Salmon lice caught with a fine-mesh orkast seine. Photo: Frode Oppedal / HI

Salmon Lice That Detach During Crowding Can Reinfect New Fish

NORWAY
Friday, March 21, 2025, 08:00 (GMT + 9)

Lice can survive up to three weeks before finding a new host.

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

“If these lice are not collected, they can quickly infect new salmon. Once detached, they can survive for one to three weeks without a host and easily reattach to new fish,” explains researcher Frode Oppedal at the Institute of Marine Research (HI).

Understanding Motile Salmon Lice

Motile salmon lice, also known as pre-adult or adult lice, move freely on the salmon’s body. In contrast, younger lice are sessile—firmly anchored to their host—and cannot reattach if they become dislodged.

Lice Detachment During Crowding

Researchers conducted experiments across various settings, including laboratory tanks, small-scale cages, and commercial farms.

Crowding experiments began with small fish in controlled conditions and gradually scaled up to include larger fish, increased stocking densities, and longer crowding durations. During these trials, fish were enclosed within a net that was progressively tightened, replicating real-world conditions before delousing or slaughter.

The results showed that between 10% and 40% of motile lice detached during crowding.

• A higher proportion of small, semi-adult lice were released compared to fully grown adults.
• Adult females exhibited the strongest grip and were the least likely to detach.
• The longer the crowding lasted, the more lice were shed.

Why Do Smaller Lice Detach More Easily?

“We don’t know for certain,” says Oppedal, “but one theory is that adult lice occupy the most secure spots on the salmon, such as behind the dorsal and anal fins, where they are better protected. Additionally, their larger size may give them a stronger grip.”

Departmental engineer Adele Dahlgren at HI measures and weighs salmon before lice counting. Photo: Frode Oppedal / HI

Lice Survival and Reattachment

To determine how long lice can survive without a host, researchers placed them in containers with a constant supply of fresh seawater.

• Semi-adult lice survived for 5 to 13 days.
• Adult males lasted 7 to 12 days.
• Adult females endured the longest, 7 to 25 days.

Temperature significantly affected survival. Lice persisted longest in cold seawater but fared worse in low-salinity environments—such as shallow fjords.

<-- Test setup to see which mesh sizes prevent sea lice from passing through. Photo: Frode Oppedal / HI

When researchers placed surviving lice onto new salmon hosts, over half successfully reattached and were alive the next day. In additional tests where lice were released into tanks with salmon, many of them also latched onto new fish.

Previous studies confirm that salmon lice, particularly adult males, frequently switch hosts in fish farms.

“If the lice reattached the same day they detached, nearly all of them survived. This highlights a high potential for reinfestation,” warns Oppedal.

Effective Lice Collection with Fine-Mesh Nets

The good news? Almost all detached lice can be collected using a fine-mesh net.

To determine the optimal mesh size, researchers carefully measured lice dimensions. While many studies focus on lice length, their width was found to be the most crucial factor.

• Semi-adult lice measure about 1.7 mm wide.
• Adult female lice are about 4.3 mm wide.
• A 1 x 1 mm mesh net can effectively trap most detached lice.

Additionally, fine-mesh nets are gentler on salmon, reducing injuries to fins, skin, and scales. However, such nets also reduce water flow, making it harder to maintain sufficient oxygen levels during trapping.

Lice Should Be Collected and Destroyed

“All lice that detach during crowding should be collected and destroyed,” emphasizes Oppedal.

Many lice are carried into well boats via water flow. Onboard filtration systems must effectively remove lice before water enters the delousing or transport system.

Collecting lice significantly reduces the risk of reinfection within the same cage, nearby cages, and even wild fish populations.

How Far Do Salmon Lice Spread?

Using current modeling data from 13 fish farms, researchers studied how detached lice disperse.

They examined facilities with:

1. Submerged cages
2. Weak water currents
3. Strong water currents

Key findings:

• Lice typically spread 200 to 2,000 meters from their original location.
• Their sinking rate—0.5 to 1.5 cm per second—limits how far they can drift before reaching the seabed.
• Most infestations occur within the same farm rather than spreading to distant sites.

Do Salmon Lice Hitchhike on Wild Fish?

Some fish farmers report lice outbreaks shortly after nearby farms conduct delousing, even when separated by over two kilometers.

Could lice drift farther than expected? Oppedal doubts it.

“The lice sink too fast for that,” he explains. “However, wild fish—such as saithe—have been observed carrying salmon lice. While lice don’t thrive on these species long-term, they might temporarily hitchhike and be transported to new farms. This is just a hypothesis for now, but it’s something we should investigate further.”

Fine-meshed orca seine with plankton net on the outside. The orca seine is usually used when a portion of the fish is to be removed from a cage. The plankton net can collect moving lice that fall off. Photo: Frode Oppedal / HI

Scuttle Lice Also Detach During Crowding

In addition to salmon lice, researchers studied scuttle lice, a related but smaller species that infests multiple fish types.

Key findings:

• 20% of scuttle lice detached during crowding.
• Unlike salmon lice, they do not sink, meaning they may spread farther in the water column.

About the Project

This research is a collaboration between the Institute of Marine Research (HI), the University of Bergen (UiB), and Deakin University in Australia.

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

This article was originally published in Norwegian by Øystein Rygg Haanæs of the Institute of Marine Research (Havforskningsinstituttet)

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