Sea louse

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For the thimble jellyfish larvae that cause itching in humans, see Seabather’s eruption.

Sea lice are copepods which parasitise fish. There are two species of sea lice commonly found on cultured salmonids, Caligus elongatus, a species of parasite that infests over fifty different species of marine fish, and Lepeophtheirus salmonis, which infests only salmon and closely related species such as rainbow trout (but current research indicates it may be spread via the three-spined stickleback). Both species are also found on wild salmon. There is widespread concern that sea lice flourishing on salmon farms can spread to nearby wild juvenile salmon and devastate these populations. Sea trout populations in recent years have seriously declined due to infestation by sea lice from salmon farms ^[1].

As sea lice develop from eggs to adults, they shed their exoskeletons in a series of moults. This creates a number of identifiable life stages. Sea lice in the first two stages are called nauplii. Nauplii can neither feed nor attach themselves to fish. In the next, copepodid, stage, the lice can attach themselves to fish. They then moult through four chalimus stages during which they are anchored to a host fish. As pre-adults (two stages) and adults (one, final stage), they can crawl about the host fish. It appears that they are most damaging to the host fish in these final, motile stages.

Sea lice have always been relatively common on adult salmon as they return to spawn. However, sea lice cannot tolerate fresh water, and drop off the fish as they encounter reduced salinities. Natural infection dynamics on wild salmon are not well understood. However, lice are typically very rare on juvenile salmon in areas far from fish farms. By contrast, relatively high abundances have repeatedly been reported on juvenile salmon that have migrated past fish farms. Evidence collected in both the Atlantic and Pacific Oceans indicates that these infestation levels can often be lethal. Smaller host fish appear to be particularly vulnerable. Hence, pink salmon (Oncorhynchus gorbuscha), which migrate out to sea immediately upon emergence from spawning gravel, are particularly at risk. Nonetheless, larger species, specifically including wild Atlantic salmon (Salmo salar) have been found to have lethal lice loads near fish farms. Sea lice are also economically damaging to the fish farms themselves; in one recent year, sea lice cost salmon farmers more than US$100 million for treatment and lost production, which represents about 20% of their total costs.

Currently, fish farmers rely heavily on the chemical emamectin benzoate (SLICE) for controlling sea louse infestation rates. Many governments now impose limits on sea louse infestation levels on fish farms. The adequacy of existing regulations, and the environmental impacts of the use of SLICE are highly controversial. Public opinion is particularly polarized in the northeast Pacific where a moratorium on expansion has been lifted by the British Columbia government while the Alaska government is maintaining a total ban on salmon farming in its waters.