Increasing numbers of oil and gas (O&G) structures are coming to the end of their operational life, and there is ongoing debate about the best ecological approaches to their removal or re-use (Bull and Love, 2019; Fortune and Paterson, 2020; Lemasson et al., 2021). In parallel, the rapid growth of offshore wind energy is leading to the installation of many new fixed artificial structures in shelf seas. Assessments of the biological costs and benefits of installing and decommissioning these structures are complicated by uncertainties over the importance of offshore artificial structures for many mobile species (Fortune and Paterson, 2020). In some regions, this constrains policy decisions over re-use of O&G structures as artificial reefs (Bull and Love, 2019). In others, policy decisions already require removal of structures (Lemasson et al., 2021), but the required EIAs are not mandated to consider effects on marine biota adhering to or dependent on the structure (Fortune and Paterson, 2020).

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. In common with studies of many aspects of artificial structures, the lack of baseline data can hinder the assessment of decommissioning effects (Fortune and Paterson, 2020).

Harbour porpoises are abundant and widely distributed across the North Sea (Hammond et al., 2013). Given their known sensitivity to anthropogenic disturbance, they are considered key receptors within EIAs underpinning extensive offshore energy activities across this region (Thomsen et al., 2011). Future assessments will need, first, to consider the extent to which attraction to redundant structures may affect local densities during decommissioning activities. Second, mitigation measures will require an understanding of how local densities vary in time to identify periods when these receptors may be more, or less, sensitive to disturbance.

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). Similar diel patterns of activity in prey (Fujii and Jamieson, 2016) and avian predators (Ronconi et al., 2015) have been observed around active O&G platforms. However, it is not known whether such diel patterns in predator-prey interactions are driven by the physical presence of artificial structures or through attraction of prey to lights and flares on operational platforms (Todd et al., 2009; Clausen et al., 2021).

Here, using passive acoustic monitoring (PAM), we studied the occurrence and foraging activity of harbour porpoises around a cluster of redundant artificial structures within Scottish shelf waters. First, we assessed the extent to which porpoises were attracted to structures of different age and complexity. Secondly, we explored whether observed increased levels of nocturnal occurrence and foraging were also evident around platforms that have been abandoned, with lighting reduced to levels required for navigational safety. Finally, we used pre-installation baseline data at one site (Thompson et al., 2010) to provide a direct assessment of how the presence of structures affected diel patterns of occurrence and foraging activity of porpoises.

The study was carried out around the Smith Bank, within the Moray Firth, NE Scotland. The area has been subject to O&G exploration and production for several decades (Addy, 1987), and has more recently seen the development of demonstration (Thompson et al., 2010) and commercial (Graham et al., 2019) offshore wind energy. Several marine mammal species occur across the area, but harbour porpoises are the most abundant and widespread of these, providing a model species for understanding interactions between wildlife and offshore energy activities (Thompson et al., 2010; Thompson et al., 2013; Benhemma-Le Gall et al., 2021).

Harbour porpoises were detected every day throughout the 31-day study period for an average of 17 hours day^-1. The complete dataset comprised 636 data days from 22 CPOD deployments, with only 23 data days (< 4%) needing to be excluded from further analyses due to excessive background noise ( Supplementary Material Table S2 ).

An extensive PAM array around four offshore structures revealed that harbour porpoise occurrence and foraging activity was higher around structures and that this effect was explained by an increase in occurrence and foraging at night. Further, we showed a change in the diel pattern of porpoise occurrence linked to the presence of an offshore structure.

Consistent with previous work (Russell et al., 2014; Clausen et al., 2021), our results provide further evidence that offshore structures attract some species of marine mammals, which use these structures to forage. A previous study found an increase in harbour porpoise activity within an operating windfarm compared to reference sites further away but raised uncertainty about the drivers of attraction as the windfarm was also subject to restricted fishing and vessel activity (Scheidat et al., 2011). Here, we also found a significant increase in harbour porpoise occurrence and foraging activity near isolated offshore structures compared to locations further away. Similarly, Clausen et al. (2021) detected an increase in porpoise occurrence within 800 m of an O&G platform compared to more distant areas. Marine artificial structures create artificial reefs that are colonised by epifaunal communities which, in turn, cause an increase in shallow- and mid-water pelagic species (Stanley and Wilson, 2000; Degraer et al., 2020). O&G platforms have been described as highly productive areas that support high fish density (Claisse et al., 2014; Love et al., 2019a). Therefore, one possible explanation for the observed increase in porpoise occurrence and foraging activity closer to the structures is that they provide more foraging opportunities. Alternatively, porpoises may perceive offshore structures as more complex areas to navigate and, since they use echolocation both to navigate and communicate (Clausen et al., 2011; Sørensen et al., 2018), the increase in occurrence and foraging activity detected in this study could partly be explained by changes in echolocation behaviour around structures. However, the attraction to man-made structures has also been detected in seals, which also use these areas to forage (Russell et al., 2014; Arnould et al., 2015). Furthermore, the increase in bentho-pelagic communities around man-made structures is well documented in the literature (Gates et al., 2019; Perry and Heyman, 2020). Therefore, we suggest that the enhanced foraging opportunities around offshore structures is the most plausible explanation for the observed increase in porpoise detections.

Our analyses showed strong diel patterns in the occurrence and foraging activity of harbour porpoises around offshore structures (< 200 m). Porpoise occurrence was significantly higher at night compared to daytime around three of four structures investigated here, while foraging activity was significantly higher at night around all four of them. Similar nocturnal increases in porpoise occurrence and foraging activity near man-made structures were found in previous studies (Todd et al., 2009; Brandt et al., 2014; Clausen et al., 2021). However, in those studies the authors highlighted that any influence of physical structures on predator-prey interactions could be confounded by the lighting on these structures attracting prey species at night (Todd et al., 2009; Clausen et al., 2021). In our study, lighting on all four structures had been reduced to minimum levels required to comply with national and international regulations on aviation and shipping navigation. Consequently, it is likely to be the physical presence of structures that shape these foraging patterns. The precise mechanisms underlying this nocturnal increase in foraging remain unclear, but it seems likely to be related to diel movements of prey or changes in their activity or schooling behaviour (Todd et al., 2009; Brandt et al., 2014; Clausen et al., 2021). Further research to directly investigate activity patterns of fish (Williamson B. J. et al., 2017) in relation to porpoise movements (e.g. Gillespie et al., 2020) are now required to better understand the drivers of porpoise activity around offshore structures.

Importantly, our study also provides direct evidence of a change in harbour porpoise diel patterns in relation to the introduction of an offshore structure. Although baseline data were available from only one of our sites, this analysis demonstrated a clear difference in nocturnal patterns of occurrence and foraging activity when the structure was present compared to the year before the structure was installed ( Figure 4 ). It should be noted that, for this comparison, we used data sets obtained with different devices: CPODs, when the offshore structure was present, and their predecessors TPODs, when the structure was absent (Thompson et al., 2010). Current CPODs include a more sophisticated detection algorithm, which results in a lower false positive rate compared to TPODs. Therefore, the absolute differences in levels of porpoise occurrence and foraging activity cannot be directly compared. Nevertheless, the difference in the diel patterns within each of these data sets should be robust to device-specific differences in detection probability. Additionally, the larger contemporary dataset also demonstrated a clear increase in nocturnal occurrence and foraging around the structures (< 200 m) compared to locations further away ( Figure 3 ). Together, these two datasets provide strong support for the hypothesis that the change in diel patterns of porpoise occurrence and foraging are linked to the presence of structures. Future studies using similar devices before and after the installation of a man-made structure could investigate this further.

While a significant increase in nocturnal foraging was detected around all four offshore structures studied here, the highest increase was detected around Beatrice Bravo (from 0.29 during daytime to 0.94 during night-time; Figure 3B ). More complex subsea structures exhibit higher fish density and greater species richness (Love et al., 2019b). Furthermore, species diversity also increases with the age of the artificial reef (Perkol-Finkel and Benayahu, 2005). Beatrice Bravo is both the oldest and the most complex structure among those investigated here, and, in line with those studies, our results suggest that higher foraging opportunities may exist around it compared to simpler structures, such as the Jacky monopile. Nevertheless, our findings are based only on individual (Beatrice Bravo & Jacky) or pairs (Beatrice Demonstrator) of similar offshore structures, where other factors could confound patterns of predator occurrence and foraging activity. Furthermore, all the structures that we studied had been in the water for at least 12 years, and we could not determine the age at which they started becoming attractive to porpoises. A recent study in the Southern North Sea found no evidence of attraction to a gas production platform in its first five years of operation (Todd et al., 2022), perhaps suggesting that it may take several years for prey communities to develop to levels at which they influence predator activity. Additionally, Clausen et al. (2021) found seasonal variability in porpoise activity around O&G platforms. Previous work in our study area has also shown seasonal changes in porpoise occurrence, with a peak in August, when the present study was conducted (Graham et al., 2019). Whether our observed attraction to structures during this month remains consistent throughout other seasons remains unclear. Further research of larger groups of similar man-made structures throughout their life cycle is now required to investigate how structure age, complexity and seasonality influence the occurrence and foraging activity of marine predators.

In conclusion, our analyses showed that harbour porpoises are attracted to isolated offshore structures and that porpoises use these structures to forage, especially at night. These findings suggest that offshore structures play an important role as foraging areas for some marine mammals, filling a key gap in the ecological understanding of offshore decommissioning in the North Sea (Fowler et al., 2020). These findings now provide important baseline to support the assessment and mitigation of future decommissioning projects.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://doi.org/10.5061/dryad.mpg4f4r2p

Ethical review and approval was not required for the animal study because it was a non-invasive, acoustic observational study of harbour porpoise activity around offshore structures. No animals were captured or tagged during this study and no research or animal ethical assessments were required. Harbour porpoise activity was determined using remote passive acoustic devices on seabed moorings licensed for scientific use by Marine Scotland and consented by the Crown Estate. Moorings were deployed and recovered using vessels with appropriate certification, accreditation, and endorsements.

OF-B: Conceptualization, data curation, formal analysis, funding acquisition, methodology, visualization, writing – original draft. IG: Conceptualization, data curation, funding acquisition, methodology, supervision, writing – review & editing. PT: Conceptualization, funding acquisition, methodology, project administration, resources, supervision, writing – review & editing. All authors contributed to the article and approved the submitted version.

This study was funded by the UK Department for Business, Energy & Industrial Strategy (BEIS) Offshore Energy Strategic Environmental Assessment research programme under contracts OESEA-20-125 and OESEA-21-133, using equipment previously purchased by UK Department of Energy and Climate Change, Scottish Government, Oil and Gas UK, COWRIE and Moray Offshore Renewables Ltd. Funding for the 2005 baseline studies was provided by the EU DOWNVinD Project and Talisman Energy (UK) Ltd., who were not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication.

We would like to thank Bill Ruck, Moray First Marine and colleagues from the University of Aberdeen for assistance with the data collection, and John Hartley for his support through the project and for his comments on the manuscript. We are very grateful to Ithaca Energy (UK) Limited, Repsol Sinopec Resources UK Limited and SSE Generation Limited for allowing access to the Jacky platform, Beatrice Bravo platform and Beatrice wind demonstrator turbine sites.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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