Abundance and distribution of planktonic Lepeophtheirus salmonis and Caligus elongatus in a fish farming region in the Faroe Islands

The abundance of planktonic sea lice in the surface waters of the strait of Sundalagið, Faroe Islands was investigated from November 2013 to June 2014. The strait is 38 km long and hosts 6 salmon farms with coordinated farming cycles. The spatial distribution of planktonic sea lice for the entire strait was examined in 2 surveys with different wind and hydrographic conditions. Temporal changes were investigated every 2−3 wk at 3 set stations throughout the study. The spatial distribution of Lepeophtheirus salmonis copepodids was clearly influenced by the actual wind direction, as the copepodids were found where winds pushed surface waters towards the shore. The same spatial pattern was not found for Caligus elongatus copepodids. This might be related to a different vertical migration pattern. The abundance of C. elongatus was seasonal. It was the dominant planktonic sea louse during winter, with a mean abundance of 0.34 ± 0.13 ind. m−3, and was virtually absent during summer. Lepeophtheirus salmonis was present throughout the study, except during the last survey when the coordinated farming sites lay fallow. During winter when the warmest seawater was deep in the water column, L. salmonis copepodids were present in 47% of the samples and nauplii were only observed in 9%. In samples where the highest seawater temperature was at the surface, nauplii prevalence attained a high value of 53%, while copepodid prevalence increased to 60%. These results indicate that nauplii might actively seek the highest possible seawater temperature.