A fish farm located in a Norwegian river system

A fish farm in Nord-Trøndelag, Norway © Kommunikasjonsavdelingen NTFK/ Wikimedia Commons (CC BY 2.0)

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The problem of sea lice in salmon farms

One third of the world's population relies on fish as a source of protein. Expanding fish farming is one way to meet the demand of the world's growing population.

Fish farms are known to experience outbreaks of sea lice that make their products unmarketable. But fixing the problem can cause damage to marine ecology, and lice can evolve resistance to the treatments. 

Prof Geoff Boxshall, a researcher of copepod crustaceans, explains the threats faced by salmon farms and the marine environment around them, and how Museum research has aided in troubleshooting the solutions.

A parasite problem

Salmon farms across the world are suffering from infestations of Lepeophtheirus salmonis, a sea louse that targets salmonids (the family of fish that includes salmon).

Sea lice are copepod crustaceans. They have been around for millions of years and have adapted to live on salmon, feeding on the fishes' skin and blood to survive. The lice have a short, free-swimming larval phase, when they need to find and attach to a fish host.

L. salmonis specimens from the Museum's collection preserved in liquid

'The infective larvae are less than a millimetre long, so in the wild finding a host is a difficult part of the life cycle,' says Geoff. 'But in aquaculture, fish are kept at unnaturally high densities, so the parasites will exploit that, and their lives become easy.

'Much like with humans, high densities mean it is easier for diseases to be transmitted.'

Although they are small, the lice can cause catastrophic damage commercially. Infected fish can't be sold due to the lesions the parasites cause. But the health of the fish is also at risk. In extreme cases, an infestation can cause mass mortality.

A chemical cleanse

Currently the most common treatment for salmon lice is harsh chemicals.

Although this treatment has been effective in managing sea lice outbreaks, it has negative effects on the fish, reducing appetite and growth. Salmon also can't be sold for several weeks after a treatment.

Over time the salmon lice have built up a resistance to the three major classes of chemical being used.

Geoff says, 'This is a big problem for the industry. It's technology versus evolution, but evolution is always going to win.'

Hydrogen peroxide has also been tested as a treatment, with limited success.

'The margin between killing the lice and killing the fish is tiny,' explains Geoff. 'So it's not really a very effective method.'

Three moult stages of the salmon louse © Thomas Bjørkan/ Wikimedia Commons (CC BY-SA 3.0) 

Growing complications

Being crustaceans, salmon lice must shed their exoskeleton to grow. So as a last resort, a treatment that halts this moulting cycle may be prescribed.

'If you're going to use a moult inhibitor, you need to know the right number of moults and the best time to treat the salmon,' says Geoff.

Before research was completed at the Museum and by external collaborators, the lice were thought go through more moults than is the case.

'The industry then had to rejig how they applied the treatments because they weren't getting the timings right,' Geoff explains.

Although moult inhibitors appear to be an ideal solution to the salmon lice problem as it doesn't negatively affect the fish, it is rarely used.

'The problem is you can't just pump a moult inhibitor into ocean farms on a large scale, because there are shrimp, lobsters and other crustaceans that would be negatively affected by it, too.'

A natural solution

Some farms are turning to the use of cleaner fish, such as wrasse, to help combat insufficient treatment plans. These fish feed on the lice and are therefore a natural, less expensive aid to end the infestations.

Corkwing wrasse (Symphodus melops) are one of the species being used as cleaner fish in aquaculture © Ben Thompson Via Flickr (CC BY-NC 2.0

Although these fish help, farms still need to treat their stock with other methods to fully control the lice. But by reducing the number of chemical treatments, the knock-on effects (such as resistance) are reduced.

Unfortunately, wrasse appear to only be effective in some areas.

'Bizarrely, the cleaner fish work better in Norway than they do in Scotland. Wrasse are unusual fish as they semi-hibernate in the winter. It might have something to do with that behaviour or their tolerated temperature range, but really it's all guesses as to what is the actual cause.'

Snorkelling salmon

A less ecologically harmful, preventative method for combatting salmon lice infestations is snorkel nets.

Keeping the fish in underwater, roofed spaces has been found to reduce infestations. Lice larvae are mostly found near the surface of the water, so keeping the salmon in deeper water lessens the opportunities for contamination. However, it isn't possible to keep the fish permanently submerged.

Atlantic salmon are widely farmed to supply the growing demand of the world's population © Hans-Petter Fjeld/ Wikimedia Commons (CC BY-SA 2.5)

Salmon are physostomes - they rely on trips to the surface to refill their swim bladder with air, otherwise they lose buoyancy. A reduction in buoyancy can lessen the salmon's appetite and growth.

To avoid this, the nets are equipped with a snorkel-like tube that allows the salmon to reach the surface to refill their swim bladder before diving back down. This temporary surface activity could reduce the potential contact between farmed salmon and the infective larvae, although it will not completely eradicate the pest.

Oceans at risk

Salmon lice occurred naturally in the wild before aquaculture began. Although mass mortality events caused by the parasites do happen outside of captivity, they are extremely rare in nature.

'It is much more a feature of aquaculture, but it really shouldn't happen these days because of strict regulations,' says Geoff.

Farm infestations cannot always be contained, however, putting wild species at risk. Salmon travel between salt water and freshwater to breed, and some have to migrate past farms to do so. This gives the infective larvae an opportunity to transfer from the captive salmon's nets to the wild fish swimming by.

Fish that escape the farms can also cause problems in the marine ecosystem.

Fish farms located in waters that salmon migrate through put the wild fish at risk of infection © Ximonis (Simo Räsänen)/ Wikimedia Commons (CC BY-SA 4.0)

'Escapes are a bad thing, because it is going to have an impact on wild fish through interbreeding. The escapees are often a different genetic stock to those in the wild, and interbreeding can cause a genetic weakening.

'Additionally, salmon are cold-water fish. None naturally live around South America, but farms in Chile have had some big escapes that resulted in several million salmon, which are top predators, cruising up and down the coast.'

Although the mortality rate of the escapee fish would have been high, as they were not adapted to the local conditions, large scale escapes will impact the ecology of the local coastal waters.

The rise of farming technology

European salmon farming only really took off in the 1970s, but in some ways has been quicker to adapt to difficult circumstances than land-based farming because of modern technology.

Geoff explains, 'Animal husbandry in terrestrial agriculture has been going on for millennia and we still have problems with diseases. So what's already been achieved with salmon farming is amazing.

'Even though these little parasites continue to come back and cause problems, overall, modern technology has been applied very effectively.'