in the northern part of Norway.
‘We have seen an explosive increase, doubling of the number of louse larvae this summer,’ she said.
Anne Sandvik collates weekly data from producers detailing louse numbers on salmon, and uses this to project the volume of salmon lice escape from farms and where they drift with the ocean currents – before they have grown into infectious lice that can settle on fish.
The life cycle progression of salmon lice depends on the temperature, as they mature faster in warmer conditions and that increases their reproduction capacity, as well as their ability to attach themselves to fish.
‘This large increase in the north is probably due to the high sea temperature this summer. It has given the lice excellent living conditions,’ Anne Sandvik said, adding that at one of the fixed stations, Eggum, an aggregate of 16.85° was recorded in August.
‘Usually the temperature here, on the outer side of Lofoten, is around 12-13°,’ she said.
The sea temperatures measured in the north this year are about the same as they usually are off western Norway.
In addition to HI’s fixed stations, the researchers have access to the salmon breeders’ temperature measurements via Barentswatch.
‘The breeders measure the temperature at a depth of three metres, these are also included when we make calculations on the amount of salmon lice. The temperature at three meters has been 3-4°’ she said.
Although salmon lice are found naturally, most lice now come from salmon farms.
‘If we start from the fact that a single cage contains between 100,000 and 200,000 fish, and each of these has 0.5 adult female lice (which is the legal limit from week 27 in the north), this cage will have 50,000 to 100,000 adult female lice before there is a requirement to de-louse the fish,’ Anne Sandvik explained.
In the south, the water temperature has been about as usual throughout the summer. Although the total number of adult female lice is as high or higher than in the north, it is not higher than it has been in recent years.
