Causal analysis of escape of Atlantic salmon and rainbow trout from Norwegian fish farms during 2010–2018

Farmed fish escaping into the wild and other environmental concerns have curbed the expansion of the Norwegian aquaculture industry. Detailed knowledge of both direct and underlying causes of previous escape episodes is crucial to ensure successful development of new technology and targeted safety-measures at fish farms. This paper provides detailed descriptions of both technological, human and organisational factors relevant to escape of fish from Norwegian fish farms during 2010–2018. Fish farmers in Norway are obliged by law to report escape incidents to the Norwegian Directorate of Fisheries. A total of 305 reported escape incidents with Atlantic salmon (Salmo salar) or rainbow trout (Oncorhynchus mykiss) were confirmed from 2010 to 2018, involving in total 1.960.000 registered escapees. Analysis of 298 of these incidents shows that most registered escapees came from sea-based fish farms (92%), while 7% were from land-based facilities and 1% from transportation between sites. Most escape incidents were directly caused by technological factors, with holes in the net as the most common cause of escape. Bad weather or handling of weights and net prior to delousing have been associated with increased probability of escape incidents. In addition to direct and contributing causes, mostly technological, escape incidents may also have underlying causes related to human and organisational factors. These causes may have triggered the incidents or prevented barriers from being effective, with technical damage and escape of fish as result. Relevant human and organisational causes were explored through interviews with employees that have experienced escape incidents. •New knowledge regarding direct and underlying causes of escape of Atlantic salmon and rainbow trout are presented.•Data sources analysed include reports of escape (n = 298) and interviews (n = 17).•Data show that holes in the net is the most common direct cause.•Bad weather and operations may increase the probability of escape.•Contributing human and organisational factors should be investigated further.