Habitat use by bottlenose dolphins: seasonal distribution and stratified movement patterns in the Moray Firth, Scotland

1. This study investigated the distribution of a population of bottlenose dolphins Tursiops truncatus resident in the Moray Firth off north-eastern Scotland. Results add to existing information from studies in tropical areas to provide a better understanding of area use in this species.

2. Boat-based surveys and photo-identification techniques were used to study the distribution and movements of individually recognizable dolphins over a 3-year period.

3. Dolphins were seen in all months of the year, but there were consistent seasonal fluctuations in the number of individuals present. Numbers were low in winter and spring and peaked in summer and autumn.

4. Dolphins were seen throughout the survey area but were concentrated in three regions. Each had similar topographic features being centred on deep, narrow channels subject to strong tidal flows.

5. Area use by dolphins changed with season. The outer part of the inner Moray Firth study area was used for most of the year and areas closer to the head of the firth were used seasonally.

6. The summer increase in numbers of dolphins in the inner Moray Firth was not simply due to incomers diluting an already resident population. Instead, there was a stratified movement of all individuals. This persistent geographical stratification suggests that competition between individuals or social groupings may shape spatial distribution in this population.

7. Individuals exhibited rapid movements across the population's range. For instance, one individual was sighted at locations 190 km apart within a 5-day period.

8. In terms of conservation, the high use of areas at the mouths of the inner firths warrants special attention. Furthermore, the stratification patterns amongst dolphins suggest that individuals do not move freely within the inner Moray Firth and therefore may be unable to move away from localized disturbance or pollution.