Fish Farm Escapes May Be Worse Than We Think

Like other types of intensive farming, commercial fish farming negatively impacts both the fishes who are farmed and the environment around them. Fish farms in Norway produce more than 65% of the world’s supply of farmed Atlantic salmon. These farmed fish pose a potentially substantial risk to the country’s large population of wild river salmon. One of the greatest threats to wild salmon is the escape of farmed fish, who can transmit disease and reduce the genetic integrity of wild populations.

This paper, published in the ICES Journal of Marine Science, assesses annual numbers and destinies of escaped Atlantic salmon from farms on the Norwegian coast. The authors evaluated data from simulated escape incidents performed from 2005 to 2011 at eight farm sites using tagged fish of varying developmental stages released at different times of the year. Recapture information (provided by farmers who returned tags) was used to calculate recapture probabilities based on locations of release sites, years, life stages of fish, and methods of capture. The resulting model provides a detailed listing of recapture data that may be useful for researchers. It includes findings such as that of recaptures of adult fish being significantly higher in fjords than the outer coast.

Additionally, and perhaps more significantly, the authors used their recapture probabilities as well as external data on reported numbers of recaptured salmon to estimate the annual number of escapees from Norwegian fish farms. The authors calculated that an average of one million salmon escaped annually from 2005-2011. This is more than double the mean of 413,000 annual escapees reported by the Norwegian Directorate of Fisheries. Other scenarios put the estimated escapees at up to 48 times the government numbers.

The authors suggest that “the official Norwegian statistics for catches of escaped farmed salmon in the sea are very incomplete.” This is largely due to low rates of reported recaptures and the failure of farms to report fish escapes. “More effort has to be put into preventing fish from escaping,” they say. They further recommend that “more detailed monitoring of the escape history of the farmed salmon that enter rivers should be implemented to provide authorities and the aquaculture industry a better basis in which to develop more effective strategies to mitigate the problems caused by escaped farmed salmon.”