Abstract
Intensity and distribution of presettlement by the copepodid of the sea louse, Lepeophtheirus salmonis, on smolts of its host Atlantic salmon, Salmo salar, were quantified for 27 infection regimes under controlled flume conditions. Each infection regime represented a level of interaction between three levels (low, medium, high) of the physical factors of light (10, 300, 800 lx), salinity (20, 27, 35), and host velocity (0.2, 7.0, 15.0 cm·s1). Light, salinity, and host velocity independently and interactively determined the distribution and number of presettled copepodids on hosts. Host surface area also influenced the number of attached preestablished copepodids. The distribution of presettled copepodids on the host body surface closely corresponded to that of settled copepodids and chalimi reported from other studies, with the greatest levels observed on the fins, in particular the dorsal, caudal, and pectoral fins. Copepodid presettlement occurred on the gills under all conditions. Differential presettlement, not selective mortality, probably produces the distribution pattern of settled stages seen in other studies.
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