The occurrence of lice on sea trout (Salmo trutta L.) captured in the sea off the East Anglian coast of England

The results of a sampling programme for salmon lice on sea trout caught in the North Sea off the East Anglian coast during 1992 and 1993 are compared with similar data from 1972 and 1973 for fish caught in the North Sea off the coast of Yorkshire. These areas are completely free of any salmonid mariculture, nor has East Anglia any significant sea trout rivers, the trout migrating into the area mostly from the north-east of England. These attributes provide an ideal basis for examining levels of infection in the absence of any potential interaction with salmonid farming activities. Counts of the two principal species of lice that occur on salmonids, Lepeophtheirus salmonis and Culigus elongatus, were made over a number of months in each year. The inter-annual and seasonal patterns of infection by both species showed mean intensity to be both low and fairly stable. The statistical distribution of the parasites amongst the host population generally showed levels of overdispersion (aggregation) typical of parasite populations. These results are discussed in terms of the dynamics of the parasite population, the interaction between parasite and host, and parasite transmission dynamics. 0 British Crown copyright 1997