Sea Lice Affect Wild Salmon
In both Scotland and Canada Parasites Harm Salmon and Sea Trout
Fish Farms in Estuaries give Parasitic Copepods to Wild Fish as they Migrate.
Sea Lice are small parasitic crustaceans (Copepods) which affect many marine fish. Two species (Lepeophtheirus salmonis and Caligus elongatus) are known to have damaging effects on wild populations of Sea Trout and Salmon. Recent research focuses on the problem in Scotland where Sea Trout are declining, and also in Canada where wild salmon species are under threat.
‘Open Net Cage’ Salmon Farming
Fish Farms situated in estuarine waters seem to be implicated in the decline of wild fish in both places (Scotland and Canada). The problem is that the dense stocking of farmed salmon in ‘open net cages’ allows the numbers of sea lice to increase dramatically. In natural conditions the fish are much more widely distributed and the parasites, while present, do not usually cause major problems. Young wild salmon and sea trout hatch in freshwater streams and then pass down through the estuaries to reach the sea. As they pass the salmon farms these young fish ‘pick up’ the parasites, which are thought to cause them serious problems.
DNA Studies
When a wild fish is captured and found to be carrying sea lice it is difficult to be certain where these parasites came from. They might be from the ‘natural’ pool of parasites in the open sea, or possibly from one of the ‘unnatural’ populations that flourish around fish farms. Recent studies in Scotland focussed on the ‘Genetic Fingerprinting’ of Lepeophtheirus salmonis attached to wild sea trout and salmon as they returned to freshwater to breed. This work shows that many of the sea lice came from the populations that surround fish farms.
Young Fish
Sea lice cannot live long in freshwater, they simply drop off adult fish as they return to spawn. This implies that any sea lice that can be shown to have come from a population found at a fish farm have been attached to the fish in question since it passed through the estuary as a young fish (or are descendants of such a sea louse). This is particularly important since small, young fish are damaged more severely than the adults. A heavy infestation might kill a weak young fish directly, but even a small number of parasites can cause ‘leisions’ which allow contact between the blood and sea water. When this happens the fish have great difficulty with their ‘water balance’ and weaken rapidly.
Lice
The term ‘louse’ is not very precise, it is rather like ‘bug’ and can refer to many different types of invertebrate animals. (The Sea Lice discussed here are neither Fish Lice nor ‘lice’).
See also: Sea Lice and Salmon
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