LICE THAT FALL OFF DURING SPAWNING CAN INFECT NEW SALMON

Lice that fall off during spawning can infect new salmon. The lice  survive for up to three weeks before they have to find new hosts.

Up to 40% of motile salmon lice are released when farmed fish are crowded together before delousing or slaughter.

“If these lice are not collected, they can infect new salmon fish. Lice that fall off can live from one to three weeks without a new host and easily attach to new fish,” says researcher Frode Oppedal at HI.

Motile salmon lice are “semi-adult” ( pre-adult in industry jargon) or adult lice that move around on the salmon’s body. Younger salmon lice are sessile – anchored to the host fish – and will not be able to attach again if they fall off.

10-40% loosened

The researchers have conducted experiments on both lab and tank scales, in small-scale cages and in commercial cages.

The crowding experiments started with few and small fish, and were expanded to include more and larger fish and longer crowding times. During crowding experiments, a group of fish is surrounded by a net that is gradually pulled together, so that the fish are gathered or “crowded”. Fish in commercial cages go through a similar process before slaughter or delousing.

The proportion of mobile lice that were released during crowding varied from 10 to 40%.

“More small, semi-adult lice were released than adults, and the number depended on the crowding time, fish size and number of fish. Adult females hung on to the fish best. The longer the crowding lasted, the more lice were released,” says Oppedal.

Why did most small lice come off?

“We don’t know for sure, but one theory could be that the adult lice have secured the best places for themselves, for example behind the dorsal fin or anal fin, where they have some protection. It may also be that the adults have a slightly better grasping and sucking ability due to their size.”

Survived for a long time and reattached easily

To check how long the lice survive without a new host to parasitize on, the lice were placed in small containers that received a constant supply of fresh water. 

“We checked how long it took before half of the lice were dead. It took 5-13 days for semi-adults, 7-12 days for adult male lice and a full 7-25 days for adult female lice,” says Oppedal.

The temperature in the experiment varied from 4-16 °C. The lice survived the longest at cold temperatures in ordinary seawater. Lower salinity, such as in shallow water in our fjords, resulted in somewhat lower survival.

After half of the lice had died, the researchers physically placed them on new salmon. Well over half attached well and were alive the next day. Others were released into tanks with salmon, and many of them also attached themselves to new host fish.

Previous studies have shown that salmon lice in cages – especially adult males – change hosts .

“If we transferred the lice to new hosts the same day they detached from the previous one, almost all of them reattached. So the potential for what we call re-infestation is very high,” says Oppedal.

Can collect 100% with fine mesh net

The good news is that almost all lice can be collected with a fine-meshed net.

To find the right mesh size, the researchers had to measure the size of the lice carefully. They assumed that the width was the most important thing. (Although it struck Oppedal that no matter how much research has been done on salmon lice, no one has measured how tall  they are.)

“The small semi-adults are about 1.7 mm wide, while an adult female lice is about 4.3 mm wide. We found that nets with meshes of one by one millimetre will catch the vast majority of them,” says the project manager.

Oppedal says that a fine-meshed net is also gentler on the salmon during trapping. Since the surface of a fine-meshed net is smoother than a coarse net, the salmon suffer less damage to their fins, skin and scales.

However, a fine-meshed net will reduce the water exchange in the net used to trap the fish. This makes it more challenging to maintain sufficient oxygen levels for the salmon.

Should be collected and destroyed

“The lice that fall off during crowding should be collected and destroyed in any case. Many lice are carried with the water into the well boat, and on board, good filter systems must therefore be used on the water that is filtered before the salmon enters the well or delousing system,” says Oppedal. 

“In connection with crowding, collection can significantly reduce the spread of mobile lice to salmon in their own cage, neighbouring cages in the facility and wild fish,” he adds.

Spreads mainly within the plant

Based on modelled current data from 13 fish farms, the researchers have modelled the spread of sea lice that have become loose. 

Three types of facilities have been studied: facilities with submerged cages, weak water currents or strong water currents.

“The maximum spread potential was mainly from 200 to 2000 metres, depending on the depth of the nets, the strength of the water current and how fast the sea lice sink. And they sink quite quickly, from 0.5 to 1.5 centimetres per second. This means that spread will mainly occur within the fish farm itself,” says Oppedal.

Do salmon lice hitchhike with wild fish?

Some fish farmers have reported that they have experienced lice infestations on their farms in the days after delousing at farms located more than two kilometres away. 

“They believe that the salmon lice must have drifted further than two kilometres, but that is unlikely. The lice simply sink too quickly for that,” says Oppedal.

What could be the reason then?

“Wild fish, such as saithe, have been observed with salmon lice. So even though the lice normally do not survive over time on the wrong species, it is possible that they hitchhike with wild fish to a new farm. In any case, it is a hypothesis and a possible future challenge that we should look into,” says Oppedal.

The shutter also comes loose

The researchers have also looked at what happens to the scuttle lice when the salmon are crowded. 

The scuttle lice are similar to sea lice, but are smaller in size. They are also not as specialized for one host species as sea lice are, but live on many different fish species. 

“Approximately 20 percent of the scuttle lice fell off during crowding. Unlike sea lice, they did not sink, which means that scuttle lice can spread further than sea lice,” says Oppedal. 

About the project

The research described in this article is a collaboration between researchers from HI, UiB and Deakin University in Australia.

The project was funded by the Norwegian Fisheries and Aquaculture Industry Research Fund (FHF).

References

Frode Oppedal et al.: “Mobile sea lice detach during crowding and can re-infest or be collected – Final report FHF #901784 “LuseOppsamlingSjø””. Report from marine research 2025-13

Barrett, L., Oppedal, F., Harvey, M., Eichner, C., Sambraus, F., Dalvin, S., 2025. Collection rates of detached mobile sea lice according to net mesh and body size: a benchtop model. Aquacultural Engineering 102514. https://doi.org/10.1016/j.aquaeng.2025.102514 

Dalvin, S., Oppedal, F., Harvey, M., Barrett, L., 2025. Salmon lice detached during aquaculture practices survive and can reinfest other hosts. Aquaculture 598, 742065. https://doi.org/10.1016/j.aquaculture.2024.742065

Salmon lice caught with a fine-mesh orkast seine. An orkast seine is a capture seine that is used when a proportion of the fish has to leave the cage. Photo credit: Frode Oppedal / HI

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