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Fine-scale hydrodynamics influence the spatio-temporal distribution of harbour porpoises at a coastal hotspot

Fine-scale hydrodynamics influence the spatio-temporal distribution of harbour porpoises at a coastal hotspot
Fine-scale hydrodynamics influence the spatio-temporal distribution of harbour porpoises at a coastal hotspot

The coastal Runnelstone Reef, off southwest Cornwall (UK), is characterised by complex topography and strong tidal flows and is a known high-density site for harbour porpoise (Phocoena phocoena); a European protected species. Using a multidisciplinary dataset including: porpoise sightings from a multi-year land-based survey, Acoustic Doppler Current Profiling (ADCP), vertical profiling of water properties and high-resolution bathymetry; we investigate how interactions between tidal flow and topography drive the fine-scale porpoise spatio-temporal distribution at the site. Porpoise sightings were distributed non-uniformly within the survey area with highest sighting density recorded in areas with steep slopes and moderate depths. Greater numbers of sightings were recorded during strong westward (ebbing) tidal flows compared to strong eastward (flooding) flows and slack water periods. ADCP and Conductivity Temperature Depth (CTD) data identified fine-scale hydrodynamic features, associated with cross-reef tidal flows in the sections of the survey area with the highest recorded densities of porpoises. We observed layered, vertically sheared flows that were susceptible to the generation of turbulence by shear instability. Additionally, the intense, oscillatory near surface currents led to hydraulically controlled flow that transitioned from subcritical to supercritical conditions; indicating that highly turbulent and energetic hydraulic jumps were generated along the eastern and western slopes of the reef. The depression and release of isopycnals in the lee of the reef during cross-reef flows revealed that the flow released lee waves during upslope currents at specific phases of the tidal cycle when the highest sighting rates were recorded. The results of this unique, fine-scale field study provide new insights into specific hydrodynamic features, produced through tidal forcing, that may be important for creating predictable foraging opportunities for porpoises at a local scale. Information on the functional mechanisms linking porpoise distribution to static and dynamic physical habitat variables is extremely valuable to the monitoring and management of the species within the context of European conservation policies and marine renewable energy infrastructure development.
Harbour porpoise Phocoena phocoena, Generalised additive model, Biological oceanography, Biophysics, Hydraulic jump, Lee waves, United Kingdom, England, Cornwall
0079-6611
30-48
Jones, A.R.
0bd0a522-f948-4b80-9c25-be108ee78147
Wynn, R.B.
72ccd765-9240-45f8-9951-4552b497475a
Hosegood, P.
0e0b8af0-add4-4f6b-aa8f-974ff31b63c2
De Boer, M.N.
6d0a6c7b-ec3a-47b0-b1e0-9f013e74640f
Butler-Cowdry, S.
c2222758-a4c9-4137-8445-2aa236d44058
Embling, C.B.
4b81299d-64ca-4fde-ab09-1fd780115b80
Jones, A.R.
0bd0a522-f948-4b80-9c25-be108ee78147
Wynn, R.B.
72ccd765-9240-45f8-9951-4552b497475a
Hosegood, P.
0e0b8af0-add4-4f6b-aa8f-974ff31b63c2
De Boer, M.N.
6d0a6c7b-ec3a-47b0-b1e0-9f013e74640f
Butler-Cowdry, S.
c2222758-a4c9-4137-8445-2aa236d44058
Embling, C.B.
4b81299d-64ca-4fde-ab09-1fd780115b80

Jones, A.R., Wynn, R.B., Hosegood, P., De Boer, M.N., Butler-Cowdry, S. and Embling, C.B. (2014) Fine-scale hydrodynamics influence the spatio-temporal distribution of harbour porpoises at a coastal hotspot. Progress in Oceanography, 128, 30-48. (doi:10.1016/j.pocean.2014.08.002).

Record type: Article

Abstract


The coastal Runnelstone Reef, off southwest Cornwall (UK), is characterised by complex topography and strong tidal flows and is a known high-density site for harbour porpoise (Phocoena phocoena); a European protected species. Using a multidisciplinary dataset including: porpoise sightings from a multi-year land-based survey, Acoustic Doppler Current Profiling (ADCP), vertical profiling of water properties and high-resolution bathymetry; we investigate how interactions between tidal flow and topography drive the fine-scale porpoise spatio-temporal distribution at the site. Porpoise sightings were distributed non-uniformly within the survey area with highest sighting density recorded in areas with steep slopes and moderate depths. Greater numbers of sightings were recorded during strong westward (ebbing) tidal flows compared to strong eastward (flooding) flows and slack water periods. ADCP and Conductivity Temperature Depth (CTD) data identified fine-scale hydrodynamic features, associated with cross-reef tidal flows in the sections of the survey area with the highest recorded densities of porpoises. We observed layered, vertically sheared flows that were susceptible to the generation of turbulence by shear instability. Additionally, the intense, oscillatory near surface currents led to hydraulically controlled flow that transitioned from subcritical to supercritical conditions; indicating that highly turbulent and energetic hydraulic jumps were generated along the eastern and western slopes of the reef. The depression and release of isopycnals in the lee of the reef during cross-reef flows revealed that the flow released lee waves during upslope currents at specific phases of the tidal cycle when the highest sighting rates were recorded. The results of this unique, fine-scale field study provide new insights into specific hydrodynamic features, produced through tidal forcing, that may be important for creating predictable foraging opportunities for porpoises at a local scale. Information on the functional mechanisms linking porpoise distribution to static and dynamic physical habitat variables is extremely valuable to the monitoring and management of the species within the context of European conservation policies and marine renewable energy infrastructure development.

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Accepted/In Press date: 11 August 2014
Published date: November 2014
Keywords: Harbour porpoise Phocoena phocoena, Generalised additive model, Biological oceanography, Biophysics, Hydraulic jump, Lee waves, United Kingdom, England, Cornwall
Organisations: Ocean and Earth Science, Marine Geoscience

Identifiers

Local EPrints ID: 367987
URI: http://eprints.soton.ac.uk/id/eprint/367987
ISSN: 0079-6611
PURE UUID: 6290b9d2-7675-4a8f-8a5f-aabb1835906d

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Date deposited: 12 Aug 2014 14:03
Last modified: 16 Sep 2019 18:31

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