Thursday, 26 September 2019

Grampus griseus: Risso’s Dolphins in a proposed Marine Protected Area off east Lewis, Scotland.

Risso's Dolphins, Grampus griseus, are a globally distributed species found in almost all marine waters except those of the Arctic and Antarctic, favouring coastal waters at mid-temporate latitudes. The species is considered to be present in British waters year round, but is distinctly uncommon in the North Sea and eastern English Channel, being more regularly seen in coastal waters around the Shetland Islands, the Outer Hebrides, the Irish and Celtic Seas, and the western portion of the English Channel. There is thought to be a notable concentration of these Dolphins off the coast of Scotland around the eastern coast of the islands within the Minch, the Little Minch and the Sea of Hebrides, and in particular along the east and north coasts of the Isle of Lewis around the Eye Peninsula and Butt of Lewis, areas that fall within the proposed North-east Lewis Marine Protected Area.

In a paper published in the Journal of the Marine Biological Association of the United Kingdom on 18 February 2019, Carloine Weir of Whale and Dolphin Conservation and Ketos Ecology, and Nicola Hodgekins, Sarah Dolman, and Alice Walters, all also of Whale and Dolphin Conservation, publish the results of a series of surveys of the waters off the east coast of the Isle of Lewis for Risso's Dolphins made between 2010 and 2017

.A Risso's Dolphins, Grampus griseus, off the coast of Scotland. Nicola Hodgekins/Scottish Natural Heritage.

The first survey of Risso's Dolphins in the waters off the coast of Lewis were carried out around the Eye Peninsula during the summers of 1995 and 1996, identifying Branahuie Bay, Tiumpan Head and Kebock Head as favoured locations. Photo identification determined that there were at least 142 individual Dolphins present in the area at some time during the surveys, with 52 Dolphins identified in both years. This led to the the waters along the north-east coast of the Isle of Lewis from the Butt of Lewis to Loch Erisort were identified as areas that should be included in the proposed  North-east Lewis Marine Protected Area.

Weir et al. colated data from surveys carried out by Whale and Dolphin Conservation between 2010 and 2017 in the waters along the east coast of Lewis, extending from Tolsta Head in the north to the Shiant Isles in the south, an area overlapped with the southern portion of the proposed Marine Protected Area. Dedicated boat-based dolphin surveys were carried out using a variety of small motor boats, giving observers an average eye level of 2 m above the sea. In addition a larger (20 m) vessel was used in 2017, which gave an average eye level of 6 m above the sea.The surveys departed from various harbours including Bayble and Portnaguran on the Eye Peninsula, Keose in Loch Erisort, and Stornoway. The route usually followed a similar outward and return track to the Eye Peninsula although a small number of trips surveyed a large loop that included inshore and offshore legs. A total of 72 boat surveys were carried out over the course of the study, almost all in August and September.

Map of the study area, shown also in a box in relation to the wider region of north-west Scotland. Bathymetry is 50 m (dot) and 100 m (dash). Weir et al. (2019).

Shore-based surveys were also carried out from Tiumpan Head between 2011 and 2017. This was done only on days when visibility was good, and the species could be identified with confidence. Photo identification was again used to try to track individual Dolphins, targeting the the dorsal fin and the region of flank located immediately forward of, behind and below the dorsal fin, an area which is visible during breaching and upon which many (but not all) Risso's Dolphins have distinctive markings.

During the boat surveys the team registered 32 primary sightings and eight re-sightings of dolphin groups, with all of these sightings happening in August and September. The sighting locations were predominantly distributed all around the south and east coasts of the Eye Peninsula, with one sighting
1.8 km off Tolsta Head at the northern limit of the study area, and one 3 km from the coast to the north-east of Loch Erisort. No Risso’s dolphins were recorded in the southern portion of the study area between Loch Erisort and the Shiant Isles. Group sizes of between one and fifty Dolphins were recorded, though most Dolphin groups comprised less than fifteen individuals. Group sizes were on average larger when calves were present, with twelve of the 52 sightings being of groups with calves. The Dolphins were most commonly sighted in waters between 20 and 40 m deep, within a kilometre of the shore.

Risso's Dolphins were spotted from the Tiumpan Head 271 times between September 2011 and December 2017. The Dolphins were spotted at every time of year, but were more commonly seen at certain times; in the Tiumpan A field (to the north) they were seen the most between May and August, while in the Tiumpan B field (to the south) they were seen the most between May and October. Sightings were rare from December to April. Calves were only recorded between April and October, with peaks during July (both areas), August (Tiumpan B) and October (both areas).

Risso's Dolphins off the coast of Gwynedd, Wales. BBC.

A total of 131 dolphins was catalogued photographically between 2010 and 2017. Most of these were seen only in a single year, though 26 were seen in more than one year, and two were seen during six different years. Four of the Dolphins spotted in 2010 were also seen in 2017. It was also noted that the Dolphin's spotted the highest number of times were also those with the most distinctive markings; this casts some doubt on the use of photographic identification with Risso's Dolphins, though the presence of the recognisable Dolphins over multiple years does suggest that they are permanent residents of the area.

The Dolphins observed by the survey were primarily distributed in nearshore (within 5 km of the coast) waters along the south and east coasts of the Eye Peninsula during the late summer. This area falls within the proposed  North-east Lewis Marine Protected Area. Only one sighting was made outside this area during the survey, a group of Dolphins that started out within the confines of the proposed protected area but moved across its southern boundary while being observed. However the 1995-96 surveys also detected Dolphins within Branahuie Bay, which is not part of the proposed protected area, and the 2010-17 survey spotted Dolphins within tens of metres of the cliffs around the southern Eye Peninsula on several occasions, potentially a vulnerable area as the proposed boundaries do not extend up to the shoreline. Weir et al. recommend that the boundaries of the proposed protected area be adjusted to include these areas.

See also...

https://sciencythoughts.blogspot.com/2019/09/phocoena-sinus-vaquita-porpoise-in.htmlhttps://sciencythoughts.blogspot.com/2015/09/isthminia-panamensis-south-american.html
https://sciencythoughts.blogspot.com/2015/01/a-fossil-porpoise-from-early-pliocene.htmlhttps://sciencythoughts.blogspot.com/2014/10/a-fossil-dolphin-from-late-miocene-of.html
https://sciencythoughts.blogspot.com/2014/10/a-new-species-of-dolphin-from-early.htmlhttps://sciencythoughts.blogspot.com/2013/09/a-fossil-porpoise-from-early-pliocene.html
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