Pearls as bycatch mitigation strategy for Harbour porpoise (Phoecena phoecena)

Abstract: One of the major threats to harbour porpoise population sustainability is thought to be the high incidence of bycatch specifically in gillnets. This study aims to examine the use of acrylic class pearls developed in an earlier study as acoustic reflectors on gillnets in an effort to increase their visibility to echolocating harbour porpoises in the lumpsucker fishery in Kattegatt. This was done by using passive acoustic monitoring (F-PODs) on a net with pearls and a control net to detect porpoise clicks in the vicinity of the two nets as a proxy for porpoise presence. The study was divided into two periods during which different spacing between pearls (30 cm and 60 cm) was used. The click data was also analysed for called buzz feeds, clicks with inter click intervals of 15 ms, as an indicator of differences in click behaviour. Porpoise presence was analysed and presented by click rate-based distribution models (GAM), and the potential role of different kinds of variables (e.g. use of pearls, pearl spacing, diel phase, wave height, water temperature, depth) as potential drivers of porpoise presence was examined. Click behaviour was analysed and presented as number of buzzes per diel phase and buzz ratio per hour and per diel phase using a custom written Matlab algorithm. The study found that the use of pearls and using different spacing between pearls had a significant effect on porpoise presence. The study also found indications for changes in click behaviour caused by the use of pearls. Using acrylic glass pearls as acoustic reflectors on gillnets show promise as a bycatch mitigation strategy for harbour porpoise.