PLANKTONIC DISPERSION OF LARVAL SALMON-LICE, LEPEOPHTHEIRUS-SALMONIS,ASSOCIATED WITH CULTURED SALMON, SALMO-SALAR, IN WESTERN IRELAND

Dispersion of Lepeophtheirus salmonis Kroyer 1838 (Crustacea: Caligida e) larvae from a single cage and also from a farm system was investiga ted in Ardmore Bay, west coast of Ireland. Surface plankton tows were taken from within a cage and also at various distances from the farm. Highest densities of larvae (maximum, 66.1 m(-3)) were consistently re corded within the cage. Less than 10% of the larval density recorded i nside the cage was found outside at any one time, indicating a high re tention of larvae within the cage throughout the sampling period. This was due to a reduction of water movement inside the cage caused by th e physical barrier of the net which was fouled during the sampling per iod. Highest densities of larvae were consistently found in samples ta ken at 10 m (maximum, 4.8 m(-3)) from the last cage on the farm in com parison to those taken further away from it (maximum, at 1 km; 0.4 m(- 3)). Regression analysis of the data from each of the sampling dates s howed a significant inverse relationship (P=0.001) between distance an d larval numbers. The models produced suggest that few louse larvae wo uld have been recovered in samples (i.e. 100 m plankton surface tows) taken more than 2 km from the last cage on the farm. Although the rati o of copepodids to nauplii increased with distance from the farm, the densities of the copepodids did not increase.