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Dispersion of Lepeophtheirus salmonis Kreyer 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, 661 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; 04 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.

The distribution of Lepeophtheirus salmonis larvae in Killary Harbour was investigated by taking plankton tows from a number of stations on a regular basis during the spring/summer period in 1995 and 1996. Current patterns, ovigerous lice loads on the resident fish and water temperature and salinity were also recorded during the study. Larvae were only found consistently at a station close to a salmon farm at the mouth of the harbour. In the inner harbour, close to the mouths of two rivers, larvae were recovered in sporadic time blocks. Highest densities of larvae were found at the beginning of the study in both years in the inner harbour with a subsequent decrease in the following months. The distribution, density and origin of the larvae are discussed in relation to the prevailing current patterns in the harbour, the ovigerous lice loads on the resident fish and the requirements of the larvae to locate a new host and complete the life cycle.

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