Predicting the population‐level impact of mitigating harbor porpoise bycatch with pingers and time‐area fishing closures

Abstract

Abstract. Unintentional mortality of higher trophic-level species in commercial fisheries (bycatch) represents a major conservation concern as it may influence the long-term persistence of populations. An increasingly common strategy to mitigate bycatch of harbor porpoises (Phocoena phocoena), a small and protected marine top predator, involves the use of pingers (acoustic alarms that emit underwater noise) and time-area fishing closures. Although these mitigation measures can reduce harbor porpoise bycatch in gillnet fisheries considerably, inference about the long-term population-level consequences is currently lacking. We developed a spatially explicit individual-based simulation model (IBM) with the aim to evaluate the effectiveness of these two bycatch mitigation measures. We quantified both the direct positive effects (i.e., reduced bycatch) and any indirect negative effects (i.e., reduced foraging efficiency) on the population size using the inner Danish waters as a biological system. The model incorporated empirical data on gillnet fishing effort and noise avoidance behavior by free-ranging harbor porpoises exposed to randomized high-frequency (20-to 160-kHz) pinger signals. The IBM simulations revealed a synergistic relationship between the implementation of time-area fishing closures and pinger deployment. Time-area fishing closures reduced bycatch rates substantially but not completely. In contrast, widespread pinger deployment resulted in total mitigation of bycatch but frequent and recurrent noise avoidance behavior in high-quality foraging habitat negatively affected individual survival and the total population size. When both bycatch mitigation measures were implemented simultaneously, the negative impact of pinger noiseinduced sub-lethal behavioral effects on the population was largely eliminated with a positive effect on the population size that was larger than when the mitigation measures were used independently. Our study highlights that conservationists and policy makers need to consider and balance both the direct and indirect effects of harbor porpoise bycatch mitigation measures before enforcing their widespread implementation. Individual-based simulation models, such as the one presented here, offer an efficient and dynamic framework to evaluate the impact of human activities on the long-term survival of marine populations and can serve as a basis to design adaptive management strategies that satisfy both ecological and socioeconomic demands on marine ecosystems.

Harbor porpoise (Phocoena phocoena) reactions to pingers

“…Here it was found that when AQUAmark100 pingers were used in areas of high porpoise usage, the habitat exclusion had a higher impact on the population than the bycatch in the fisheries (van Beest et al . ). These kinds of model predictions will, however, depend on the pinger type and specific effects of the pinger used.…”

Environmental drivers of harbour porpoise fine-scale movements

“…Independent case studies have highlighted two common impacts of such anthropogenic activities on harbour porpoises, namely, behavioural alterations (Tougaard et al 2012 ; Dyndo et al 2015 ) and direct mortality or injury of individuals (Vinther and Larsen 2004 ; Lucke et al 2009 ). To assist in the conservation of harbour porpoises, there has been increased effort to develop predictive simulation models to assess any consequences of anthropogenic stressors and disturbances on individuals and populations (Nabe-Nielsen et al 2014 ; King et al 2015 ; Aarts et al 2016 ; van Beest et al 2017 ). Such simulation models are valuable tools to highlight areas of potential conflict with current or planned anthropogenic activities and to evaluate the effectiveness of potential mitigation measures.…”

Distribution and Habitat Use of a Cryptic Small Cetacean, the Burmeister's Porpoise, Monitored From a Small-Scale Fishery Platform

“…The results of our study suggest that the protection of key habitats for porpoises may be more straightforward than for dolphins, given their more restricted range. Successful mitigation of SSF bycatch is likely to be achieved through a combination of multiple approaches; for example, restricting fishing activities in near-shore waters, or use of acoustic alarms (pingers) on nets (e.g., van Beest et al, 2017). Indeed, a previous study has shown that pingers reduce bycatch of Peruvian small cetaceans (Mangel et al, 2013), and future work should refine the efficacy of using pingers to deter porpoises and dolphins from nets (Dawson et al, 2013).…”