Seafood Media Group - Worldnews - Norwegian Institute of Marine Research: 'Explosive increase in salmon lice'

Welcome

Impact on Trout and Char

This sudden rise in lice numbers poses a significant threat, particularly to sea trout and Arctic char.

“When the larvae develop into infectious copepodites, they are ready to attach themselves to salmon, sea trout, or Arctic char,” Sandvik explains.

The red line shows how the production of louse larvae has increased in recent weeks compared to the last three years. In the same period, the sea temperature has also been higher than normal. The figure shows the situation in Production Area 12, which is Western Finnmark in week 34, but other areas also have a lot of lice. If you are interested in other production areas, you can check the status in our weekly report (updated once a week, pdf).

While salmon swim out to sea early in the summer, sea trout and char remain for extended periods in fjords and coastal waters, making them particularly vulnerable. Sea trout, which stay farther out in the fjords, face a heightened risk.

“The likelihood of sea trout contracting harmful amounts of lice has sharply increased in recent weeks,” Sandvik warns.

As an example, in week 30, there were more than double the number of adult female lice on farmed fish in the PO8 region compared to the same time in 2023. Salmon lice feed on the fish, and in severe cases, heavy infestations can be fatal.

Sjørret (archive photo). Photo: Rune Nilsen / Institute of Marine Research

The Role of Rising Temperatures

The significant increase in lice populations in the north is largely attributed to unusually high sea temperatures this summer. These warmer conditions have provided ideal living environments for lice, Sandvik notes.

"The lifecycle of salmon lice speeds up with higher temperatures, meaning they mature faster and begin reproducing sooner. Additionally, their ability to latch onto fish increases with warmer water, leading to a simultaneous rise in both adult and young lice," she explains.

Unprecedented Warmth in Northern Waters

This summer, sea temperatures in the northern regions have been much higher than average. At one of HI’s fixed temperature stations in Eggum, for instance, the water temperature in August reached 16.85°C, compared to the usual 12-13°C typical for the area, which lies on the outer coast of Lofoten.

“This year, sea temperatures in the north are comparable to what we normally see in western Norway,” Sandvik observes.

The breeders' temperature measurements, collected via Barentswatch, confirm this trend. At a depth of 3 meters, water temperatures have been 3-4°C higher than usual, giving the lice a significant advantage, Sandvik adds.

The breeders measure the temperature at a depth of 3 metres. The figure shows the median temperature for all the facilities in Production Area 12 through week 34. The red line is this year's measurements, the others are from previous years. If you are interested in other production areas, you can check the status in our weekly report (updated once a week, pdf). In several of the other production areas, the summer's heat wave shows even more clearly in the temperature graph than it does in this one.

Farms as Lice Hotspots

While salmon lice are naturally present in the sea, the majority now originate from fish farms. This year, the north is setting an unwanted record for lice production.

To put things into perspective, Sandvik provides a calculation: “If a single cage holds 100,000 to 200,000 fish, and each fish carries 0.5 adult female lice (the legal limit from week 27 in the north), that equates to 50,000 to 100,000 adult female lice per cage before delousing becomes mandatory.”

Many farms have recently reported surges in the number of adult female lice. For instance, in PO 12, there are now four times as many female lice as in previous years. By week 34, 56 out of 558 facilities along the Norwegian coast exceeded the legal lice limits (source: Barentswatch).

On this map from 15 August, the heat wave in the sea in the north is clearly visible, the temperature is partly as high or higher than what they usually are in Western Norway. The scale on the right shows °C.

Southern Regions: Stable, but Not Improving

In southern Norway, water temperatures have remained relatively stable throughout the summer. While the total number of adult female lice is as high—or higher—than in the north, it hasn’t exceeded previous years' levels.

Ongoing Monitoring

The Institute of Marine Research continues to monitor salmon lice populations on wild salmon species (salmon, sea trout, and Arctic char) in selected areas along the coast. Monitoring starts earlier in the south, where temperatures rise first. Although the results from the north are still being reviewed, they are expected to be available this autumn.

Author: Beate Hoddevik

editorial@seafood.media
www.seafood.media