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Assessing small-scale heterogeneity in harbour porpoise distribution in energetic environments: the case for PAM arrays
Marine renewable energy (MRE) projects typically require site-specific baseline assessment of cetacean presence and abundance. Given the relatively high financial costs and logistical complexity of such assessments, appropriate data collection decisions are vital to avoid wasting resources. For cetaceans, long-term datasets are often collected using a single passive acoustic monitoring (PAM) instrument deployed on-site. This approach assumes that data collected by one device will be broadly representative of the entire site, but this is rarely verified. Small-scale (<5 km) site usage by harbour porpoise (Phocoena phocoena) was investigated by deploying arrays of C-POD passive acoustic detectors in different Scottish sites (wave-exposed: A] the European Marine Energy Centre [EMEC], Orkney, and B] Sound of Mull; tidal-stream: C] Kyle Rhea and D] Scarba channels & adjacent waters). Array designs were variable, containing 4- 19 moorings with 1-2 C-PODs each, which were deployed for up to 61 days; mean closest inter-mooring distances ranged from 336 – 874 m. Porpoise encounter data were analysed using standard statistical methods (e.g. Analysis of Variance, F-tests). Although porpoises were observed regularly across all arrays, significant variation was noted between adjacent detectors’ daily encounter rates. For example, average daily encounter rates at one end of the EMEC wave site array (<1 km across) were nearly half of those at the other end (F = 6.52; p<<0.001). This variation in encounter rates may be driven by anthropogenic influences (e.g. boat traffic, infrastructure) or by small-scale environmental variability (e.g. diel cycle, tidal phase and distance from shore). Results indicate that porpoises display considerable spatiotemporal variability in distribution across scales of several hundred metres and hours which is unlikely to be adequately captured by isolated detectors yet may have implications for impact assessment. MRE site developers and regulators should consider this variability when designing future site-specific data collection programmes for cetaceans.