Vastly more salmon could be escaping from aquaculture farms (pictured) than is officially reported, say Ove Skilbrei and his colleagues at the Institute of Marine Research in Bergen, Norway.

Credit: Jean Gaumy/Magnum Photos

Farmed salmon that escape could mate with wild populations and make them less fit for survival. The researchers tagged more than 90,000 farmed Atlantic salmon (Salmo salar) and released them along the Scandinavian coast in 2005. Over the next five years, the team collected data on how many tagged animals were recaptured by fishermen. In some cases, immature fish were found near their release sites, but for adults, the recapture rate after 1–2 years was less than 0.1%.

Using this probability for recapturing escapees, the team estimates that as many as 1.5 million farmed salmon escape from farms in Norway each year — significantly more than the 413,000 escapees that are reported annually.

ICES J. Mar. Sci. http://doi.org/t6t (2014)