Echolocation activity of harbour porpoises, Phocoena phocoena, shows seasonal artificial reef attraction despite elevated noise levels close to oil and gas platforms

Abstract

1. Harbour porpoises frequently alter their behaviour in response to underwater sound from shipping, seismic surveys, drilling and marine renewables. Less well understood is the response of porpoises to sounds emitted from oil and gas (O&G) platforms during routine operations. 2. The responses are not easily predicted as platforms can act simultaneously and to varying degree as a source of disturbance through noise and attraction through an artificial reef effect with increased prey abundance and diversity. 3. To investigate the presence and feeding behaviour of harbour porpoises around platforms, autonomous acoustic loggers were placed for up to 2 years, at 21 stations 0-25.6 km from the largest platform in the Danish North Sea. 4. Harbour porpoises were detected at all distances year round in two distinct seasonal activity patterns. During July-January, porpoises were attracted to the platform as indicated by high foraging activity within 800 m of the platform. Echolocation activity levels were up to twofold higher than those observed at 3.2-9.6 km from the platform. 5. Similar high echolocation activity was observed 200 m from neighbouring offshore installations located within 15 km, regardless of their size, during May-July. 6. This study shows that porpoises may be attracted to offshore O&G platforms despite confirmed elevated underwater noise and are likely exploiting higher prey abundance in the vicinity of such structures. This is possibly due to increased prey availability created by the combined effect of the artificial reef formed by the underwater structure and the local protected area around all platforms where fishery is banned. 7. Hard substrate and untouched seabed are rare and valuable habitats to many organisms in heavily trawled waters like the North Sea, and the ecological importance of these structures should be considered in the development of decommissioning strategies. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Reef effect of offshore structures on the occurrence and foraging activity of harbour porpoises

“…To date, just one study has demonstrated that harbour porpoises are locally attracted to O&G structures. In this case, within the Danish sector of the North Sea, porpoise echolocation activity was up to twofold higher within 800m of an operational O&G platform compared to reference sites (Clausen et al, 2021). As demonstrated in earlier work from a jack-up barge around a gas platform in the German sector of the North Sea, porpoise echolocation activity was also highest during the night (Todd et al, 2009).…”

“…It is recognised that marine mammals may be attracted to artificial offshore structures in certain situations (Russell et al, 2014;Clausen et al, 2021), but the generality of these findings is less clear. This is partly because studies have focused on a limited number of the many designs of structure currently installed offshore, but also because research has been conducted only in a few ecological regions.…”

Influence of offshore oil and gas structures on seascape ecological connectivity

“…echolocation around offshore O&G installations may indicate foraging behaviour, particularly at night (Clausen et al, 2021;Todd et al, 2009), and were closer to the installations than at control sites further away (Clausen et al, 2021). Satellite tracking of pinnipeds (harbour Phoca vitulina and grey seals Halichoerus grypus)…”

“…Attraction included infrequent visits over a 6‐week period, to high residency, potentially for feeding (Thomson et al, 2021), with feeding observed at offshore platforms in the Arabian Gulf (Robinson et al, 2013). Passive acoustic monitoring detection of harbour porpoise ( Phocoena phocoena ) echolocation around offshore O&G installations may indicate foraging behaviour, particularly at night (Clausen et al, 2021; Todd et al, 2009), and were closer to the installations than at control sites further away (Clausen et al, 2021). Satellite tracking of pinnipeds (harbour Phoca vitulina and grey seals Halichoerus grypus ) shows that some individuals can spend prolonged periods at O&G infrastructure for foraging (Russell et al, 2014).…”

“…On the positive side, the presence of O&G infrastructure, particularly in oligotrophic environments or on seabeds where natural hard substrata are scarce, provides a physical structure for marine ecosystems to develop. In such instances, structures can facilitate larval settlement and recruitment (Gass & Roberts, 2006; Love et al, 2006; Nishimoto et al, 2019b), increase the biomass of fish (Claisse et al, 2014; Clausen et al, 2021; Meyer‐Gutbrod et al, 2019), and promote biodiversity (Bond, Partridge, Taylor, Cooper, et al, 2018; Bond, Partridge, Taylor, Langlois, et al, 2018; Broadbent et al, 2020; McLean et al, 2017; Todd, Williamson, et al, 2020). The perceived beneficial value of these structures as habitat has led to ‘rigs to reefs’ (RTR) policies for decommissioned O&G structures in some parts of the globe (Bull & Love, 2019; Smyth et al, 2015).…”