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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Approximately 300 salmon smolts were stocked in each of four cages moored at various distances between an Atlantic salmon farm and a river system in Killary Harbour on the West Coast of Ireland. Samples of fish were taken from each of the cages on three occasions and examined for the presence of sea lice (Lepeophtheirus salmonis). Results from one-way ANOVA tests and a multiple comparison of the means of infection |evels in the cages indicated that there was a significant difference between the infection levels in the cages on each of the sampling dates. Cage 1, positioned nearest the river, exhibited the highest levels of infection on the first sampling date and was highly significantly different (p -0.001) from the other three cages which showed no significant difference between each other. This situation was again repeated on the second sampling date. On the final sampling date, cage 4, positioned nearest the salmon farm, was highly significantly different (p -~ 0.001) from the other three cages; cage 1 was significantly different (p -0.05) from cages 2 and 3 which showed no significant difference from each other.The population structure of the lice recovered from the smolts indicated that the lice settled in pulses rather than continuously over the trial period. The results are discussed in relation to the hydrography of Killary Harbour, the output of lice larvae from the farm, the impact that these may have on infestations in the inner area of Kil|ary and probable areas of lice larval production.

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