A method of estimating in situ salmon louse nauplii production at fish farms

Abundance and depth distribution of planktonic sea lice were investigated in relation to hydrodynamics and diurnal solar insolation at a salmon farm in Sundalagio, Faroe Islands. Plankton surveys were conducted by surface tows with a plankton net around the farm and by using a plankton pump at 1, 4 and 6 m depth in a fish cage. The entire sample content was investigated under a stereomicroscope and sea lice were identified. Sea lice of the species Lepeophtheirus salmonis and Caligus elongatus were present at the farm. Nauplii dominated the planktonic stages (>95%) while copepodids were absent from most samples. The highest observed copepodid density was 0.3 ind. m(-3), which is within the range found in open water. No diurnal vertical distribution pattern was observed for salmon lice nauplii in the net cages, which were most abundant in the top meter of the water column, and density decreased with depth. At 1 m depth, nauplii density was inversely proportional to the current speed at the same depth. From this relation, and the abundance of adult female sea lice on the farmed fish, the in situ nauplii production was calculated to be within 26-68 nauplii female(-1) d(-1). The lower end of this range is similar to production rates suggested by laboratory studies at similar temperatures (7.8 degrees C).