Large-scale use of fish traps for monitoring sea trout (Salmo trutta) smolts and sea lice (Lepeophtheirus salmonis) infestations: efficiency and reliability

ABSTRACT

Lice-infected sea trout populations were monitored using fish traps in the Romsdalsfjord (Norway). The reliability and efficiency of this capture technique, which allows estimation of lice infestation rates without killing the fish, was evaluated through a mark–recapture study. A total of 2447 sea trout smolts were captured, tagged and released over a three-year period. There was a considerable variation in capture rates (range: 0.4–17.7 fish per day) and sea lice numbers (number of lice per fish: 2.8–30.3; number of lice per gram body weight: 0.02–0.69) among localities, sampling times and years. Recapture rates of tagged fish with traps, which were low (2% or 0.11 fish per day), showed that the risk for pseudoreplication was minor, in terms of counting lice on the same fish several times. Most of the tagged sea trout (90%) were recaptured within the first two months after release, and no significant variations in lice numbers were found between tagging and recapture. The lack of differences in lice levels between tagging and recapture during the first week after tagging indicated that the method most likely would not significantly underestimate the lice infestations due to loss of lice during handling. Therefore, our results confirm that the use of fish traps is a suitable method for estimation of lice numbers on wild salmonids.

KEYWORDS:

• Fish populations
• fish traps
• monitoring
• parasite
• salmonids
• tagging

Acknowledgements

Arne Kvalvik, Peder Naalsund, Birk Rosvoll Finstad, Reidar Kvam and Alison Harvey are thanked for excellent support during the fieldwork and Patricia Puerta for her help with statistical concepts and procedures. The experiment described was approved by the local responsible laboratory animal science specialist under the surveillance of the Norwegian Animal Research Authority (NARA) and registered by the Authority (FOTS ID 2014/29314).

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

The Norwegian Research Council (project no. 221404), the Norwegian Institute for Nature Research (Strategic Institute Programme, project no. 160022/F40), the Institute of Marine Research, the fish farmers in the Romsdalsfjorden, Møre and Romsdal County Community (Marint miljøsikrings- og verdiskapningsfond), the County Governor of Møre and Romsdal, the Norwegian Food Safety Authority and the NILS Science and Sustainability programme (ES07 – 231 EEA Grants; ABEL-IM-2014A) funded this project.