Inter-specific and seasonal comparison of the niches occupied by small cetaceans off north-west Iberia

• The niches of five odontocetes were compared using PCA and CART.
• Significant differences were found; niche partitioning was stronger in the summer.
• Harbour porpoise shows the narrowest ecological niche during the summer.
• Pilot whale shows a preference for colder waters than the other species.
• This method is an effective multivariate approach to assess niche partitioning.

Knowledge of species' ecological niches can be used to assess ecological interactions between different taxa. Sixteen species of cetaceans have been recorded in Galician waters and niche partitioning is expected to occur among these species in order to allow them to co-exist. In this study, the niches occupied by five of the most commonly encountered odontocete species off Galicia (NW Iberia) were compared, based on seven ecogeographic variables, using a PCA-based methodology and Classification trees. Significant differences in niche centres and niche widths were found among all the species. During the summer, the harbour porpoise occupied the narrowest and most differentiated niche when compared to the rest of the species. Three species could be compared during the winter, when long-finned pilot whales preferred colder and less variable water temperatures than did common dolphins. Seasonal differences in habitat preferences were found for bottlenose dolphins. A higher degree of specialisation was found during the summer, resulting in stronger differences in habitat use in this season, which may be related to an increment in resource availability during the upwelling period (April–September). The PCA-based methodology used in this study provides an effective multivariate approach to explore niche partitioning between co-existing species.