Recapture of cultured salmon following a large-scale escape experiment

OT Skilbrei,T Jørgensen

doi:10.3354/aei00011

Abstract

A large-scale escape experiment using 1031 adult Atlantic salmon Salmo salar L. was performed in the Hardangerfjord in western Norway to study the dispersal of escaped salmon, eval- uate the effect of a gill-net fishery targeting escaped salmonids and test whether surface trawling is an effective way of recapturing escaped salmon in a large fjord system. The salmon of mean weights 1.56 and 5.5 kg were released from 2 commercial fish farms in late September 2006. All fish were tagged with external tags, and 48 were also equipped with acoustic transmitters. A surface pair-trawl (50 m wide and 8 m deep) was constructed to optimize catchability and maneuverability in the fjord environment. Trawling was unsuccessful, and caught only 6 simulated escapees. Telemetry data con- firmed that the fish were available along the towing tracks, and we assume that towing speed and/or trawl size may have been suboptimal with regard to avoidance by fish in the fjord environment. Gill- netting proved to be an efficient method of recapture. The total reported recapture rate (of 114 fish- ers) was 40%, but a significantly higher recapture rate (67%) of the more highly rewarded acoustic transmitters, and the distribution of the fish in time and space, suggest that the actual catch may have been substantially higher. Approximately 90% of the catches were taken within 40 km of the release sites over the course of 4 wk. We conclude that a significant proportion of escaped adult salmon can be recaptured if the catch effort within the fjord basin is widespread and lasts for at least 4 wk.

Highlights

Farming of Atlantic salmon has grown rapidly since the mid-1960s, and farmed salmon that escape from cages have become a serious problem for the fishfarming industry and for the conservation of wild stocks
This study of 2 realistically scaled escape incidents in a large fjord shows that a high percentage of both medium-size and large adult salmon were gill-netted within 40 km of the release site over the course of 4 wk, and that surface trawling was an ineffective method for recapturing escapees in a fjord environment
The post-release behaviour of the fish equipped with acoustic transmitters was similar to that observed in previous studies in Hardangerfjord, when limited numbers were released (Skilbrei et al 2009, 2010); the majority stayed within the fjord basin for several weeks and swam close to the surface above the halocline, especially at night

Summary

Introduction

Farming of Atlantic salmon has grown rapidly since the mid-1960s, and farmed salmon that escape from cages have become a serious problem for the fishfarming industry and for the conservation of wild stocks. Escapees may spread diseases and parasites and interfere with the genetic make-up of wild stocks if they manage to interbreed (Lura & Sægrov 1991, McGinnity et al 2004, Naylor et al 2005, Hindar et al 2006, Jonsson & Jonsson 2006, Skaala et al 2006, Ferguson et al 2007, Skilbrei et al 2010). Technical and operational failures of fish-farming equipment are the main causes of the escape events reported to the Norwegian Directorate of Fisheries (Jensen et al 2010); numbers have varied from less than 200 000 to more than 900 000 individuals yearly from 2001 to 2009 (www.fiskeridir.no). Small-scale unreported escape events may make up a large portion of the escaped farmed fish (Skilbrei & Wennevik 2006), and genetic assignment (Glover et al 2008) has recently been developed as a tool to find the farm of origin of larger unreported escape events

Results

Discussion

Conclusion