Identifying critical areas and habitat preferences of bottlenose dolphins Tursiops truncatus

We present the findings of a 2-yr study of habitat use by bottlenose dolphins in the outer Shannon estuary on the west coast of Ireland. Data were collected during repeated, standardised, boat-based surveys of dolphins in a predefined 150 km(2) area of the outer estuary. Areas found to exceed a uniform-use pattern were termed 'critical areas' and delimited using the 50% contour derived from harmonic mean transformation of sighting locations. Dolphins exhibited preferential use of areas of the estuary with the greatest benthic slope and depth, highlighting the influence of environmental heterogeneity on habitat use by this species. Additionally, photo-identification was used to catalogue sightings of uniquely marked dolphins which provided distribution data at an individual level. Although the ranges of frequently sighted dolphins overlapped, a degree of partitioning was found in the use of up-river parts of the study area. The methods presented here could be applied at a range of spatial scales and we suggest that the identification of critical areas within a population's range is a priority when planning any conservation management strategy for marine mammals.