Inadequate scientific evidence of the effect of cleaner fish at full commercial scale

Many millions of cleaner fish are put into farm cages to eat salmon lice off salmon, but there is limited research into how efficiently they do this job. Our lab, working with the Institute of Marine Research, Norway, has gone through all published studies in the field – and found large knowledge gaps.

Lumpfish and several species of wrasse are widely used today as cleaner fish. In a new review study, we systematically analyzed published scientific studies that have tested the effectiveness of cleaner fish.

Very few representative studies

The review shows that:

• the reported effect of cleaner fish on the number of salmon lice on salmon in the different studies varied greatly,
• only 11 published studies had examined lice predation in experimental setups with and without cleaner fish and in general replication was low,
• almost all studies were done in small experimental cages or tanks and only one published study was performed in large, commercial-sized cages.

Taken together, the body of evidence is not representative of today’s use of cleaner fish by the industry, where up to 200000 salmon swim in a single cage of enormous volume and depth. Here, close contact between salmon and cleaner fish is not guaranteed, as it is in small scale tanks and cages with small volumes where promising results of the effect of cleaner fish are most often recorded.

Knowledge gaps must be closed

The study reveals knowledge gaps that should be filled with further research on various topics for all species used as cleaner fish.

First and foremost, cleaner fish must be offered an environment they can thrive and survive in. The focus should be on understanding what are the best environmental conditions and optimal densities, lice removal efficiency under different environments, and breeding of cleaner fish that are better suited for life in the cages.

The industry is gradually gaining experience with the use of cleaner fish in commercial cages, and several farms report good results from time to time. In the future, it is important that knowledge of when cleaner fish worked well, and when they did not work well, is developed and shared, and documented in scientific studies at representative scale in commercial cages. An evidence-based approach will help the industry improve most rapidly without the pitfalls of mis-steps.

There is a particular need for research on the effectiveness of several wild caught wrasse species, where many millions are used each year but there are very few studies to support their lice removal effects. Lumpfish, the species most commonly used, are also the species with the best scientifically documented effect, but here too the evidence of their effects is limited to a few locations. Research should be extended to different environments and conditions. There is also a knowledge gap for how cleaner fish work together with other preventative and control measures against lice.

Improving the effects of cleaner fish

More targeted, knowledge-based use of cleaner fish should increase their lice-eating effect, and lessen economic, sustainability and ethical concerns about their use. Increased welfare for cleaner fish will likely make them better lice-eaters, but it is also absolutely necessary for the use of cleaner fish to be able to be defended both legally and ethically.

Cleaner fish facts:

• Cleaner fish is a common name for lumpfish and several different wrasse species which are kept in sea cages with salmon to consume salmon lice.
• Approximately 50 million cleaner fish were used in 2018 in Norway, of which 18 million were wild caught and 31 million farmed. Globally, over 60 million cleaner fish are used each year by the salmon farming industry.
• According to the Institute of Marine Research’s 2019 risk assessment for salmon aquaculture, poor welfare and high mortality of cleaner fish is one of the biggest challenges in Norwegian aquaculture.
• Most lumpfish used are farmed, while most wrasse species are caught in traps in shallow water along the coast.