Egg strings of salmon lice, Lepeophtheirus salmonis (Krøyer 1837), collected from farmed and wild Atlantic salmon had similar length and number of eggs string−1. Egg production was investigated at water temperatures from 7.1 °C to 12.2 °C. A regression model indicated that at low temperatures egg strings were longer and had more eggs. Mean length of single eggs was significantly smaller and the percentage of non‐viable eggs in the strings was higher at 7.1 °C than at 12.2 °C. Adult females survived for up to 191 days at 7.2 °C, and during this period 11 pairs of egg strings were produced.

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Both the salmon louse Lepeophtheirus salmonis and its close relative Caligus elongatus are ectoparasitic on Salmonidae in salt water in the northern hemisphere. In this study we monitored population dynamics of these parasites on anadromous brown trout, i.e. sea trout, on the Norwegian Skagerrak coast in the winters of 1998–1999 and 1999–2000. The low salinity was expected to reduce sea lice populations as lice do not tolerate sojourns of more than a few weeks, at most, in freshwater. Results confirmed the presence of both parasite species on estuarine sea trout in winter, and showed that the lice populations go through a bottleneck in this period. Prevalences of infection of both parasite species were very different in the two sampling periods, but fell below 10% in March in both winters. Median infection intensity was 1–2 fish−1. Salinity was statistically related to the presence of C. elongatus both winters, and to L. salmonis in 1999–2000. Temperature appeared to be less important for the abundance of lice.

ABSTRACT. The significance of ultraviolet light as a cue in diel vertical migration of free-swimming copepodids of the ectoparasite salmon louse Lepeophtheirus salmonis (Krnyer) and adults of the holopelagic Calanus finrnarchicus (Gunnerus) were examined in the laboratory. Copepodids of salmon lice were photopositive in 1 m deep water columns when illuminated with visible light (VIS) only, but when VIS was combined with ultraviolet light, with a spectral irradiance maximum at 313 nm, the copepodids gathered significantly deeper in the water column. In similar expenments C. finrnarchicus did not show upward migration as a response to VIS, and a combination with ultraviolet light did not alter their normal behaviour. In 250 ml beakers, however, C. finmarch~cus showed avoidance behaviour upon being exposed to UV-light; the copepods swam vigorously with prosome pointing downwards as if they were trying to escape from the irradiation. We conclude that these copepods must have sensory structures not yet descnbed which can detect UV-radiation. Possible implications of these findings are discussed in relation to vertical rnigration and to host-finding in the salmon louse.