UW Aquatic & Fishery Sciences Quantitative Seminar

Brendan O'Connor

Aqua-Fact International Services Ltd., 12, Kilkerrin Park, Liosbaun, Galway, Ireland

A review of field studies on larval Lepeophtheirus salmonis Kroyer off the Irish west coast

This presentations reviews findings of studies on the larvae of Lepeophtheirus salmonis initiated in 1994 at a number of locations on the Irish west coast. The work was undertaken to provide information on i) larval densities in selected bays with and without salmon farms ii) densities in transects away from salmon farms along the axis of the main current flow and iii) levels of infection of L. salmonis on smolts placed in cages close to and at distance from farms.

The plankton studies include findings on the density, vertical/horizontal distributions and population structure of the larval copepod both close to farms and at greater distances from the farms. Results show that densities rapidly decrease with distance from the farm e.g. by two orders of magnitude over a distance of 50m.

Larval production appears to be continuous over the Summer months though larval densities show a strong correlation with the lunar cycle. This feature may explain pulses of settlement recorded in other parts of this work and reported by farmers. Larvae exhibit a pattern of vertical migration which is apparently tuned to parts of the tidal cycle.

Results from experiments using salmon smolts placed in cages at varying distances from salmon farms showed that cages which were placed furthest away from a farm but closest to a river showed much higher initial infestation levels than the cage closest to the farm (ca 0.5 km).

The findings are discussed in the light of parasite-host interactions and are used to explore the possible evolution of the relationship between L. salmonis and its salmonid hosts.