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 predictable 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 two-fold higher than that 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, during May-July, regardless of their size.
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.

Study site and experimental design

Acoustic monitoring was carried out from July 2013 to July 2015, to record noise levels and harbour porpoise echolocation activity around DanF, located 200 km west of Denmark (Fig. 1). The study area is characterised by sandy sediments and relatively flat bathymetry ranging between 40 and 46 m (Fig. 2, Delefosse et al. 2017). Around all Danish O&G installations, a 500-m fishing exclusion zone is enforced, which corresponds to an area of approximately 1.9 km² around DanF. To investigate spatial variation in porpoise echolocation activity, 18 acoustic monitoring stations were deployed around DanF at various distances and with replicates along two transects for most distances to avoid data loss (2x0 m, 2x0.2 m, 2x0.4 km, 2x0.8 km, 2x1.6 km, 2x3.2 km, 2x6.4 km, 9.6 km, 12.8 km and 2x25.6 km; Fig. 2). Station locations were chosen to avoid overlap with other O&G installations and minimize variation of environmental factors (e.g. depth). The station located at 12.8 km was placed at Regnar, an inactive subsurface wellhead with dimensions 7.5 m x 6.5 m x 4.5 m that may act as an artificial reef, but does not emit any sound. One station was placed at 25.6 km from DanF as a control stations for the central North Sea (no reef effect and no noise from platforms are present. Note that only six months recording is available, Table S1). At all stations, harbour porpoise presence was monitored using C-PODs (Chelonia Ltd, Penzance, UK); these are calibrated acoustic click detectors (Clausen et al. 2018). Furthermore, five stations (0, 0.2, 0.8, Regnar and 25.6 km away from DanF) were equipped with calibrated broadband loggers (SM2M+ or SM3M+, Wildlife Acoustics, Boston, USA) to record noise emitted from the platform and other sources in the area. All equipment was deployed 2-3 m above the seafloor with the hydrophone pointing upwards to cover the entire acoustic water column. The potential effect on echolocation and noise recordings of instruments being placed near the bottom was considered insignificant (see Supplementary material).

To verify findings from DanF and Regnar, C-PODs data deployed at 200 m from three other platforms (HalfdanB, Skjold and Kraka), were available from a different project during May-June 2014 (Fig. 2). Together with DanF and Regnar, the three platforms represent a relatively wide range of sizes, industrial activity, light and sound level typical of O&G installations in the shallow (<100 m) central North Sea (seabed footprint 93-6996 m², in the following order from small to large: Regnar only subsea, Kraka small and unmanned, Skjold, HalfdanB and DanF large platforms with light and manned, Delefosse et al., 2017).

Data analysis – porpoise detections

C-POD data were processed with CPOD.exe v2.043. Following Clausen et al. (2018), the click train filter was set to include click trains with a minimum of five clicks and a mean instantaneous frequency between 100 and 160 kHz, avoid transient erroneous clicks from unknown sources to be included. Data were exported as Porpoise Positive Minutes (PPM, i.e. a minute where at least one click train is detected). In addition, all occurrences of click trains with inter-click-intervals (ICI) shorter than 15 ms were extracted. Whilst such short ICIs have been shown to mediate social communication in porpoises (Sørensen et al. 2018), they are more frequently characteristic of echolocation buzzes (74% vs. 26%; Sørensen et al. 2018), which are used during prey pursuits (Verfuss et al. 2009; Wisniewska et al. 2016). We therefore assume that the majority of such click trains are a measure of foraging effort, and they are presented here as Buzzing positive minutes (BPM).

Only full recording days (1440 minutes) were considered in spatial and temporal analyses. The instruments were set to stop recording when it reached a maximum value of 4095 clicks per minute to avoid memory overload. Due to this truncation, minutes with more than 4095 clicks were excluded from the analysis, as they were considered saturated and incomplete. Each minute within 24 hours was assigned to either ‘day’ or ‘night’ using civil twilight (http://aa.usno. navy.mil). To reduce any potential influence of an unbalanced data design, only C-PODs with data from a minimum of three days within a month were included in the analyses.

We initially explored spatial (distance to platforms) and temporal (diel to month-to-month scale) patterns in porpoise echolocation activity using generalized additive mixed models (GAMM, see Supplementary material). Based on these preliminary results, we found two distinct porpoise activity periods and therefore grouped data from February-June and July-January.

We then calculated the % PPM for day and night (D/N) conditions at all stations for the two identified periods separately. Percentage of PPM was logit-transformed to fulfil the assumption of normality of residuals (Warton and Hui 2011) and analysed using linear mixed effects models (one model for each period) with the interaction between distance to platform and D/N conditions classed as fixed effects. For all mixed models (here and below), C-POD ID, transect ID, and year were fitted as nested random effects. Temporal dependence among observations was modelled using an autocorrelation structure of order 1 (corCAR1), because this structure provided the best model compared to other correlation structures based on lowest AIC (Pinheiro and Bates 2000). Moreover, for all mixed effects models constructed, any significant differences in the response variable between and within fixed effects groups were estimated using the Bonferroni-corrected Tukey honest significant difference test.

We computed the percentage ratio of BPM to PPM (i.e. percentage of time with echolocation clicks that was dedicated to buzzing) for D/N at all stations for the two identified periods separately. To simplify interpretation of the results, stations were grouped as follows: ≤ 800 m from DanF (the platform), 6.4 km from the platform (control), Regnar (12.8 km from platform) and 25.6 km from the platform (control), for this analysis the other distances were excluded to make a simple comparison between platforms vs. control stations. These station groups were, a posteriori, identified as representing "reef/platform noise”, “no reef/ low platform noise”, “reef/no background noise” and “no reef/no background noise”, respectively. Percentage of BPM/PPM was logit transformed to fulfil the assumption of normality of residuals and analysed using linear mixed effects models (one model for each period) with an interaction between distance and D/N conditions as fixed effects.

For inter-platform comparison, we calculated the %PPM for each day of May, June and July 2014 using data recorded at 200m from the five platforms and at a control area (6.4 km from DanF). Percentage of PPM was logit transformed to fulfil the assumption of normality of residuals and analysed using linear mixed effects models with the interaction between month and platform (+control) as a fixed effect.

Data analysis – noise

The broadband acoustic data were analysed using custom-written routines in Matlab R2015a (Mathworks, Inc., Natick, MA, USA). For ease of processing, the 46-minute-long SM2M+ or SM3M+ sound files were subdivided into sections of 10-second duration that were subsequently averaged over 5 minutes. Prior to averaging, the routines excluded logger artefacts and porpoise clicks from further analysis (Clausen et al, 2018). Up to 10 10-second-long sections per 46-minute file were removed this way. 

The mammalian auditory system is typically modelled as a filter bank approximated by one-third octave bands (Richardson et al., 1995). Hence, we calculated the received third-octave rms sound pressure levels (TOLs, dB re 1 µPa rms), using a third-octave filter bank implemented in Matlab according to the ANSI standard S1.6-1984 (Christophe Couvreur, Faculte Polytechnique de Mons, Belgium). Occasionally, very brief noise events exceeded the clip level of the broadband recording device, but given the temporal resolution of the data, this should not have affected the average TOL values.

Our analysis focused on bands with the highest noise levels (centred at 315 Hz, 5.0 and 31.5 kHz), and on a subset of bands highlighted in the EU’s Marine Strategy Framework Directive (centred at 63, 125 and 2,000 Hz). For each frequency band, we tested for differences in ambient noise levels (mean TOL over 5 minutes) as a function of distance to the platform, taking into account both D/N conditions and the two porpoise activity periods identified with the GAMMs. To do so, we fitted linear mixed effects models with the 3-way interaction between distance to platform, D/N condition and porpoise activity period as a fixed effect.