Feeding associations between Grey Phalaropes Phalaropus fulicarius and Basking Sharks Cetorhinus maximus over a tidal-topographic front off southwest Cornwall

Russell B Wynn

National Oceanography Centre, European Way, Southampton SO14 3ZH, UK.

Full paper

Abstract

This paper documents a previously unreported feeding association between Grey Phalaropes Phalaropus fulicarius and Basking Sharks Cetorhinus maximus. The observations were made during effort-based visual monitoring from Gwennap Head in southwest Cornwall, in October 2008. Flocks of up to 50 Grey Phalaropes were seen to follow and aggregate around surface-feeding Basking Sharks, mostly in an area characterised by a prominent tidal-topographic front. It is proposed that Grey Phalaropes were initially attracted to visual manifestations of upwellings and convergence zones over and adjacent to this front (with enhanced surficial prey availability), and then opportunistically used the tall dorsal fins of foraging Basking Sharks as a visual cue to home in on particularly dense prey patches.

Introduction

The 'SeaWatch SW' project, which ran from 2007 to 2011, included intensive effort- based visual observations from Gwennap Head, southwest Cornwall (e.g. Wynn & Brereton 2009; Wynn et al. 2010). This site is at the southwest tip of the UK mainland, and is ideally situated for observing migrating seabirds moving around the southwest peninsula. A primary aim of the project was to assess the flux of seabirds passing the watch-point during the autumn migration period, from 15 July to 15 October. Between these dates, the watch-point was manned daily from dawn until dusk, and over the five years of survey ~5,000 hours of effort-based data were collected. This short paper describes a series of observations of Grey Phalaropes Phalaropus fulicarius associating with foraging Basking Sharks Cetorhinus maximus off Gwennap Head in October 2008. The watch-point overlooks Runnelstone Reef, a horseshoe-shaped rocky platform that delimits an inshore zone of shallow water (< 10-20 m depth) from deeper water (> 50 m depth). A visible tidal-topographic front is associated with the depth change approximately 1.5 km offshore (Figure 1; Jones 2012).

Acknowledgements

SeaWatch SW was financially supported by the following organisations: Total Foundation, RSPB, BTO, SAHFOS, The Seabird Group, Royal Naval Bird-watching Society, Birdguides, and Marine Information Ltd. I would particularly like to thank the large number of volunteer observers who assisted with data collection, and the assistant co-ordinators, John Swann and Alice Jones. Two Seabird reviewers provided useful comments that helped to improve the manuscript.

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