Planktonic Dispersion of Larval Salmon-Lice,Lepeophtheirus Salmonis, Associated with Cultured Salmon,Salmo Salar, in Western Ireland

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Abstract
Dispersion ofLepeophtheirus salmonisKreyer 1838 (Crustacea: Caligidae) larvae from a single cage and also from a farm system was investigated 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 recorded within the cage. Less than 10% of the larval density recorded inside the cage was found outside at any one time, indicating a high retention of larvae within the cage throughout the sampling period. This was due to a reduction of water movement inside the cage caused by the physical barrier of the net which was fouled during the sampling period. Highest densities of larvae were consistently found in samples taken at 10 m (maximum, 4·8 m−3) from the last cage on the farm in comparison 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 showed a significant inverse relationship (P=0·001) between distance and larval numbers. The models produced suggest that few louse larvae would 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 ratio of copepodids to nauplii increased with distance from the farm, the densities of the copepodids did not increase.