Salmon are a Sink and Source of Infestation

Salmonlouse by Thomas Bjørkan CC BY-SA 3.0 


The Price of Lice

Are salmon lice even a real thing? 

Heck yeah! Salmon lice are a biblical plague on farmed fish. Salmon lice are marine crustaceans, smaller than a fingernail. They have a shield-shaped body and the females bear two long egg strands. Their mouth closely resembles a hypodermic needle, which they use for puncturing the skin and sucking blood and juices. They literally bite onto the fish and eat the fish alive. In severe infestations, they can strip much of the skin and flesh off a fish's face, fins, and genital area, such as in the photos herehere, and here.

These lice are not related to head lice or pubic lice in humans; rather, they are a type of small marine crustacean called a copepod. Most species of copepods are free-living and provide a rich food source for pretty much everything bigger than a copepod, but these are voraciously parasitic. It is not sensationalistic to call them salmon lice, in fact, their scientific name -- Lepeophtheirus salmonis -- denotes the fact that they specialise in terrorising salmon. 

Salmon lice spread from fish to fish quite easily, so intensive farming is a dream-come-true for these parasites. They are a scourge in most regions where salmon farming occurs.


Do Tasmanian salmon have lice?

Yes and no. In Australia, salmon lice have been reported in salmon aquaculture in very low numbers and unique circumstances [1]. However, these species occur naturally in wild fish populations here. In fact, one species of fish lice, Caligus epidemicus, was originally discovered during an explosive infestation on bream and mullet in Victoria [2]. That infestation was associated with a period of high temperature and salinity, portending bad mojo for Tasmania's coastal salmon industry in a climate changing world. 

There is a profound risk that fish lice will again 'jump' to farmed salmon, where they can proliferate in the crowded pens, then these farms can act as reservoirs for parasites and pestilence affecting native species. This is not speculative: this is how it usually happens. 

Fish lice have already jumped from native fish to farmed salmon in Tasmania at least once [1], so it is now just a numbers game as to when it next occurs. Climate warming, overcrowding, and expansion all serve to hasten it. Warmer water makes the fish more stressed and therefore more vulnerable, and the parasites more numerous, and overcrowding makes it easier for the parasites to transfer from fish to fish. Expansion just means more fish for the lice to parasitise. Watch this space, we have created the perfect context for a plague. 


Are salmon lice dangerous to humans?

No. There is no evidence that humans are harmed by lice on fish. The lice typically fall off or are washed off during processing. But perhaps the thought of your food being eaten alive by blood-sucking parasites is unappetising: when you buy a fillet at the supermarket, you have no way of telling what condition the fish was in when it was harvested. 


Are salmon lice dangerous to native fish?

The short answer is yes. Normally in nature, lice stay in fairly low numbers because an infected fish may not be close enough to other fish for the larvae to find suitable hosts. But industrial salmon farming packs tens of thousands of fish into each net, offering parasites and diseases an unnaturally high density breeding ground. Using COVID-19 as an analogy, think of it as the difference between social distancing and thousands of people in a crowded nightclub. Salmon farming enables lice to multiply into ongoing epidemics, which pose a significant threat to native fish. 

Besides the threat to fish caused by lice eating their tissues, it also appears that lice can be reservoirs and vectors of disease [1]. Moreover, a recent study also found that salmon infested with sea lice were more susceptible to infectious salmon anemia virus [3]. 


What can be done to get rid of the lice?

A range of harsh pesticides and medications have been used in salmon farming to combat the lice problem. However, over time the lice have developed resistance to the drugs and chemicals [4], meaning that they are less effective and ever-greater quantities need to be used. "The margin between killing the lice and killing the fish is tiny," explains a salmon lice expert at the Natural History Museum in London [5]. This is an arms race that will not go well for humans. 

Where we discuss parasites, we must also talk about chemicals in the environment. Just like locusts in a corn farmer's field, or fleas on Fido or worms in Fluffy, or cancer growing in our own body, we always turn to chemical eradication of the pests that bug us. In open-cage sea farms, a significant percentage of these chemicals end up in our waterways. And just like with antibiotic resistance that now threatens the foundation of our medical treatments, no matter how much poison you throw at an infestation, often a few of the heartiest individuals survive and become the breeders for the next generation [6]. This is how we inevitably travel the one-way journey from arms race to pest fest. 

The name of the game is to avoid infestation in the first place. In the face of uncertainty, the only safe option is to take it slow – the precautionary principle. But alas, with so much money to be made with intensive farming, moderation does not seem to be on the cards. The other alternative is to convert from the sea-based feedlots to land-based fully-recirculating facilities where they have more control, but salmon companies seem resistant to that too. 

When having to choose between parasites eating into the fish or moving on-land eating into the profits, one has to wonder, is industry just chasing the profits?



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