Sampling strategies, distribution and concentration of planktonic salmon lice copepods in the Outer Hardangerfjord and the Altafjord

Abstract

Salmon lice are parasitic copepods with three planktonic larvae stages, consisting of two nauplii stages and a copepodite stage. The parasite spread during these stages as plankton, and with the increased number of host represented by salmonid fish in aquaculture it is important to know the concentrations and ecology of the fee-living stages. Both Lepeophtheirus salmonis and Caligus elongatus are parasitic lice on salmon and trout, and will be referred to as salmon lice through this thesis.

Finding suitable sampling strategies to collect salmon lice copepods makes it possible to obtain field observed concentrations of salmon lice in their infective stage and in open water. Through this thesis, three different kinds of gear (Go-Flo water sampler, net hauls and a provisional bilge pump) and in total seven different strategies (different depths and volumes sampled) have been tested. In total 117 samples were collected and analysed. The vertical net haul proved to be best suited for the task of collecting salmon lice copepods under sub-optimal weather conditions, as often is the case in Norwegian fjords. In the two fjords investigated, the outer Hardangerfjord and the Altafjord concentrations ranged between 0-30 ind. m-3 and between 0-13 ind. m-3, respectively. These field data were compared with results from a hydrodynamic salmon lice model, and concluded that the range of concentrations found in the field was within the same range of concentrations simulated by the model. The concentrations obtained were also similar to concentrations found during previous studies in aquaculture impacted regions around Scotland and the Faroe Islands. This study found that areas less influenced by aquaculture had lower concentrations of salmon lice copepods (<2 ind. m-3), as seen in the samples from Talvik, situated within a National Salmon Fjord. The copepodite size and their vertical, horizontal and seasonal distribution were also investigated. The size range of the copepodite salmon lice caught during this study was smaller than expected from earlier studies. This could be because the two common salmon lice species, L. salmonis and C. elongatus, may both have been present in the samples, and C. elongatus is normally smaller during their copepodite stage. From the vertical distribution of lice, a patchy aggregation in the upper 5 m was found, while the only observed trend in the horizontal distribution was that the concentrations were lower at stations that were >10 km from the nearest salmonid farm. Due to rough weather during the October cruise and only sampling through the autumn in Altafjord, this thesis had insufficient data to determine any seasonal distribution of salmon lice.

Knowledge on field concentrations and the spatial distribution of salmon lice is important to ensure a sustainable growth and management of the salmon farm industry in Norway.