Recapture of cultured salmon following a large-scale escape experiment

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, evaluate 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 confirmed 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. Gillnetting proved to be an efficient method of recapture. The total reported recapture rate (of 114 fishers) 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.