Cleaner fish: a role in question

Wrasse and lumpfish – called collectively “cleaner fish” – are effective “pickers” that help keep salmon in sea pens clean of parasitic sea lice, reducing the need for physical, chemotherapeutic or thermal delousing treatments.

 

Ballan wrasse (Labrus bergylta) are either wild-caught and then deployed into salmon sea pens, or bred and reared in specialist onshore facilities. 

 

Lumpfish (Cyclopterus lumpus), also known as lumpsuckers, are typically reared in specialist facilities. Lumpfish eggs may be collected from the wild, or broodstock are captured from the wild or kept at hatcheries.

 

How much the Scottish salmon farming industry uses cleaner fish was revealed by the Scottish Animal Welfare Commission, which gives scientific and ethical advice to the Scottish Government, when it published its report on cleaner fish welfare last November.

 

One section, titled “Numbers of cleaner fish deployed and losses on Scottish salmon farms”, began with the trade body Salmon Scotland’s calculation that “two million lumpfish and one million wrasses are used in salmon farming”. 

 

But use is falling. A further recent study, titled “The rise and fall of cleaner fish use in Norwegian salmon farming”, found that: “Cleaner fish use (wild-caught wrasse, and farmed lumpfish and ballan wrasse) peaked in 2019, but has since declined.”

 

The latter paper, published in this year’s issue of Aquaculture Environment Interactions, reads: “Using cleaner fish to remove parasitic lice from farmed salmonids was heralded as an effective, environmentally sound method. 

 

“However, we contend that the use of cleaner fish was adopted too rapidly and lacked sound evidence on their efficacy and welfare at commercial scale.”

 

It set out possible causes. “Four main factors likely contributed to the decline,” it said: “(1) uncertainty surrounding efficacy; (2) poor welfare and high mortality rates; (3) availability of new preventative and control methods; and (4) changes in regulations and enforcement.

 

“Since 1998, over half a billion cleaner fish have been deployed in sea-cages in Norway, with experimental and farmer-reported evidence indicating consistently poor welfare.”

 

The paper went further, making recommendations. “Based on uncertain efficacy, poor welfare, and our assessment that improvements to cleaner fish use will not reduce mortality rates to the Norwegian government’s targets, we recommend that the use of lumpfish and wild-caught wrasse be phased out,” it urged. 

 

“Furthermore, as farmed ballan wrasse are regarded by some farmers as effective delousers, we recommend that the Norwegian salmon industry secure their welfare, reduce mortality levels to 5%, and demonstrate efficacy through multiple commercial-scale trials by 2029, or discontinue their use.”

 

Meanwhile back in Scotland, the SAWC report looked deep into cleaner fish’s effectiveness. “Cleaner fish are seen by many in the salmon farming industry as the first line of defence against sea lice,” it said. 

 

“However, there may be instances when the level of sea louse infestations can ‘overwhelm’ the ability of cleaner fish to remove the lice. At this point, some other treatment would be necessary.

 

“A review reported a wide range of efficacies of sea louse removal by cleaner fish from a 28% increase to a 100% reduction in sea louse numbers. Higher efficacies were reported for ballan wrasses, with lumpfish being more variable in their success. 

 

“A study using data from national louse counts, delousing treatments and cleaner fish stocking events from Norwegian salmon farms (which largely used lumpfish) showed that the use of cleaner fish reduced the lice populations only by a small amount, but their use delayed the use of other treatments.

 

“In comparison, a Scottish study showed that the efficacy of cleaner fish in reducing sea lice numbers was rated by experts as better than some other methods, such as in-feed medicine or hydrogen peroxide treatments, but lower than for Hydrolicer or Thermolicer treatments.

 

“A combination of methods, including the use of cleaner fish, was considered to be the most effective with efficacies reported to reach 100%.

 

“The use of cleaner fish as a sea louse control method has a lower impact on the welfare of the salmon. Negative impacts of cleaner fish on salmon are not reported, but significant salmon mortality can be observed when thermal, physical or chemical treatments are used.”

 

The welfare of the cleaner fish themselves was the subject of the study, following concerns. Concluding its inquiry into salmon farming early in 2025, Holyrood’s Rural Affairs and Islands Committee had called for more comprehensive, consistent and transparent mortality figures, including cleaner fish.

 

On mortality figures, the SAWC report said one company told them: “Overall survival is 78.6% across the cycle (compared to 82% for salmon)”, while “other companies state that losses can be high, but many do not have the figures available”.

 

“High levels of loss have also been reported in research projects, (eg, 57% for ballan wrasse and 27% for lumpfish),” it added.

 

“The loss of cleaner fish is one of the biggest concerns of the salmon companies and of welfare NGOs. It is not clear what causes the losses or disappearance. Predation by the salmon, mortality due to handling, escapes or disease are possible causes.”