Sealice 101
Below is an article originally published in 2005. The sea lice debate still rages on in British Columbia and other areas where salmon are farmed. More will be discussed in future postings.
The Story of the Salmon Louse – originally published in BC Naturalist Magazine in 2005
Sea lice are a marine parasite of Pacific salmon. In British Columbia, two species infect salmon at measurable levels. Caligus clemensii, a species that infects numerous fish in coastal marine waters including salmon, sticklebacks, and herring, and Lepeoptheirus salmonis, a more specialized parasite that is essentially only a parasite of salmonids. Lepeoptheirus salmonis, or the salmon louse, is three times the size of Caligus clemensii and has more pronounced effects on salmon. Sea lice have a complex life cycle that consists of five phases (free living, infective, attached feeding, and mobile feeding (pre-adult and adult) and numerous developmental stages.
One of the key characteristics of the relationship between parasites and hosts is the state of dynamic balance that exists between them. Without this balance, parasites especially ones with lethal effects would drive their hosts and themselves to extinction. Nature has created this balance in the form of a never-ending dance. The dance results in a see-saw battle between the hosts trying to defend themselves against the parasites and the parasites trying to overcome the defenses of the host.
Sea lice begin their lives as free-living nauplii popping into existence like a popcorn kernal off of an eggstring protruding from the back of their mother. Depending on temperature free-living lice larvae moult over the next 8-14 days before becoming infective and seeking a host using a variety of light and chemical cues. Once salmon lice land on a salmon they insert an anchor, which holds them firmly attached to the salmon. The attached lice then begins feeding and developing through their developmental stages completing the lifecycle as mobile adult lice in approximately 30-40 days depending on temperature. A female salmon louse will live as long as eight months and produce up to ten pairs of eggstrings.
In the case of the salmon louse, they are picky about their host as they need a salmon in order to grow into full maturity and reproduce. Imagine how unlikely it would be for a sea lice larvae no bigger than a sesame seed to first land on a fish and then for it to be a salmon, yet this is the strategy that salmon lice employ and have been successful with over time. The most likely reason for the specific preference of the salmon louse is that it gives them a competitive advantage over other generalist species like Caligus clemensii.
The picture of how salmon lice interact with salmon is not complete. Anyone who likes to fish for salmon will know that it is not unusual to catch an adult salmon with salmon lice on it. Studies have shown that adult salmon are infected with salmon lice in the offshore regions and bring that infection to coastal marine areas when they return to spawn. However, sea lice are killed when adult salmon enter the rivers as they cannot survive in freshwater. Thus, the salmon louse has not traditionally had an overwintering host with the exception of a few overwintering coho and/or chinook salmon.
In contrast to the adult salmon, the question of what “normal” salmon lice levels are for young salmon is not well understood. Young salmon begin their ocean lives undertaking a spectacular migration through the coastal marine areas of British Columbia and out to the continental shelf. In the case of young pink and chum salmon, they enter coastal marine areas no bigger than a house key.
Previous to 2001, an outbreak of salmon lice had never been recorded on young pink and chum salmon. In 2001, the first of four salmon lice outbreaks over a five-year period was recorded in the Broughton Archipelago, an area home to the highest density of salmon farms on the B.C. coast. In the middle of these outbreaks pink salmon returns in the Broughton Archipelago experienced a dramatic decline that was unique to the area. This sequence of events touched off a firestorm of debate over whether or not salmon lice levels enhanced by salmon farms may be affecting the health of wild salmon populations.
How do salmon farms artificially enhance the natural levels of salmon lice and affect the natural balance between salmon lice and young salmon? There are several reasons. First, salmon farms represent the perfect habitat for salmon lice that has not been previously available to salmon lice, especially over the winter. Salmon farms contain a high-density population of relatively stationary hosts that are generally under higher stress levels. Higher stress equals a greater chance of being infected by disease or parasites.
Second, salmon farms use open-net pens in their operations. These large nets, which hold anywhere from 30,000 to 100,000 salmon depending on lifestage, allow a free flow interaction between the farm environment and the surrounding marine ecosystem. Thus when adult Pacific salmon return to spawn in coastal marine areas it is likely that they will infect the salmon farms with salmon lice as they pass them on their way to their spawning rivers. Similarly, if a salmon lice infection on a farm persists throughout the winter then it will expose young migrating salmon to extremely high salmon lice levels as they pass by the farm on their way out to sea. One study from Norway found that a given farm could introduce over two billion lice larvae to the environment with a lice per farm fish level as low as three. Given their small size during migration, it is likely that young pink and chum salmon are particularly vulnerable to enhanced levels of salmon lice.
In British Columbia, the debate over the impacts of salmon farming rages on while low returns of pink salmon continue to show up in the Broughton Archipelago. While both the federal and provincial governments are preaching that everything is fine, it is clear that they care more about maintaining their legitimacy and getting re-elected than they do about addressing environmental issues. Now they are pushing ahead with the expansion of salmon farms despite the uncalculated risks posed to wild salmon through enhanced parasites and disease created by salmon farming. Every country that has had salmon farms has had serious problems with salmon lice both on the farmed salmon and the wild salmon migrating nearby. The difference between those countries and British Columbia is that we have significant populations of wild salmon to lose.