Annex I: Movements and Diving Behaviour of Juvenile Grey Seals in Areas of High Tidal Energy

Report

Title: Annex I: Movements and Diving Behaviour of Juvenile Grey Seals in Areas of High Tidal Energy
Authors: Thompson, D.
Publication Date:
July 01, 2012
Document Number: JER3688
Pages: 55
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Citation

Thompson, D. (2012). Annex I: Movements and Diving Behaviour of Juvenile Grey Seals in Areas of High Tidal Energy. Report by Sea Mammal Research Unit (SMRU). pp 55.
Abstract: 

The Marine Renewable Energy Strategic Framework for Wales (MRESF) provides for the sustainable development of marine renewable energy in Welsh waters. As one of the recommendations from the Stage 1 study, a requirement was identified for further evaluation of the occurrence and use of Welsh waters by marine mammals (cetaceans and pinnipeds), and for an initial assessment of the potential risks to these receptors from marine renewable devices. The Phase 1 report for the 'Assessment of Risk to Marine Mammals from Underwater Devices in Welsh waters' (Wilson and Gordon, 2011) is a desk based review of the potential impacts from marine renewables devices on marine mammals. The Phase 2 Report (Gordon et al., 2011), to which this report forms an Annex, focuses on providing additional baseline characterisation information of the occurrence and use of Welsh waters by marine mammals. The Phase 2 report covers both cetaceans (whales, dolphins, propoises) as well as pinnipeds, in this case, grey seals (Halichoerus grypus). This Annex provides an assessment of the grey seal tagging study undertaken in autumn 2009 and 2010.

 

With the exception of a few visual sightings made during cetacean surveys, all the information on seal movements and behaviour has been collected using high resolution fastloc Global Positioning System (GPS) and depth tags which relay data back via the Global System for Mobile Communications (GSM) mobile phone network. These were attached to newly weaned grey seal pups at breeding beaches close to high tidal current sites at the Skerries, Bardsey Island and Ramsey Island in October of 2009 and October and November of 2010. Typically, pups spent the first month or so in waters close to their breeding beaches, spending most of this time in tidal rapid areas, apparently drifting with the current and repeatedly diving to the bottom in a pattern characteristic of foraging dives. With time, animals travelled more widely, one ranging as far as the west of Brittany. In several cases however, seals found other high tidal current areas and appeared to drift and forage within these in a similar way. It is therefore clear that during the first few months of life, when individuals might be expected to be most vulnerable, young seals are making extensive use of high tidal current areas.

 

Pups from both Ramsey and Anglesey dispersed widely by the end of the study, with seals from both sites moving to south east Ireland and Cornwall.

 

Dive patterns show that grey seal pups spend the majority of their time either at the surface or close to the maximum dive depth which is usually the sea bed. They swim directly to the bottom and therefore spend relatively little time in mid water where they would be at risk from collision with tidal turbine blades risk. Movement patterns in areas of high tidal flow close to the Skerries and in Ramsey sound show that there is wide variation in rates of transit between individuals. Most seals transited through the tidal rapids but one seal at each of the sites performed approximately 70% of the transits. This extreme variation would presumably translate into a wide variation in the likely exposure to collision risk.

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