Developing deterrent devices designed to reduce the mortality of small cetaceans in commercial fishing nets

Abstract

The incidental mortality of cetaceans in commercial fisheries is a matter of international concern and whilst some by‐catch may occur in almost any type of fishery the threat from gillnets has attracted particular criticism. The causes of this phenomena are complex and effective solutions elusive. Acoustic alarm devices, based on noise‐makers developed in Canada to reduce the incidence of humpback whale collisions with static nets and traps, have recently been shown to be beneficial in reducing harbour porpoise by‐catch in bottom set gillnets. In Britain the development of passive sonar reflectors, ‘cats eyes’, optimised to match dolphin echolocation signals, have been demonstrated to make gillnet‐like structures detectable to echolocating wild bottlenose dolphins at ranges between 50 and 170 m. These reflecting devices induce avoidance behaviour once detected and have not been observed to attract animals towards them. This paper discusses limitations, both behavioural and acoustic, that need to be appreciated when attempting to develop effective by‐catch mitigation methods.