Using simulated escape events to assess the annual numbers and destinies of escaped farmed Atlantic salmon of different life stages from farm sites in Norway

Abstract

To improve assessments of the environmental risks of aquaculture, a series of simulated escapes of farmed Atlantic salmon (Salmo salar L.) from seawater netpens were performed. Individually tagged post-smolts and adult Atlantic salmon were released from various locations at different times of the year. Post-smolts that escaped during their first summer were capable of rapid migration towards the open sea. A small fraction returned to spawn and were recaptured after 1–3 years at sea (0.4%, range 0.0–1.1%). A total of 13% of the post-smolts that escaped during autumn were reported in nearby fisheries during subsequent months, partly because they had grown large enough to be caught in the gillnets used, but more importantly because migratory behaviour diminished towards the end of the year. The mean recapture rate of adult salmon was high after releases in fjords (7–33%), lower after coastal releases (4–7%), and zero on the outer coast. Most of these recaptures were immature fish recaptured in sea relatively close to the release site during their first months post-release. Recaptures of adult escapees after 1–2 years in the wild were very rare (0.09%), probably because of their low survival. A Monte-Carlo method was developed to estimate the annual numbers of escapees from Norwegian fish farms based on reported catches of escaped farmed salmon in the sea and in rivers and the recapture probabilities reported here. The model provides a tool to estimate numbers of escapees independently from the reported numbers. Importantly, our analysis suggests that the total numbers of post-smolt and adult escapees have been two- to fourfold as high as the numbers reported to the authorities by fish farmers, depending on whether the incomplete sea fishery statistics are compensated for.

Use of fatty acid profiles to monitor the escape history of farmed Atlantic salmon

“…This is in accordance with the following tagging/ release experiments. Post-smolts that escape during their first summer in net pens in seawater may return to freshwater to spawn after 1 to 3 yr at sea (Skilbrei 2010a,b), with much higher long-term survival rates than adult escapees (Skilbrei et al 2015). Reports have showed that the stomachs of most escaped Atlantic salmon are empty (Hislop & Webb 1992, Soto et al 2001, Morton & Volpe 2002, Abrantes et al 2011) and 2 studies demonstrated that the FA profile in adult escapees was similar to those given fish feed (Olsen & Skilbrei 2010, Abrantes et al 2011.…”

“…According to Norwegian escape statistics, < 4% of escapees escaped as smolts between 2005 and 2011 (Skilbrei et al 2015). However, escaped salmon can leave the net pens unreported and possibly also undetected by the fish farmer (Skilbrei & Wennevik 2006, Glover 2010, Zhang et al 2013, and therefore estimates of actual escapes of both smolts and adults are significantly higher than reported numbers (Skilbrei et al 2015). Knowledge of the escape history of these salmon is important for risk assessments and for the development of strategies to prevent future escapes and reduce their ecological impacts.…”

Siblingship tests connect two seemingly independent farmed Atlantic salmon escape events

“…Although tens to hundreds of thousands of escapees are reported to the NDF annually, underreporting represents a challenge and the true numbers of escapees have been estimated to be 2 to 4 times higher, putting these numbers in the millions in some years (Skilbrei et al 2015a). Despite efforts to minimise and monitor escapes, large numbers of farmed salmon have been observed in native spawning populations in Norway for several decades (Fiske et al 2006), to the extent that in some rivers, escaped farmed salmon have accounted for 50% of the total brood stock across different years (Saegrov et al 1997, Fiske et al 2006.…”

“…However, this information alone does not permit identification of the farms from which the escapees originated. This presents a challenge given that salmon can disperse over large distances after escaping (Hansen 2006, Skilbrei 2010, Skilbrei et al 2015a. In order to address this issue, Glover et al (2008) developed a method based upon genetic assignment statistics called the 'DNA stand-by-method', which permits the identification of the farm of origin for salmon escapees.…”

Environmental problems and regulation in the aquaculture industry. Insights from Norway

“…The yearly escapement numbers are uncertain, but according to the official statistics, farmed salmon escapes constitutes around half of the total yearly in-run of wild Atlantic salmon to Norwegian rivers. In addition, a recent meta-analysis of catch statistics and tagging studies has estimated that the actual numbers of escapees in Norway were 2-4 times higher than the numbers reported by the farmers during the period spanning 2005 to 2011 (Skilbrei et al 2015).…”