Investigating the influence of fine-scale physical processes on the spatio-temporal distribution of marine megavertebrates off southwest UK

Abstract

The primary aim of this PhD research is to describe environmental controls on distribution patterns of free-ranging marine top predators in tidally-dominated coastal waters off southwest UK, at a spatial resolution of metres to hundreds of metres, and a temporal resolution of hours to months. As human impacts increase in the nearshore zone (e.g. wet renewables), the need to better understand such fine-scale controls on distribution is critical, particularly as highly mobile marine megavertebrates in southwest coastal waters are amongst some of the region’s most threatened species (e.g. basking sharks Cetorhinus maximus, harbour porpoises Phocoena phocoena, and the critically-endangered Balearic shearwater Puffinus mauretanicus). Current policy drivers (e.g. Marine Protected Area designation and management) require the development of appropriate low-cost methodologies for land-based data collection, in order to provide robust scientific evidence on fine-scale habitat use of these threatened species.
To improve understanding of how the physical environment influences habitat use of vulnerable marine predators, a complementary and multi-disciplinary suite of techniques for data collection and analyses was developed. This methodological ‘toolbox’ included: a theodolite to collect highly-accurate (<50 m) positions of animals at sea; acoustic data loggers to collect continuous, subsurface echolocation data on cetacean presence; fine scale (1-m) resolution seafloor bathymetry; novel radar-derived sea surface metrics; and temporally highly-resolved (30-min) met-ocean data (e.g. weather and tides). These data were integrated into statistical predictive models to identify significant drivers of distribution; information that can be used to inform local policy at the study sites and provide new knowledge on the target species.
To test the utility of the methods, two contrasting locations of different physical habitat were selected as study sites: the tidally-swept, high-energy, bedrock-dominated Runnel Stone Reef off the southwest tip of the UK mainland; and St Ives Bay, a shallow, sediment-dominated, gently-sloping bay on the northwest Cornish coast. Both sites are known ‘hotspots’ for top predators, and both have an urgent requirement for species’ distribution data to meet current and future management requirements.
Theodolite sightings data were collected at both study sites for a variety of small cetaceans and seabird species. All target species were influenced by small-scale topographic features at the scale of metres to tens of metres, with areas of steeper slopes and strong tidal flows (i.e. tidal-topographic fronts) forming particular hotspots. At St Ives Bay, shallow sheltered nearshore habitats were preferentially utilised. Small-scale commercial fishing activity showed significant overlap with target species’ distributions, highlighting the additional importance of these sites for human predators. Significant relationships with tidal flow parameters (e.g. current direction and tidal range) provided further insights into the physical processes driving these clustered sightings.
Acoustic data collected from the Runnel Stone Reef provided valuable supporting data on porpoise and dolphin activity, including a strong diel pattern and fine-scale spatial variation in habitat use that was species-specific. The importance of accounting for variability in survey conditions (e.g. acoustic noise, and wind, cloud and sea state) across land-based and subsurface analyses was also highlighted.
The methodological toolbox developed and successfully applied in this study, comprising high-resolution visual and acoustic data, and a variety of environmental parameters, provides a relatively low-cost and effective method for determining fine-scale habitat use of mobile top predators in dynamic, topographically-complex nearshore environments, where vessel-based surveys may be impractical.

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