Harbour porpoise movement strategy affects cumulative number of animals acoustically exposed to underwater explosions

Abstract

Anthropogenic sound in the marine environment can have negative consequences for marine fauna. Since most sound sources are intermittent or continuous, estimating how many individuals are exposed over time remains challenging, as this depends on the animals' mobility. Here we explored how animal movement influences how many, and how often, animals are impacted by sound. In a dedicated study, we estimated how different movement strategies affect the number of individual harbour porpoises Phocoena phocoena receiving temporary or permanent hearing loss due to underwater detonations of recovered explosives (mostly WWII aerial bombs). Geo-statistical distribution models were fitted to data from 4 marine mammal aerial surveys and used to simulate the distribution and movement of porpoises. Based on derived doseresponse thresholds for temporary (TTS) or permanent threshold shifts (PTS), we estimated the number of animals affected in a single year. When individuals were free-roaming, an estimated 1200 and 24 000 unique individuals would suffer PTS and TTS, respectively. This equates to respectively 0.50 and 10% of the estimated North Sea population. In contrast, when porpoises remained in a local area, fewer animals would receive PTS and TTS (1100 [0.47%] and 15 000 [6.5%], respectively), but more individuals would be subjected to repeated exposures. Because most anthropogenic sound-producing activities operate continuously or intermittently, snapshot distribution estimates alone tend to underestimate the number of individuals exposed, particularly for mobile species. Hence, an understanding of animal movement is needed to estimate the impact of underwater sound or other human disturbance.

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.…”

Population‐level consequences of seismic surveys on fishes: An interdisciplinary challenge

“…IBMs can be applied in combination with sound propagation models, using species‐specific data on typical swimming depths, hearing sensitivity and spectral range to assess exposure probability and consequences for over‐exposure and behavioural responses of fish for any particular sound event (Erbe & King, ; Hovem et al., ; Southall et al., ). An example of this approach can be found in harbour porpoises and their exposure to the loud sounds of underwater detonations of explosives (Aarts et al., ; Von Benda‐Beckmann et al., ). In this work, the authors estimated that detonations in the Dutch part of the North Sea (WWII explosives) cause permanent hearing damage for 800–8,000 porpoises per year and many more animals with some temporary impact on their hearing.…”

Spatial distribution and habitat preference of harbour seals (Phoca vitulina) in the Dutch North Sea

“…Shipping noise, explosions at sea and seismic surveys may also deter seals. No field or data analysis studies have been carried out to examine the impact of these other anthropogenic activities on harbour seals (but for porpoises see (Thompson et al 2013;von Benda-Beckmann et al 2015;Aarts et al 2016), and ringed seals see (Harris, Miller & Richardson 2001). If these activities have an effect in the North Sea, this may also have influenced the observed distribution of harbour seals.…”