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

G Á Norði,K Simonsen,Ø Patursson

doi:10.3354/aei00185

Abstract

Abundance and depth distribution of planktonic sea lice were investigated in rela- tion to hydrodynamics and diurnal solar insolation at a salmon farm in Sundalagið, 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 nau- plii 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°C).

Highlights

The salmon louse Lepeophtheirus salmonis is a naturally occurring parasitic copepod in the northern hemisphere and has been a subject in the salmon farming industry since the onset of commercial salmon aquaculture (Pike & Wadsworth 1999)
The free water exchange includes exchange of pathogens such as salmon lice, which transmit between hosts as planktonic larvae (Pert et al 2014)
The study was conducted at a fish farm in the northern part of Sundalagið, Faroe Islands, towards the end of the farming cycle when sea lice prevalence on farmed fish was at its maximum

Summary

Introduction

The salmon louse Lepeophtheirus salmonis is a naturally occurring parasitic copepod in the northern hemisphere and has been a subject in the salmon farming industry since the onset of commercial salmon aquaculture (Pike & Wadsworth 1999). On February 7, 2014 the spatial distribution of sea lice was investigated at 1 m depth at 14 different sites inside and around the fish farm (Fig. 1). The second survey investigated temporal changes in sea lice abundance in one of the net cages at the farm.

Results

Conclusion