Caligus elongatus ( Nordmann) is probably the most common species of parasitic copepod in British waters, (Kabata, 1979) if not the North Atlantic. It parasitises over eighty host species, including several commercially important species of teleost and elasmobranch. C. elongatus contributes to sea lice infestations in farmed salmonids throughout the North Atlantic. It has been recorded from farmed fish in Norway (Bristow and Berland, 1991), Scotland (Wootten et al., 1982), Canada (Hogans and Trudeau, 1989b) and Ireland (Jackson and Minchin, 1992). This species has been responsible for serious epizootics on farmed salmonids in both Scotland (Wootten et al., 1982) and Canada (Hogans and Trudeau, 1989b).
In Ireland, lice levels on farmed salmonids have been monitored since 1991 (Jackson and Minchin, 1993). From the time monitoring commenced C. elongatus has been recorded in low numbers on Irish salmon farms. This is in strong contrast to the experience in Canada where it was recorded as accounting for over 90% of copepods recorded in a 1988 study by Hogans and Trudeau (op. cit.). The situation in Canada has since altered and Lepeophtheirus salmonis (Kroyer) is now a serious parasite of farmed salmon in Atlantic Canada. In the autumn of 1994, the first epizootic of L. salmonis which required treatment was recorded (Hogans, 1995).
The pattern of occurrence of C. elongatus on farmed salmonids in Ireland is very different from that of L. salmonis in a number of ways. These differences do not appear to be related to competitive exclusion by the larger and more specialist salmonid ectoparasite L. salmonis (Jackson and Minchin, 1992). C. elongatus is often the first parasite to occur on new farms or on recently fallowed sites (op. cit.). Ovigerous female C. elongatus have been found on smolts in their first summer at sea, within 6–8 weeks of transfer (Minchin and Jackson, 1993).