Sea lice thrive poorly in environments with many fish available

In the new study from the Department of Biological Sciences at UiB, researchers recreated two types of environments: one resembling natural ocean conditions and one corresponding to the high fish densities seen in modern aquaculture facilities.

“Even though there were many salmon and plenty of food available, there were not more parasites per fish. The lice didn’t reproduce faster either. Those that survived to adulthood were actually smaller and had reduced reproductive output. This finding surprised us,” explains Associate Professor Adele Mennerat. She is leader of the research group behind the study published in Proceedings of the Royal Society B (external link).

Adaptable parasites

Mennerat is an evolutionary biologist who studies how species change through natural selection. Parasites are particularly interesting, she says, because they live short lives, reproduce rapidly, and evolve quickly when the environment changes. Antibiotic resistance is a well-known example of such rapid evolution.

Salmon lice are marine parasites that live on salmon and sea trout and are completely dependent on a host for nourishment. In fish farms, humans have created a new environment by gathering large numbers of animals at high density.

For parasites, this means an environment with abundant access to hosts—essentially, food. This makes salmon lice the biggest challenge for the aquaculture industry.

A method that makes it possible to study each louse’s life cycle

In the research project that began in 2021, researchers Alexius Folk and Adele Mennerat tracked individual salmon lice across multiple generations using a new tagging method.

Tiny sensor chips with unique numbers were attached to the lice using medical-grade superglue, similar to how birds are ring-tagged. The method makes it possible to follow each parasite throughout its entire life cycle.

By combining evolutionary theory with existing knowledge of salmon farming, the researchers investigated how aquaculture might influence the evolution of the parasites.

Evolution on speed

The researchers found that salmon lice from farmed areas had developed shorter life cycles and faster development than lice from wild populations.

This suggests that evolution has already taken place as a consequence of aquaculture, which has existed for more than 50 years. Salmon lice can have five or more generations per year, depending on sea temperature, so evolution happens quickly.

“It is true that more fish in farms lead to more salmon lice. But the picture may be more complex than we thought. The new study shows that female lice actually lay fewer eggs in environments with many fish and many larvae. One possible explanation is that the fish change their behavior or mount a stronger immune response when exposed to many larvae. At the same time, we must be cautious about generalizing from a single laboratory experiment,” says Mennerat.

Better equipped against the fish’s immune system?

The study also shows that more salmon lice die in high-density fish environments. This exposes the lice to strong selection pressure: only lice that can withstand the host’s immune response survive long enough to lay eggs.

“This may mean that the lice evolve traits that make them both more resistant to the fish’s immune system and potentially more harmful. This is important to investigate further. If true, it is concerning and could have consequences for both farmed and wild salmon,” Mennerat says.

Wild salmon are already heavily affected by salmon lice when they migrate out to sea as juveniles. A development toward more robust lice could worsen the problem. 

Need for an evolutionary perspective

Mennerat believes the study can contribute knowledge that can be used in models to estimate the effects of salmon lice from aquaculture.

The research also provides extremely rare and valuable information about parasites in general—a type of organism that is often very difficult to study at the individual level over time.

“Better understanding of which traits salmon lice are evolving could help us develop more targeted measures and models. This increases predictability for both the industry and the environment,” she concludes.