Why and where are sea lice believed to be serious parasites of sea trout?

Sea lice (mainly the salmon louse, Lepeophtheirus salmonis (L.)) are naturally occurring and widespread skin parasites of salmon and sea trout in their marine phases. Other lice (Caligus spp.) also can be found on sea trout and salmon at sea, but are more common on other fish species. At the low background levels of infection found in areas away from marine salmon culture, Leps. cause sea trout few problems.  However, coastal rearing of salmon in net cages leads to a sharp increase in the potential for louse infestation. Given that these parasites are extremely well-adapted to finding and attaching to their normally relatively scarce salmonid hosts, it is unsurprising to find that in areas of intensive salmon culture louse infection levels can become very heavy on coastal sea trout post-smolts.  [Wild salmon post-smolts are vulnerable too, less so those than can migrate out quickly to the open sea.]  Sadly, recent history, in Scotland Ireland and Norway, has shown that marine mortality levels soon escalate among sea trout that migrate into bays or sea lochs containing salmon farms and their spawning stocks may collapse.  Control of sea lice on the farms has improved lately, but is still largely limited to chemical treatment within the cages and the use of fallow periods after the salmon reach a marketable size in order to limit opportunity for cross-infection between cage sites and age groups.  Inevitably, there is a tendency for the lice to develop immunity to the few lice removing chemicals that are sanctioned for use.  In recent years, better control of sea lice has been achieved through the use of SLICE, but this may be only a temporary success as resistance to the treatment has already been detected. Under the present prevalent rearing regime involving open cages, there is an urgent need for new chemicals to be found regularly, their efficacy and safe use examined and authorised.  [In the longer term, through local and international agreements, commercial salmon culture may have to move to totally enclosed onshore rearing facilities].