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

The harbour porpoise Phocoena phocoena is the most common cetacean around the British Isles, but knowledge of its ecology, habitat preferences and inter-annual variability is still inadequate. Here, sightings collected by the Sea Watch Foundation during vessel surveys in West Scotland (August during the years 1993, 1994, 1996 and 1997) were critically analysed and used to construct a predictive habitat model for harbour porpoises in the Greater Minch. Generalised additive models were used to analyse relative abundance in relation to environmental variables; a preference for waters within 15 km from the shore and between 50 and 150 m depth was clearly identified. A relationship between tidal variables and porpoise distribution was also recognised with more sightings predicted for high tidal stream speed areas as well as during times of high tide. Maps constructed from the model were used to identify potential 'hotspots' and compare between years. Four areas with high relative abundance were identified in (1) the region between Ardnamurchan, Coll and the Small Isles, (2) southeast of Barra, (3) northeast of Skye to Gairloch, and (4) west of Pairc Peninsula (Isle of Lewis) to Shiant Islands. Number of sightings fluctuated up to 4-fold between consecutive years; such extreme variability in relative abundance is offered as a bench-mark for comparing trends in the future as well as evidence that the Greater Minch represents only a small part of the effective range of this population.