Regional-scale patterns in harbour porpoise occupancy of tidal stream environments

Abstract

As harbour porpoises Phocoena phocoena are abundant within tidal stream environments, mitigating population-level impacts from tidal stream energy extraction is considered a conservation priority. An understanding of their spatial and temporal occupancy of these habitats at a regional-scale will help steer installations towards locations which maximize energy returns but reduce the potential for interactions with populations. This study quantifies and compares relationships between the presence of harbour porpoise and several hydrodynamic characteristics across four tidal stream environments in Anglesey, UK—a region that has been earmarked for extensive industrial development. Within sites (0.57–1.13 km2), encounters with animals were concentrated in small areas (<200 m2) and increased during certain tidal states (ebb vs. flood). In sites showing relatively high maximum current speeds (2.67–2.87 ms−1), encounters were strongly associated with the emergence of shear-lines. In sites with relatively low maximum current speeds (1.70–2.08 ms−1), encounters were more associated with areas of shallow water during peak current speeds. The overall probability of encounters was higher in low current sites. It is suggested that the likelihood of interactions could be reduced by restricting developments to sites with high maximum current speeds (>2.5 ms−1), and placing turbines in areas of laminar currents therein. This study shows that a combination of local and regional hydrodynamic characteristics can partially explain variations in occupancy patterns across tidal-stream environments. However, it was found that such hydrodynamic characteristics could not comprehensively explain these occupancy patterns. Further studies into the biophysical mechanisms creating foraging opportunities within these habitats are needed to identify alternative explanatory variables that may have universal applications.

Oceanographic drivers of marine mammal and seabird habitat-use across shelf-seas: A guide to key features and recommendations for future research and conservation management

“…In near-shore coastal regions, marine mammals and seabirds frequently forage within tidally active areas (Nol and Gaskin, 1987;Marubini et al, 2009;Anderwald et al, 2012;Benjamins et al, 2015;Warwick-Evans et al, 2016;Waggitt et al, 2018), resulting in distinct regularities in their distributions and behaviours that coincide with particular tidal phases (Becker et al, 1993;Irons, 1998;Isojunno et al, 2012;De Boer et al, 2014;Ijsseldijk et al, 2015). Specifically, areas such as narrow channels, headlands, islands, reefs and bays often function as periodic foraging hotspots, where interactions between strong tidal currents (often exceeding 1.5ms -1 ) and complex topography create prosperous foraging opportunities for marine predators (Cairns and Schneider, 1990;Coyle et al, 1992;Zamon, 2003;Benjamins et al, 2015;Couperus et al, 2016).…”

First in situ passive acoustic monitoring for marine mammals during operation of a tidal turbine in Ramsey Sound, Wales

“…The site-specific and spatiotemporally variable distributions of porpoises in energetic tidal areas (Gordon et al 2011, Benjamins et al 2017, across tidal state, emphasizes the value of understanding occupancy patterns when conducting risk assessments (Scott et al 2014, Waggitt et al 2017a. For example , Macaulay et al (2015) showed varying porpoise behaviours at 3 different tidal rapid sites in Scotland, with regard to both depth distribution and swim direction relative to the current.…”

“…Previous studies (e.g. Hastie et al 2016, Macaulay et al 2017, Waggitt et al 2017a have investigated marine mammal habitat use and behaviours in tidal areas in the absence of any tidal energy infrastructure. While such knowledge is relevant given that collision risk depends on animal density and diving behaviour (Hastie et al 2014), these studies did not measure avoidance behaviour.…”

No leading-edge effect in North Atlantic harbor porpoises: Evolutionary and conservation implications

“…Santos & Pierce, 2003; but see Nielsen et al 2018). Coined the ‘aquatics shrews’ of the sea, prey availability has been shown to be an important driver of porpoise movements (Johnston et al, 2005; Sveegaard et al, 2012; Wisniewska et al, 2016) and local densities (Hammond et al, 2013; Marubini et al, 2009; Waggitt et al, 2018). These animals are thus expected to be highly susceptible to environmental changes, increasing sea temperature, and prey displacements or modifications (Lambert et al, 2014; MacLeod et al, 2005).…”