X Conference of the Spanish Cetacean Society 2018, University of Valencia 29 - 30 September 2017 Does interspecific competition control local density and habitat use of harbour porpoises? Bruno Díaz López1, Séverine Methion1,2 1. Bottlenose Dolphin Research Institute (BDRI), Av. Beiramar 192, 36980, O Grove, Pontevedra, Spain 2. Université Bordeaux, UMR CNRS 5805 EPOC, Allee Geoffroy St Hilaire, 33615 Pessac Cedex, France
Abstract Determining the drivers that are responsible for the fine scale distribution of cetacean species is fundamental to understand better how they respond to changes in their environment. Coastal cetaceans regularly exist in sympatry; nevertheless, ecological studies have been often based on a single species approach while ignoring the potential effects of inter-specific competition. In this paper, we used a generalized additive modelling (GAM) framework to carry out a comprehensive investigation of the key environmental (including the distribution of sympatric cetacean species) and anthropogenic correlates of habitat use and local relative density of harbour porpoises. The field effort entailed over three consecutive years of fieldwork from April 2014 to November 2017. In all, 273 daily boat surveys over a period of 38 months were spent in the field covering 9 417 km along the coastal and shelf waters of Galicia (NW Spain). A total of 1015 hours were spent in satisfactory conditions and 73 groups of harbour porpoises were monitored during 20 h. The unequal use of available habitat confirms that harbour porpoises present a fine-scale pattern of habitat selection along the study area. Our results indicate that there is a competitive exclusion of the harbour porpoises by common bottlenose dolphins in near-shore locations with high density of preys consumed by both species. Ours is the first study to show that both inter-specific competition and food availability play a significant role in determining the use of habitat and local relative density of a coastal cetacean species. These findings highlight the importance of considering the interactions between multiple variables in ecological studies on temporal and spatial scales, in addition to highlighting the importance of using a multi-species ecology approach in research and conservation management planning. Keywords: inter-specific competition, sympatric species, cetaceans, harbour porpoises, bottlenose dolphins, Atlantic Ocean
