Field methods

The study area comprised the waters of the Minch, situated off the west coast of Scotland (Figure 1). This proposed study area was identified as a region where white-beaked dolphins were expected to occur based on previous studies (Reid et al., 2003). The Minch comprises a 35–60 km wide stretch of water located between the Scottish mainland and the Isle of Lewis in the Outer Hebrides. 

The survey platform was the 16m long Hebridean Whale and Dolphin Trust (HWDT) research ketch ‘Silurian’.  During the visual survey, single dedicated cetacean observers were positioned on the port and starboard sides of the vessel to maintain a continuous watch for cetaceans within the 90° area of water on their side (from the beam to the bow of the vessel), using a combination of naked eye and binoculars.  Standardised effort and cetacean sighting forms were completed throughout the survey. Water depth for each sighting was recorded using the vessel’s ST60 Autohelm echosounder, and a Minilog 8-bit data logger (www.vemco.com) was deployed from the Silurian daily to record sea surface temperature (SST) at 1-min intervals. 

A towed hydrophone array comprising high and low frequency elements was deployed daily to record the sounds of dolphins and porpoises.  Wherever possible dolphins were approached to attempt photo-identification work, whereby individual animals are photographed and identified from their unique markings, primarily the size and location of notches and cuts in the trailing edges of the dorsal fin and also body scarring and pigmentation patterns.  An SNH licence permit for photo-identification work had been granted to Ketos Ecology and the HWDT for this work.  During photo-identification encounters the vessel was manoeuvred slowly towards the group at a shallow angle to their heading, with the aim of travelling parallel alongside the animals.  Photographs were taken using a range of camera equipment including Nikon and Canon digital SLR cameras and zoom lenses in the 75–300, 28–200 and 100–400 mm range.

Figure 1.  Location of the study area

Data analysis

The occurrence of each cetacean species was described in terms of the spatial distribution of the sightings (all data) and their relative abundance within ¼ ICES rectangles.  To compensate for differences in the amount of survey coverage in each ¼ ICES rectangle, an index of relative abundance (individuals/km) was used to analyse cetacean occurrence.  For each species the mean water depth, SST and distance from shore were calculated.  For species with ten or more sightings, a Chi-square analysis was carried out to test for differences in the occurrence of the species within categories of depth and SST.

A database of photo-identification images was established, and the images from each encounter were analysed for the presence of distinctive individuals.  The best image of each identifiable individual was manually compared (by a single observer) to the catalogue of existing animals.  If a match was found, the image was filed under the appropriate Identification (ID) Number.  Where the individual was deemed to be a new animal, it was assigned a new ID Number and entered into the catalogue.  Animals were assigned an ID Number according to the type and likely permanence of their diagnostic feature(s).  Individuals were divided into those exhibiting permanent features (i.e. nicks, notches, damaged fins or diagnostic fin shape) and those exhibiting only temporary features (e.g. de-pigmentation, skin lesions, scars, scratches, tooth rakes).