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Variation in harbour porpoise activity in response to seismic survey noise
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Enrico Pirotta *,a, Kate L. Brookes b,c, Isla M. Graham b, Paul M. Thompson b
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Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen AB24 2TZ,
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UK
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Lighthouse Field Station, Institute of Biological and Environmental Sciences, University of
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Aberdeen, Cromarty IV11 8YL, UK
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Marine Scotland Science, Marine Laboratory, Aberdeen AB11 9DB, UK
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*Corresponding author: enrico.pirotta@abdn.ac.uk.
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Short title: Seismic survey affects porpoise activity.
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Summary
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Animals exposed to anthropogenic disturbance make trade-offs between perceived risk and the cost of
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leaving disturbed areas. Impact assessments tend to focus on overt behavioural responses leading to
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displacement, but trade-offs may also impact individual energy budgets through reduced foraging
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performance. Previous studies found no evidence for broad–scale displacement of harbour porpoises
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exposed to impulse noise from a 10-day 2-D seismic survey. Here, we used an array of passive
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acoustic loggers coupled with calibrated noise measurements to test whether the seismic survey
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influenced the activity patterns of porpoises remaining in the area. We showed that the probability of
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recording a buzz declined by 15% in the ensonified area, and was positively related to distance from
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the source vessel. We also estimated received levels at the hydrophones and characterised the noise
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response curve. Our results demonstrate how environmental impact assessments can be developed to
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assess more subtle effects of noise disturbance on activity patterns and foraging efficiency.
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Keywords: activity budget, anthropogenic disturbance, environmental impact assessment, foraging
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efficiency
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Introduction
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Human disturbance may not lead to acute, easily-measurable behavioural responses in exposed
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wildlife [1,2]. Less overt effects can arise from trade-offs that animals make [3], such as deciding to
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remain in disturbed areas and tolerating higher exposure levels where prey are abundant [4].
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However, these trade-offs may also influence activity budgets, potentially affecting fecundity and
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survival [1,5] or having biologically significant consequences at a population level [6–9].
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There is particular concern over the potential impacts of underwater anthropogenic noise on marine
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mammals [10,11]. Aversive behaviour towards intense noise sources such as pile driving (e.g. [12])
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and seismic surveys [13] may not necessarily lead to broad-scale displacement [14]. However, it is not
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known whether there are more subtle changes in the activity budgets of animals remaining in exposed
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areas.
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Patterns of echolocation clicks can be used to characterise the activity of both bats [15] and
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odontocetes [16–19]. Here, we use an array of passive acoustic loggers coupled with calibrated noise
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measurements to assess the effects of a seismic survey on the activity pattern of harbour porpoises
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(Phocoena phocoena).
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Material and Methods
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The study was conducted around a commercial 2-D seismic survey in NE Scotland. The survey vessel
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used a 470 cu inch airgun array with a shot point interval of 5-6 seconds to survey a 200 km2 area
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between 1st and 11th September 2011 (see electronic supplementary material). Full details are provided
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in [14].
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Patterns of porpoise echolocation clicks were characterised at 22 sites in and around the seismic
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survey area throughout August and September 2011 (Fig. 1). Data from V.0 and V.1 CPODs
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(www.chelonia.co.uk) had previously been used to characterise the occurrence of porpoises [14,19].
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While the sensitivity of C-PODs at different sites could vary between different units or different local
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conditions, our sampling design and mixed modelling approach were designed to account for this (see
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electronic supplementary material).
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We calculated the inter-click intervals (ICI) from the time series of detected clicks. A Gaussian
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mixture-model was fitted to log-transformed ICIs to identify different patterns of echolocation clicks
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[16]. Each ICI was classified as either a regular ICI (regular clicking for navigation and prey
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searching), a buzz ICI (buzzes associated with attempted prey captures or social communication), or
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an inter-train ICI (pauses between click trains) [20,21] (see electronic supplementary material).
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Following [14], we used data from CPODs in 25 km by 25 km control and impact areas (Fig. 1; see
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electronic supplementary material) to assess the effect of seismic surveys on the occurrence of buzz
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ICIs when porpoises were present (i.e. in hours in which at least one ICI was detected). The presence
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of buzz ICIs was modelled as a function of seismic period (before or during) and experimental block
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(control or impact). Crucially, we also evaluated the interaction between these covariates to test
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whether seismic surveys influenced buzzing activity. We fitted a binary mixed-effects model in R
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13.01 [22], with location included as a random effect. Model selection was based on AIC and the
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significance of retained covariates evaluated using Wald’s tests.
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The presence of buzz ICIs for each minute in which at least one ICI was detected was then modelled
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as a function of distance from the seismic vessel. Following previous studies [12,14] on likely spatial
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scales of porpoise responses to noise, we used data from within 25 km of the vessel (see electronic
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supplementary material). A binary Generalized Linear Model was fitted using Generalized Estimating
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Equations (GEE-GLM) [23] (see electronic supplementary material). We used QICu for model
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selection [24], and Wald’s tests to assess significance [25].
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Calibrated noise measurements were made at 15 sites between 1.6 km and 61.8 km from the seismic
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vessel [14]. Airgun noise was characterised in terms of broadband sound exposure level for single
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pulses in dB re 1 μPa2s (using the region of the waveform that contained the central 90% of the
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pulse’s energy; hereafter SEL). Peak-to-peak source levels were estimated to be 242 –253 dB re 1 μPa
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at 1 m. We tested whether there was a relationship between buzz occurrence and estimated noise
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levels at the corresponding CPOD location using a bootstrapping procedure (see electronic
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supplementary material).
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Results
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There was a significant interaction between experimental block and seismic period (Wald’s test:
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χ2=7.2; df=1; p=0.007), indicating that the seismic survey had a differential effect in the control and
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impact block. This resulted in a 15% reduction in the occurrence of buzz ICIs in the impact block
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during the survey (Fig. 2). Wide confidence intervals highlight high levels of natural variability in
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detection of buzzes.
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The probability of occurrence of buzzes increased significantly with distance from source (Wald’s
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test: χ2=9.6; df=1; p=0.002), ranging from approximately 0.15 at 0 km (95% confidence interval (CI):
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0.11-0.19) to 0.35 at 40 km (95% CI: 0.25-0.48) (Fig. 2).
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Noise measurements allowed us to characterise this relationship in terms of estimated received noise
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levels (Fig. 2). This probability increased from 0.07 (95% CI: 0.03-0.17) to 0.31 (95% CI: 0.16-0.52)
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for SEL that varied from 165 to 130 dB re 1 μPa2s (Fig. 2).
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Discussion
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Short-term responses to this seismic survey did not result in broad scale displacement, suggesting that
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impact assessments should focus on sub-lethal effects within affected sites [14]. Our results indicate
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that porpoises remaining in the impact area reduced their buzzing activity by 15% during the seismic
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survey. Moreover, the probability of detecting buzz ICIs when porpoises were present increased with
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distance from the source vessel, suggesting that the likelihood of buzzing was dependent upon
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received noise intensity. The baseline probability of occurrence of buzzes was around 0.4 in the
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impact block before the survey, although with high natural variability. This probability declined to
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0.1-0.2 at estimated received SEL of 150-165 dB re 1μPa2s (Fig. 2). Such changes were unlikely to
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result from the presence of seismic noise, since this was mostly below 400 Hz [14], while most energy
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in porpoise clicks is within 110-150 kHz [26].
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Porpoises may use high-repetition click trains for prey capture or for social communication [27].
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Thus, observed changes in buzzing occurrence could reflect disruption of either foraging or social
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activities. Despite this uncertainty, these data provide a worst-case indication of the extent to which
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foraging was disrupted by noise exposure. These effects may result from prey reactions to noise [28],
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leading to reduced porpoise foraging rates. Alternatively, foraging effort may change if porpoises
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adjust time budgets or diving behaviour to avoid noise. Irrespective of the proximate mechanism(s)
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through which seismic surveys reduced buzzing rates, this clearly has the potential to affect the
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energy balance of exposed animals.
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High metabolic rates [29] mean that porpoises have limited ability to cope with prolonged starvation
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[30]. Our results provide an estimate of the noise levels at which porpoise activity patterns are
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disrupted, and an indication of the scale of potential reductions in foraging activity [6]. Porpoise
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occurrence and activity is typically characterised by large seasonal and diel variability [19,31,32], as
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also reflected in our results (Fig. 2). Further studies are now required to explore the environmental
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conditions that drive this variation, and develop energetic models to assess whether this scale of
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disturbance has long-term consequences for individual energy budgets [33].
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Acknowledgements
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We thank colleagues at Kongsberg Maritime, Moray First Marine, Gardline Geosurvey and the
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Scottish Fishermen’s Federation for essential support in the field, and those at DECC, PA Resources
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and Caithness Petroleum Ltd for ensuring that this research was effectively integrated into their
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regulatory and commercial activities. The project benefitted at all stages from a scientific steering
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group and broad stakeholder group with statutory and NGO representation. We thank them, and Tim
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Barton, Keith Needham, Alex Douglas and the many other colleagues who provided support during
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both field work and data analysis. Thanks to David Lusseau and two anonymous reviewers for their
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useful comments and suggestions.
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Data accessibility
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The data set used in this paper is available in the Dryad data repository: doi:10.5061/dryad.1847s
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Funding statement
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Data collection was funded through a partnership project that was co-ordinated by the UK Department
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of Energy and Climate Change (DECC) and jointly funded by DECC, Scottish Government, Oil &
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Gas UK Ltd, and COWRIE. Enrico Pirotta was supported through the MASTS pooling initiative (the
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Marine Alliance for Science and Technology for Scotland). MASTS is funded by the Scottish
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Funding Council (grant reference HR09011) and contributing institutions. None of the analysis nor
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write up of the paper involved or was influenced by the sponsors of the research.
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Figure
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Figure 1. Map of the study area showing the seismic survey lines and the locations of CPOD
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deployments.
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Figure 2. Modelling results. (a) Changes in the probability of feeding buzz occurrence in control and
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impact blocks, before and during the survey. (b) Estimated relationship between the probability of
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buzz occurrence and distance from the source vessel. (c) Estimated relationship between the
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probability of buzz occurrence and unweighted sound exposure level (SEL).