Reduced migratory performance of farmed Atlantic salmon post-smolts from a simulated escape during autumn

Abstract

Escape of migratory finfish species from fish farms, such as Atlantic salmon Salmo salar L., offer a challenge when considering the environmental risks of cage culture. The migratory behaviour of groups of cultured salmon smolts and postsmolts after a simulated escape was studied in a small fjord from May to October using telemetry. Five groups of fish were released approximately every 6 wk from mid-May to late October, each of them composed of 15 to 20 fish equipped with acoustic transmitters and 1780 to 3700 individuals tagged with T-bar anchor tags. The migratory behaviour of the smolts was well developed in postsmolts for at least 6 wk after transfer to sea cages in May, but was less evident in August and was almost absent 2 mo later, when residency was prominent. In contrast to the first 2 groups that moved out of the 21 km-long fjord within a few days, fish released from August to October were being recaptured in the fjord several months after release. The actions required to mitigate the negative environmental impacts of cage-rearing therefore appear to depend on the developmental stage the fish has reached when it escapes. During the first summer months in the sea, efforts to prevent rapidly migrating smolts and postsmolts from escaping should be given priority. The chance of recapture in the small fjord used in this study increased as fish grew and dispersal rate decreased in autumn.

An effective method for the recapture of escaped farmed salmon

“…Just over half of the tagged fish went out towards the open ocean, not following the in-migrating wild sal mon in the fjord at the same time. The leading hypothesis for the random dispersal of escaped adult farmed salmon is that the migratory performance of cage-reared salmon declines during the post-smolt stage at the end of their first summer in net pens (Skilbrei 2010). Another influence may be the fact that the hatcheries and smolt-rearing areas of the experimental fish were in another fjord system, which may affect their imprinting and migratory instincts.…”

“…This strategy included (1) mandatory reporting, (2) mechanisms to analyse and learn from the reporting, (3) cause assessments, (4) technical standards for equipment coupled with independent en forcement and (5) mandatory training of fish-farm staff. Skilbrei (2010) suggested that the prevention of escapes during the first marine summer should be a top priority, as the migratory ability of post-smolts was well developed until at least 6 wk following transfer to sea cages in May, whereupon their dispersal rates declined and chances of recapture increased. Other ideas, including the sterilization ) and conditioning of farmed fish (Tlusty et al 2008), may also reduce the risk of negative effects on wild stocks in the future.…”

“…Past studies on escapee behaviour found that recapture fisheries are a realistic option for reducing the impact of escape events . 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).…”

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

“…Fork length of all fish and weight of samples were recorded. In addition, a small number of salmon were equipped with 9 mm telemetry transmitters for studies of migratory behavior (Skilbrei 2010).…”

“…The target area chosen was Masfjorden, Western Norway. This is a well controlled area with respect to fish farming activity and fishing efforts, and where migratory behavior of released farmed fish is well documented (Skilbrei 2010).…”