Pingers reduce harbour porpoise bycatch in Norwegian gillnet fisheries, with little impact on day-to-day fishing operations

A field trial was conducted to determine the effect of acoustic deterrent devices (ADDs, or pingers) on harbour porpoise (Phocoena phocoena) and harbour seal (Phoca vitulina) bycatch in three Norwegian commercial gillnet fisheries targeting cod (Gadus morhua), saithe (Pollachius virens) and monkfish (Lophius piscatorius). Catch data on 3500 net-km-days were collected by 8 fishing vessels operating gillnets in high bycatch regions over two years. A total of 20 harbour porpoises and 9 harbour seals were bycaught, with 19 harbour porpoises and 6 harbour seals taken in control (non-pingered) nets. Bycatch was modelled using a generalized additive mixed modelling approach and fitted with penalized maximum likelihood. Modelling results indicated that using pingers on gillnets reduced the risk of bycatching a harbour porpoise by an estimated 94% (95% confidence interval CI 77–100%) compared to ordinary pinger-free nets. The effect of pingers was not significantly different between different fisheries. The pingers also had no significant effect on catch rates of fish (Wilcoxon rank sum test, p = 0.24) or harbour seals (Wilcoxon rank sum test, p = 0.19). Self-reported pinger-associated extra time costs on day-to-day fishing operations were low, averaging about 2.8 min per operation. These results add to a growing body of scientific evidence that pingers can lead to substantial reductions in harbour porpoise bycatch rates in gillnet fisheries, and that extra time costs associated with operating nets with pingers are low.