Cute, colourful and not that tasty – but the cleaner fish could save salmon farmers millions

What’s for dinner? Salmon parasites. Stirling Institute of Aquaculture, Author provided

A stunning little fish that eats salmon parasites could revolutionise aquaculture by providing an eco-friendly alternative to fish medicines.

The Atlantic salmon (Salmo salar) is one of the world’s favourite fish, and there’s big money to be made farming it – Scotland alone exports £400m worth per year. As in most agrifood sectors, the big rewards on offer have caused the industry to consolidate into fewer, larger players. It’s also lead to major technological breakthroughs and most of the big salmon farming challenges have now been addressed, except one: sea lice.

Sea lice are small crustacean parasites that live on salmon, damaging their scales, reducing growth and helping diseases spread. The industry in Scotland is spending more than £30m per year on mitigation with approved veterinary medicines being a core element of parasite control.

A salmon infected with sea lice. PBS, CC BY-NC-SA

There is a better way. My research group, in collaboration with leading salmon farming companies, has focused on an alternative “greener” approach – the use of cleaner fish, especially ballan wrasse.

Wrasse make great cleaners

Controlling parasites with a biological agent – the cleaner fish – would be unique among livestock farmed for human food, but there are good reasons to think these wrasse are up to the task as we’ve known about their cleaning powers since the 1970s. Previously evidence was mainly anecdotal however, based on observing what happened when wild cleaner fish broke into salmon cages.

If we want scale things up, to harness these powers to make fishing more sustainable then the challenge now is to farm the cleaner fish themselves. This should protect numbers in the wild as well as being a safer option for salmon farmers – farmed wrasse would be disease-free, with a standardised size and robustness.

However, we first had to determine farming (domestication) would not mean these fish stopped going after the sea lice. Would a farmed wrasse still be interested in foraging on sea lice if it had never seen one during the two years spent in fish tanks and fed on pellets?

Two Stirling colleagues and I recently published a study confirming the strong innate delousing behaviour of farmed ballan wrasse. Surprisingly, even though the wrasse were each able to eat up to 175 lice per day we didn’t find their hunger levelled off. The fish enjoy eating parasites, but they still don’t feel full. Supplementary feed is still needed.

Young farmed ballan wrasse, before they’re let loose on the sea lice. Stirling Institute of Aquaculture, Author provided

We also found size did not really matter, larger fish not being more effective than smaller ones. This means we can figure out the optimum wrasse deployment – numbers and size relative to salmon and sea pens and so on – and it also means the wrasse can be fed while cohabiting with salmon.

A different kind of aquaculture fish

Ballan wrasse aren’t the tastiest fish, though they are often used in the French “bouillabaisse” soup. However the species does have a number of striking features – and not just its looks.

Their reproduction strategy is unusual. All ballan wrasse are born females and then undergo a sex inversion into males at a later stage of their life cycle (a phenomenon known as hermaphroditism protogynous). Their behaviour can be odd: they truly sleep at night which is not common in fish, the younger fish have a habit of grouping together in a ball, and of course they enjoy delousing. The fish also have no stomach, and a bile pigment causes their plasma to be blue. All this means scientists find ballan wrasse fascinating but it doesn’t make farming them any easier.

In marine aquaculture, the first stages of life are generally the toughest to get right as the supply of eggs can be unreliable and lots of fish die off before they are fully developed.

But things are improving. We can now better manage breeders and optimise egg production and fish survival, thanks to a range of tools developed to identify gender and manipulate sex on demand if required, assess reproductive conditions, remove the stickiness from the newly spawned eggs which simplifies the disinfection from potential pathogens, synchronise hatching and successfully produce robust fish in the hatchery. All of these resulted in the first commercial batches of farmed ballan wrasse in the hatchery and significant numbers of fish deployed to salmon farms.

Over the past few years we’ve made significant progress and the industry has even completed full 16-18 month salmon production cycles at sea without using any medicine to control sea lice. We’ve just been given a grant to continue our research so look out for further developments. It’s clear that cleaner fish really can be an effective way to keep salmon clean and healthy.

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