Avoidance and escape behaviour by herring encountering midwater trawls

Abstract

The main objective of this study was to assess how and to what extent Baltic herring avoid and escape midwater trawls under commercial fishing conditions. Information was obtained by acoustic, visual and direct sampling techniques. The majority of herring schools in the path of an approaching trawl were observed to enter the trawl. When they did avoid the trawl it was by swimming downwards. The frequency of strong avoidance reactions was significantly higher during daytime than at night. Herring entering the trawl passed quickly into the funnelling rear part of the trawl, where they swam vigorously, trying to maintain their position in relation to the trawl. Fish often struck against the netting and their scales were seen passing through the meshes. Some herring escaped through the meshes of the upper rear panels of the trawl body, especially in the dark. Escaping herring were significantly smaller than those caught in the codends. Nevertheless, most herring ended up in the codend. We conclude that, although herring occasionally avoid and escape midwater trawls, the majority of fish in the trawl path are caught in the codend.