Summer distribution and relative abundance of cetacean populations off north-west Scotland

Abstract

A line transect survey was conducted in July/August 1998 to investigate the distribution and abundance of cetaceans off north-west Scotland. Over 2156.5 km of survey effort, 304 sightings were recorded of which 184 were identified to species. Nine species were identified: Atlantic white-sided dolphin Lagenorhynchus acutus, fin whale Balaenoptera physalus, sei whale Balaenoptera borealis, long-finned pilot whale Globicephala melas, sperm whale Physeter macrocephalus, common dolphin Delphinus delphis, harbour porpoise Phocoena phocoena, white beaked dolphin Lagenorhynchus albirostris and Risso's dolphin Grampus griseus. The Atlantic white-sided dolphin was the most relatively abundant species. The relative abundance of large whales and dolphins was greatest in the Faroe–Shetland Channel. The continental slopes of this area are undergoing development by oil industries and concerns have been raised about the potential impacts of these activities on cetaceans. These waters are used year-round by cetaceans and provide feeding and breeding grounds and are a migration route for large whales.

Habitat preferences and interannual variability in occurrence of the harbour porpoise Phocoena phocoena off northwest Scotland

“…It is of interest that increased predicted sightings occur also south of Barra. Harbour porpoises have been sighted further west of the Outer Hebrides to the shelf edge (Macleod et al 2003, MacLeod et al 2007; this might indicate a preference to use this area between Mingulay and Barra when entering the Sea of Hebrides, maybe as part of a seasonal pattern of movement.…”

“…It is of interest that increased predicted sightings occur also south of Barra. Harbour porpoises have been sighted further west of the Outer Hebrides to the shelf edge (Macleod et al 2003, MacLeod et al 2007; this might indicate a preference to use this area between Mingulay and Barra when entering the Sea of Hebrides, maybe as part of a seasonal pattern of movement.In the north of our study area, high numbers of sightings of harbour porpoise are predicted north of Skye to Gairloch on the east and off Piarc Peninsula to Shiant Islands on the west. Whatever the mapped percentile of the credible interval chosen, the location of the centre of the 2 areas with higher porpoise relative abundance does not vary, but the degree of separation between these 2 centres does.…”

The habitat preferences of marine mammals west of Scotland (UK)

“…While the spatial distribution of marine mammals in this region is relatively well described (e.g. Pollock et al, 2000;Weir et al, 2001;MacLeod et al, 2003;MacLeod 2004;Reid et al, 2003), there has been little work examining and, particularly, comparing the habitat preferences of different species. This is somewhat surprising given the large amount of data on marine mammal distribution that has been collected in this region.…”

Techniques for cetacean–habitat modeling

“…Therefore, correcting sighting frequencies for effort, using relative indices of abundance or encounter rates (Kasamatsu et al 2000b, Griffin & Griffin 2003, MacLeod et al 2003, or producing stratified estimates of cetacean densities is recommended. Alternatively, categories of habitat variables may be defined so that they contain equal effort.…”