I’m obsessed with fish that clean other fish: they remember their clients, much like a hairdresser

I was in my 50s when I first became aware that cleaner fish existed, when I met a fisher who sold them to Scottish salmon farms. Each year, around the world, such farms use more than 60 million cleaner fish to eat – or “clean” – parasites off other fish. But the natural habitat of the cleaner fish is the reef.

On a reef, each cleaner fish has clients that visit them to have their parasites removed – sometimes much bigger fish or predators such as sharks and rays. I was intrigued to discover the cleaner fish would gently massage these clients with their fins and make sure they were comfortable.

A ballan wrasse swollen with eggs. The fish is used to remove sea lice from farmed salmon. Photograph: David Ainsley/Coastal Communities Network

Some fish seem to feel so pampered by the cleaner fish, they go back again and again for more – one Australian study has found that some will visit a cleaner fish up to 144 times a day, on average. There’s no way those fish are doing that just to get rid of parasites, so one can only assume they are also doing it for pleasure.

Sometimes, the cleaner fish will nudge their clients as if to say: this is what I want you to do. And then their clients – even huge fish such as the grouper – will happily spread their fins or open their mouths, and the cleaner fish will go inside, which I find extraordinary.

Grouper fish will even protect cleaner fish from predators by closing their mouth with the cleaner inside. Then the grouper will open its mouth just enough for the cleaner fish to escape into a hiding place.

A potato cod grouper with a bluestreak cleaner wrasse cleaning inside its mouth on the Great Barrier Reef, Queensland, Australia. Photograph: Nature Picture Library/Alamy

Cleaner fish will occasionally nip their clients, to get mucus from the fish, which we think has nutritional value. But they have to be careful, because they can lose potential clients who witness that behaviour.

Even though each cleaner fish can get about 2,000 clients a day, they are intelligent enough to differentiate between these clients, recognise them and remember their last interaction with them, much like a hairdresser. They tend to be more cooperative with clients near their home range, because they are more likely to encounter them again.

It is as if they are aware of their reputation and know that if they cheat somebody, they won’t come back. They also tend to give the predators more caresses than most clients.

Cleaner fish are increasingly sought after by salmon farms because they remove sea lice and are seen as the biological alternative to anti-parasitic chemical treatments. This has led to a rapid rise in their wild capture and subsequent release in salmon farms.

I’m campaigning for legal protection for these fish, because no one has done a proper assessment of the impact of removing cleaner fish from Scottish reefs or whether the reef ecology is going to survive if we keep removing them.

Melanie Watt is concerned at how ‘brave, highly intelligent creatures are being sacrificed’ to supply commercial salmon farms. Photograph: Lynne Kennedy/The Guardian

Also, many farms will kill their cleaner fish after a production cycle in the salmon farm, to stop the spread of infection. I find it upsetting that these brave, highly intelligent creatures, who are very social and do good work on the reef, are being sacrificed to improve the welfare of another fish that is being commercially produced. It’s a very wasteful and inhumane practice – and I hope one day to put a stop to it.