Currently, one-third of the world population depends on fish as their main source of protein. With salmon being the most marketed. Meeting the demands of the world’s population has required an expansion in salmon fish farms.
It’s not uncommon for a fish farm to regularly experience sea lice outbreaks that make their products unmarketable. With concern for the marine ecology, fixing the outbreaks become more difficult. Further, sea lice can and have become resistant to the treatments.
The family name for sea lice is copepod crustaceans. Having been around for millions of years, they have been feeding on salmon’s skin and blood to survive. When they are in their free-swimming larval phase, they need to find and attach to a fish host in order to survive.
The larval are less than a millimeter long, and in the wild, finding a host is difficult. This makes for a difficult life for them. But that all changes with the fish farms, as the fish are kept in unnaturally high densities. Consequently, the sea lice exploit the situation and their lives become easy. And just like with people in high densities, the disease spreads much more quickly.
Commercial catastrophic damage can be caused by sea lice even though they are small. This is because infected fish can’t be sold with the lesions caused by parasites. The fish are also at risk, and in extreme cases, the entire farm can be wiped out from an infestation.
Unfortunately, the most common treatment for salmon with sea lice is harsh chemicals. While this effectively manages the outbreaks, it causes a reduced appetite and growth in the fish. Another downside to using this treatment is, fish can’t be sold for several weeks after the treatment. Further over time, the sea lice have built up a resistance to the chemicals being used.
A few farms are starting to use a cleaner fish such as the corkwing wrasse. This fish feeds on the lice and are a less expensive aid to the end an infestation. The downside to using the corkwing wrasse is that it only appears to be effective in some areas.
Another option that is a preventative method is snorkel nets. By keeping the fish in underwater roofed spaces, there has been a reduction in infestations. The primary reason is lice larvae are mostly found near the surface of the water. The problem with this method is, it isn’t possible to keep the fish permanently submerged.
Salmon rely on trips to the surface to remain buoyant since they physostomes. If they don’t make regular trips to the surface, they will have a reduction in buoyancy, and this will reduce the salmon’s growth and appetite.
In the wild, salmon lice did occur before fish farms began, and while mass mortality happens in the wild, it is extremely rare.
Containing farm infestations is difficult and puts wild species at risk. In order to breed, salmon travel between saltwater and freshwater. At times, they migrate past farms to do so, and this gives the infected larvae an opportunity to transfer to the wild fish swimming by.