Sea Lice

The following has been kindly supplied by Professor Graham Shaw, Chairman of Save Our Sea Trout.

The Sea Louse Problem

The sea louse (Lepeoptheirus salmonis) is a natural parasite on salmonid fish (sea trout and salmon). A few sea lice on a wild sea trout or salmon used to be welcomed as a sign of a freshly run fish since lice cannot survive for more than a few days in fresh water. With the advent of sea cage fish farming, the presence of sea lice has a more sinister connotation in that unprecedented levels of sea lice infestation has accompanied the collapse of sea trout populations in areas where fish farms are present. This has been observed in Ireland , Scotland and Norway and, from these latter two countries, there is strong evidence that wild salmon populations have also been adversely affected. Heavily infested sea trout return prematurely to fresh water, perhaps in an effort to shed the lice which causes them severe head and fin damage and ultimately kills them.

It has been clearly established that sea lice infestation from fish farms is the primary cause of sea trout population collapses since sea trout populations will begin to recover if the farms are fallowed or if they control lice to very low levels during the critical March - May period while sea trout and salmon are migrating to sea. This cause and effect relationship has been accepted by the Norwegian Government and by the government's Environmental Protection Agencies in both Ireland and Scotland.

Lice infestation on salmon farms cannot be prevented since wild fish act as the initial source of infestation. Keeping massive numbers of farmed salmon in the close confinement associated with the intensive sea cage rearing process results in very rapid amplification and spread of infestation so that the farms become a reservoir of infestation and a source of re-infection of wild sea trout and salmon. It is a characteristic of the infestations on sea trout observed near fish farms that relatively freshly shed immature lice (chalimus) are the dominant infesting form.

Anglers are naturally concerned to ensure that farms control their lice infestation to a very high standard - effectively eliminating the mature egg-bearing (ovigerous) lice which are the source of these chalimus larvae.

Chemical Control

Chemical control is the most effective method of lice control and fish farmers now have access to several fully Authorised (Licensed) treatments. The National Licensing Authority is the Irish Medicines Board but the European Medicines Evaluation Agency (EMEA) may also issue Authorisations which are accepted throughout the E.U. Licensed products fall into the following categories:

I. Bath Treatments

Organophosphates
Dichlorvos (Nuvan, Aquaguard) is an organophosphorous insecticide which was the first treatment to be Licensed in Ireland. It is seldom used now because sea lice have developed resistance to it and because organophosphates are out of favour because of their potential toxicity to the nervous system. Organophosphates are toxic to other marine species, especially lobsters, but disperse fairly quickly. Azamethiphos (Salmosan) is a similar organophosphate which has been used for lice control.

Pyrethroids
Cypermethrin (Excis) is a pyrethroid insecticide of synthetic origin. It is specially formulated for use in sea water (in an emulsion form) and quickly disperses. However, it is also highly toxic to crustaceans and problems with lobster kills have arisen in the past in overseas fish farms where unauthorised products containing pyrethroids have been used instead of the Licensed product. Resistance to pyrethroids used as insecticides is well established.

Hydrogen Peroxide
Hydrogen peroxide (Salartect) is arguably the most environmentally suitable treatment since it degrades to form oxygen and water. It appears to have been little used in Ireland, possibly because of high cost and also because it needs to be used carefully as the difference between the dose which kills lice and that which is toxic to fish is not large.

II. In-Feed Treatments

Emamectin
Emamectin (Slice) is a member of the same family as ivermectin and is effective against all lice stages. It is not as persistent as ivermectin but still controls lice for 1-2 months after dosing. Ivermectin has never been the subject of a Product Licence for use in farmed salmon and its use is illegal.

Teflubenzuron
Teflubenzuron (Calicide) is a moult inhibitor which is only effective against the moulting juvenile stages of sea lice and not against adults.

Environmental / Health considerations

As part of the Product Authorisation process, data concerning the toxicity of the treatment to other marine species must be submitted and the Licensing Authority must consider this. The weakness of this process is that it relies on strict compliance with instructions for dosage and conditions of use.

An issue with in-feed treatments is that some unconsumed feed is liable to get beneath the cages where it can be scavenged by susceptible marine species. The issue as to whether it is a good idea to pour chemicals into the sea remains a matter of concern to environmentalists and others including some shellfish interests. Anglers see it as a necessary short-term evil pending the arrival of more sustainable methods of farming such as land-based operations. The development of resistance to the treatments is an ever-present threat which argues against the sustainability of sea cage farming.

For all approved treatments, maximum residue levels in the flesh of the farmed fish for human consumption have been set and a withholding time (time between drug treatment and slaughter) established. This again relies on compliance with instructions for use and it does not foresee the issue of treated fish escaping.

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Sea Lice