Arctic angst

Marine Institute researcher Anne Sandvik, who collects data on a weekly basis, says lice numbers in the north have never been higher.

She uses the numbers to calculate how many 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.

She told the institute’s news site there had been an explosive increase in the region and a doubling of numbers. Trout and char were also affected.

The timing of the increase means that sea trout may also have been exposed to lice attack.

“When the louse larvae have developed into infectious copepodites, they are ready to attach themselves to salmon, sea trout or char,” she explained.

“While the salmon swim out to sea in early summer, the trout spend a longer period of time in the fjords and become particularly vulnerable. 

“The risk of sea trout becoming infected with harmful amounts of lice has also increased in recent weeks,” Sandvik added.

As an example, she points out that in midsummer, there were more than twice as many adult female lice on the farmed fish in one area (PO8) than at the same time a year ago.

In addition to the institute’s fixed stations, the researchers have access to the breeders’ temperature measurements via the information organisation Barentswatch.

Temperatures are taken at depths of three metres and this year the water temperature has been a great deal higher (up to 4ºC above normal) which gives the lice a huge advantage, she said.

This is broadly the same as the seas around western Norway which are generally much warmer than around the Arctic Circle.

One northern farm produced millions of larvae. And although lice are found naturally in the sea, most are now found on salmon farms. And this year, an unwanted production record is being set in the north.

Sandvik explained: “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.”

The institute’s scientists say the pace of the life cycle of salmon lice depends on the temperature. At high levels, it goes faster; that is, the lice become adults sooner and begin to reproduce. In addition, their ability to attach to a fish increases with increasing temperature. This in turn leads to more lice, both adults and young, at the same time.

Sea temperatures, which are quite different from normal land weather trends, have been rising gradually for some time but this year they have been exceptionally high.

Scientists fear this is becoming the norm and water temperatures will continue to rise setting new challenges for salmon companies, and not just involving lice.

Strangely, in the south, the water temperature has been fairly normal during the summer when it might have been expected to be correspondingly higher.

Although the total number of adult female lice has gone up in the north, again it has not happened further south.