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.

An effective method for the recapture of escaped farmed salmon

“…from a torn pen, escapees may form schools and migrate in a more directed manner. However, the present results are consistent with both small-and large-scale releases in another large Norwegian fjord, which also demonstrated multidirectional migration patterns in escaped salmon , Skilbrei & Jørgensen 2010.…”

“…However, fishing efforts need to be initiated immediately following the event and cover a large area, as salmon may either remain near the farm site (Olsen & Skilbrei 2010) or disperse rapidly following release (Skilbrei et al 2010). A study in southern Norway found that trawling for escapees was not very efficient, whereas gillnetting had greater success, especially when carried out over a large area for an extended period (Skilbrei & Jørgensen 2010). As salmon tend to be surfaceoriented, nets set near the surface were most effective (Skilbrei et al 2009).…”

Siblingship tests connect two seemingly independent farmed Atlantic salmon escape events

“…4a,b) suggests that fish would have had enough time to cover the ~150 km distance that separates both sites. This is in agreement with field experiments in this region reporting that the majority of the escapees disperse from the escape area after 1 mo (Skilbrei 2010, Skilbrei & Jørgensen 2010. Likewise, Chittenden et al (2011) experimentally showed that fish dispersed rapidly (9.5 ± 19.2 km d −1 ) in the days following escape, travelling outward to coastal waters along the edges of the fjord.…”

Feeding preference of recaptured Atlantic salmon Salmo salar following simulated escape from fish pens during autumn

“…The estimated total catch of farmed fish in salmon rivers throughout Norway is below 10 000, which is low compared to the total reported escapees (Anon 2010). In accordance with this, high recaptures of tagged adult salmon have been reported in the vicinity of release sites for the first months after release, but none or very few have been found in freshwater (Hansen 2006, Skilbrei & Jørgensen 2010. It therefore appears that survival until fish mature and enter rivers is low.…”