Harbour Porpoise Bycatch: Determining Spatial Distribution of Risk to Inform Management Measures

Bycatch in static nets (i.e., gillnets, trammel nets and tangle nets) has been identified as the leading cause of harbour porpoise mortality globally. Various options are available for mitigating and managing this risk. However, selecting the most effective management measures to balance harbour porpoise conservation needs with sustainable fishing practice is challenging. By understanding the spatial variation of bycatch risk, it is possible to tailor mitigation and management options for a specific localised area or region. This study identified areas of potential interaction between harbour porpoise and static net fishing activity. An interaction index, a measure of the coincidence of porpoise and fishing activity, was developed. Using this approach, it was possible to differentiate between areas that potentially represented concentrations of higher bycatch risk and other areas with a lower but more widespread level of risk. For the Irish and Celtic Seas, an area recognised as having the highest harbour porpoise bycatch rate in European waters, there was a low to moderate interaction index, with the risk spread over a relatively large area. In the North Sea, in contrast, high values of the interaction index occurred in smaller areas, indicating that bycatch risk was more concentrated in this region. With the exception of some coastal areas, the interaction index was generally low for the West of Scotland and Ireland region. The identification of potential areas of interaction between harbour porpoise and static net fishing provides an opportunity to focus monitoring efforts and inform management decisions. For example, the use of area-based management encompassing small areas may be appropriate when spatially concentrated risk of bycatch is evidenced, whereas the use of technical measures, such as pingers, could be considered for areas with widespread lower risk of bycatch.