The occurrence of Sea Lice (Lepeophtheirus salmonis Krøyer) on Farmed Salmon in Ireland (1995 to 2000)

Abstract

Lepeophtheirus salmonis is the most frequently recorded ecto-parasite on farmed salmon in Europe, and parasitises only salmonid fish species. It is regarded as being commercially damaging to farmed salmon, with major economic losses to the fish farming community resulting per annum. Lepeophtheirus salmonis is a member of the Family Caligidae and has a direct life cycle. Annual data from around Ireland are analysed, as well as per region and per bay. Data is compiled up to the year 2000 and results are based on lice inspections undertaken bimonthly for the months March to May inclusive, and monthly for the remainder of the year, with one exception, December/January, when only one sample was taken. Mean ovigerous and mean mobile lice levels are presented. These estimate, respectively, successful breeding females and successful infection. Results obtained indicate, to some extent, that control methods on different farms differ in the efficacy they have on sea lice infestation, and that depending on which treatment type is used rates of reduction can be different for various life cycle stages. Overall mean ovigerous and mean mobile lice levels were lower in the year 2000 than in 1999. It was apparent that lateral transfer of sea lice during harvesting did occur at a number of sites in the country. It was also apparent that some individual bays appeared to have a greater control over lice infestation levels than others, especially during the critical spring period March to May. The decrease in the control of infestation levels can possibly be attributed to changes in treatments that occurred during the study period, and also to difficulties in achieving effective treatments due to inclement weather and low water temperatures. Since the initiation of monitoring in 1991, improved control of sea lice infestation has always been one of the goals of the programme. Single Bay Management (SBM), introduced in 1993 by the Marine Institute, implemented new measures to minimise re-infection by these parasites, with protocols agreed by all salmon producers within each bay. These plans were later extended and incorporated in 1998 into the Co-ordinated Local Aquaculture Management System (CLAMS), aiming to optimise environmental conditions within each bay for all users of the bay.