MIND THE GAP: Pingers, spacing, and sound levels influence bycatch rates of harbour porpoises

Incidental bycatch of harbour porpoise ( Phocoena phocoena ) in set-net fisheries poses a threat to their populations. Pingers, acoustic devices designed to deter cetaceans, are currently the primary tool used to reduce porpoise bycatch without impeding fishing effort. This study aimed to evaluate the effectiveness of different pinger spacing configurations at reducing porpoise bycatch in the Danish North Sea turbot ( Scophthalmus maximus ) fishery. Three trials were conducted in 2021, 2023, and 2025, testing standard Banana pingers with varying spacings of 200 m and 500 m (B-200 and B-500, respectively), and a third alternative using a louder Banana pinger version (LB-500) with a 500 m spacing. Another, even louder, pinger type (DDDs) was tested only in 2025 with a spacing of 1,800 m. Results from the combined trials indicated that B-200 reduced bycatch per unit effort (BPUE) by 66%, B-500 reduced BPUE by 30% and STM-1800 reduced BPUE by 52%. Across years, pingers showed inconsistent results. B-200 reduced porpoise bycatch by 56% in 2023 and by 73% in 2025, while B-500 reduced bycatch by 55% only in 2025. LB-500 reduced bycatch by 74% in 2025, and STM-1800 reduced bycatch by 64% in 2025. These results show that, while increasing pinger spacing reduces pinger implementation costs, effectiveness declines significantly unless the pinger's source level is increased. This research underscores the need for precise pinger spacing regulations to effectively reduce harbour porpoise bycatch.