Follow-up study after four years shows reduced deterrence effect of pingers on harbour porpoises in Norwegian gillnet fishery

We report results from follow-up field trials testing acoustic deterrents (pingers) as a measure to reduce harbour porpoise (Phocoena phocoena) by-catch rates in coastal gillnet fisheries in Norway. Twelve gillnet fishers conducted a total of 308 fishing trips, fishing with and without pingers in alternating weeks between January and April 2024. Harbour porpoises were by-caught in both pingered (9 porpoises) and control nets (25 porpoises), with most (~73%) taken in control nets. Average by-catch rates were calculated using a generalised linear mixed model (GLMM) approach. The average harbour porpoise by-catch rate was estimated at 0.061 (95% CI 0.034 – 0.098) porpoises per net km day in control nets and 0.031 (95% CI 0.011 – 0.060) porpoises per net km day in pingered nets. This shows that the by-catch rate in pingered nets was about 50% lower than in control nets (95% CI 0 – 83%). This estimate is lower (i.e. the pinger effect size is smaller) than a directly comparable study (Moan & Bjørge, 2023). Possible causes include limited sample size, the use of bait bags to attach pingers to the nets, reporting biases, and/or desensitisation/habituation of porpoises to the pinger sounds. Extra time use due to pingers was low, about two minutes per haul. The use of bait bags seems to have mitigated some, but not all the practical issues reported earlier. The pooled pinger effect size estimate using data from both Moan and Bjørge (2023) and the present study, was estimated at 72% (95% CI 44 – 89%).