The spatio-temporal distribution and habitat associations of marine mega-vertebrates off southwest UK

Abstract

This thesis uses data collected between 2007 and 2010 in an intensive, effort-based visual monitoring survey in southwest Cornwall, UK. The survey was carried out from a strategic watchpoint overlooking a regionally unique seabed feature, the Runnelstone Reef, which has previously been identified as key site for all three of the study’s target species: harbour porpoise Phocoena phocoena, basking shark Cetorhinus maximus and Balearic shearwater Puffinus mauretanicus. The location of the survey site is perceived as a productive, coastal marine ‘hotspot’ by local wildlife observers, eco-tour companies, conservation bodies and commercial fishermen. The aim of the study was to use a multidisciplinary approach to investigate the drivers behind the increased abundance and diversity of mega-vertebrates at the site. A wide range of environmental data, from fine-scale bathymetry to remote-sensed oceanographic data, were utilised in an attempt understand the potential interactions between the target species and the environment at a variety of scales.
Although the target species have very different ecologies and each represent a different taxonomic Class; they each face significant threats throughout their range due to human impacts and are all listed as species of conservation concern on a number of UK, European and International Directives and Conventions. Therefore, improving our understanding of their distribution and highlighting interactions between the animals and their environment is an important objective, both for science and conservation.
Harbour porpoise sightings showed significant fine scale temporal clustering associated with tidal flow, as well as spatial clustering around parts of the survey area with steepest seafloor slopes. The timing and location of highest porpoise densities coincided with tidal-topographically controlled hydrodynamic features identified using fine-scale current profile data (ADCP). The sightings and acoustic monitoring (C-POD) data both showed a high amount of temporal variability at seasonal, daily and hourly scales, highlighting the complex nature of the fine scale animal-environment interactions.
Daily patterns in basking shark sightings data were investigated as a function of physical environmental covariates, with particular focus on the effect of meso-scale thermal fronts. In line with previous national scale studies of the species, shark sightings were significantly affected by sea surface temperature (SST) and there was evidence for a change in seasonal abundance compared to the long-term pattern. In addition, the variance of SST over the preceding weeks was identified as being a key predictor of the abundance of sharks in the survey area. Surprisingly, there was not a significant effect of the presence, persistence or thermal gradient strength of fronts on daily shark sightings. The implications of this result are discussed with reference to results of previous studies and the focus of marine protected area policy in the UK.
The broad scale spatio-temporal analyses of Balearic shearwater sightings data from the UK, Ireland and France indicate that the birds continue to be recorded in significant numbers throughout areas previously considered to be at the northernmost extent of their range. Record counts of passing birds were recorded off southwest UK in the last two years, along with unprecedented aggregations in bays along the Brittany coast, comprising approximately 20 % of the estimated global population. The data presented provide much-needed quantitative information on the at-sea distribution and behaviour of this Critically Endangered species during the interbreeding period, and support earlier studies suggesting a northwards shift in their migratory distribution.

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