National Survey of the Sea Lice (Lepeophtheirus salmonis Krøyer and Caligus elongates Nordmann) on Fish Farms in Ireland - 2000

Abstract

Fanned fish can be divided into three distinct groups, rainbow trout and two year classes (or generations) of salmon. In terms of husbandry and lice management, salmon which are at sea for a year or longer in April (growers/one-sea winter) are treated separately from younger salmon (smolts) and rainbow trout. Those salmon that were put to sea in winter 1999/spring 2000 are referred to as smolts, or 2000 year class fish. The farms were inspected twice a month in March, April and May and once a month thereafter, with one exception, December/January where sites were visited only once. Two species of lice are commonly found on cultured salmonids, Caligus elongates Nordmann, a species of parasite that infests over fifty different species of marine fish, and Lepeophtheirus salmonis Krøyer, which infests only salmon and closely related species such as rainbow trout. Lepeophtheirus salmonis, the Salmon Louse, is regarded as the more serious of the two species and occurs most frequently on Irish cultivated salmon (Jackson and Minchin, 1992). Results for both species are given for each sampling period. These sea-lice inflict damage to their hosts through their feeding activity on the host's body (Jones et al., 1990; Jonsdottir et al., 1992; Kabata, 1974) and significant economic losses were attributed to these copepod ectoparasites by Roth et al. (1993). Lepeophtheirus salmonis is a member of the Family Caligidae and has a direct lifecycle (i.e. a single host). This life-cycle comprises ten stages. Following hatching from paired egg strings, two free-living nauplius stages are dispersed into the plankton. These stages are followed by a copepodid stage where contact with the host takes place. The copepodid then moults through four chalimus stages before becoming a pre-adult male or female. This pre-adult phase comprises two stages and is followed by the fully mature adult phase. The adult female can produce a number of batches of paired egg-strings which in turn hatch into the water column to give rise to the next generation (Kabata, 1979; Schram, 1993).