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Does Interspecific Competition Drive Patterns of Habitat Use and Relative Marine Biology (2018) 165:92 https://doi.org/10.1007/s00227-018-3345-8 ORIGINAL PAPER Does interspecifc competition drive patterns of habitat use and relative density in harbour porpoises? Bruno Díaz López1 · Séverine Methion1,2 Received: 8 September 2017 / Accepted: 4 April 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Determining the drivers that are responsible for the fne-scale distribution of cetacean species is fundamental to understand better how they respond to changes in their environment. We utilized information theoretic approach to carry out a compre- hensive investigation of the key environmental and anthropogenic correlates of habitat use and relative density of harbour porpoises. In all, 273 daily boat surveys over a period of 38 months, between April 2014 and November 2017, were spent in the feld monitoring 9417 km along the coastal and shelf waters of Northwest Spain. Throughout this period, there were 70 encounters with harbour porpoises and 712 encounters with common bottlenose dolphins. The observed unequal use of available habitat indicates that harbour porpoises present a fne-scale pattern of habitat selection along the study area, which is likely related to the variation in oceanographic variables and human disturbance mainly caused by marine trafc and fsheries. While diferences in habitat use between harbour porpoises and bottlenose dolphins were observed, interspecifc competition with bottlenose dolphins (as competitive exclusion hypotheses) did not appear to play an important role in the distribution and relative density of harbour porpoises. These fndings highlight the importance of considering both environmental and anthropogenic variables in ecological studies, in addition to highlighting the importance of using a multi-species ecology approach in research and conservation management planning. Introduction While some surveys have recorded this species in ofshore waters (MacLeod et al. 2003; Bjørge and Øien 1995), the Habitat choice is the outcome of decisions that balance the harbour porpoise is generally distributed in highly produc- trade-of between resource availability and predation risk tive coastal areas with upwelling and high tidal fow (Ham- (Lima and Dill 1990). Understanding the habitat use and mond et al. 2002; Skov and Thomsen 2008; Marubini et al. determining the factors that are responsible for the distri- 2009), at depths up to 200 metres (Otani et al. 1998; Booth bution of marine predators, such as the harbour porpoise et al. 2013). (Phocoena phocoena, Linneaus 1758), are fundamental to With the expansion of human activities in coastal and understand better how these potentially vulnerable species shelf waters, there has been general concern over the status respond to changes in their environment. The harbour por- of harbour porpoises in European waters and the establish- poise is a highly mobile cetacean species found throughout ment of Special Areas of Conservation (SACs) has been sug- the waters of the Northern Hemisphere (Gaskin et al. 1974). gested as a method to protect this cetacean species (Embling et al. 2010; Hammond et al. 2013). The designation of these SACs for harbour porpoises, as required under the EU ‘Hab- Responsible Editor: D.E. Crocker. itats Directive’ (Directive 92/43/EEC), is especially chal- lenging, because harbour porpoises are small, highly mobile, Reviewed by S. Isojunno and an undisclosed expert. spend limited time at the surface, and change behaviour in * Bruno Díaz López response to boats (Westgate et al. 1995; Otani et al. 1998). [email protected] Fluctuations in the use of habitat of harbour porpoises have been the subject of several studies in diferent parts 1 Bottlenose Dolphin Research Institute (BDRI), Av. Beiramar of its range, almost exclusively from analysis of the rela- 192, O Grove, 36980 Pontevedra, Spain tionship with oceanographic features that could determine 2 Université Bordeaux, UMR CNRS 5805 EPOC, Allee food availability and abundance (e.g., Johnston et al. 2005; Geofroy St Hilaire, 33615 Pessac Cedex, France Vol.:(0123456789)1 3 92 Page 2 of 11 Marine Biology (2018) 165:92 Marubini et al. 2009; Edrén et al. 2010; Embling et al. 2010; often unclear and the ultimate causes may be complex (Ross Booth et al. 2013). Few studies have included anthropogenic and Wilson 1996; Díaz López et al. 2017). explanatory variables (such as the presence of fshing nets, In this paper, we present a comprehensive investigation fshing vessels, and marine trafc) in the habitat models for of the key environmental and anthropogenic correlates of harbour porpoises, although the species is known to be sen- habitat use and relative density of harbour porpoises along sitive to anthropogenic noise (Johnston 2002; Dähne et al. the Northwestern coast of Spain. As such, investigating these 2013; Thompson et al. 2013). Moreover, all these previous fuctuations in their coastal habitat utilization and relative studies were only based on a single species approach that density is essential for a better understanding of the ecol- ignores potential efects of sympatric competitors on harbour ogy and for the conservation of this potentially vulnerable porpoises’ habitat use. It is likely that harbour porpoises small cetacean species. Moreover, a competitive exclusion select habitats that have high food availability (Johnston hypothesis was tested including, as part of the environmental et al. 2005; Sveegaard et al. 2011). Likewise, other ceta- correlates, the presence of bottlenose dolphins in the model cean species with similar food preferences and commercial framework. Indeed, we examined temporal and spatial link- fsheries will be present in these rich habitats (Ross and Wil- ages between the presence and relative abundance of harbour son 1996; Jepson and Baker 1998; Díaz López and Methion porpoises and the presence of bottlenose dolphins in the 2017), in which harbour porpoises may have to trade of area. food and the safety when selecting a habitat to occupy (see Lima and Dill 1990 for a review of behavioural choices made under risk). Little or no attention has been given to Methods the efects caused by the presence of sympatric cetacean species on the fne-scale distribution and numbers of harbour Study area porpoises. Recent studies suggest that harbour porpoises from the The shelf waters of Galician coastline (Northwest Spain) Northwestern coast of the Iberian peninsula (Spain and Por- correspond to the northern zone of the Canary Upwelling tugal) and Mauritania could well represent a distinct ecotype System, one of the four major upwelling regions in the world from those from the NE Atlantic continental shelf waters (P. (Santos et al. 2011). The complex shoreline and topography phocoena meridionalis) (Fontaine et al. 2014). This area along this coastline is penetrated by a series of fooded tec- has been identifed as a zone of high density of harbour tonic valleys (named rías), which support very high primary porpoises (López et al. 2004; Pierce et al. 2010; Fernández production (Prego et al. 1999). et al. 2013), and it has been suggested to be suitable for the The study area extends approximately 450 km2, inshore creation of possible Special Areas of Conservation (SACs) and ofshore the largest and most productive of the Galician under the EU Habitats Directive. Harbour porpoises and rías, the Ría of Arousa (Fig. 1). Along this area, upwelling common bottlenose dolphins (Tursiops truncatus, Montagu practically reaches the coastline, penetrating the Ría of 1821) are distributed along the coastal and shelf waters of Arousa (Prego et al. 1999). The entire system is subjected Galicia (Northwest Spain) with diferent geographical pat- to a strong tide regime, with a tidal range of 1.1 and 3.5 m terns of relative abundance (López et al. 2004; Pierce et al. during neap and spring tides, respectively (Alvarez et al. 2010; Díaz López and Methion 2017). The diets of these 2005). These waters are subject to signifcant use by humans two species follow a similar pattern in these waters (San- including professional fsheries, shellfsh aquaculture indus- tos and Pierce 2003; Santos et al. 2007; Méndez-Fernández try, and important marine trafc (Vieites et al. 2004; Díaz et al. 2012), which is consistent with all two marine top López and Methion 2017). predators exploiting the same locally abundant resources. Complementary approaches based on the analysis of for- Data collection aging niche segregations (Méndez-Fernández et al. 2012; Fernández et al. 2013) proposed that trophic competition Data for this study were collected as part of a longitudinal occurs between harbour porpoises and common bottle- study carried out by the Bottlenose Dolphin Research Insti- nose dolphins in Galician waters as also suggested in other tute BDRI (http://www.thebd ri.com) about the ecology of adjacent areas of the Northeast Atlantic (Spitz et al. 2006). cetacean species inhabiting the Galician waters. Boat-based Aggressive, non-predatory interactions between these two surveys on-board a 12 m research vessel were conducted sympatric species have been often documented in diferent between April 2014 and November 2017. In all, 38 months regions including the Northwestern coast of Spain (López were spent in the feld. Surveys were done when the sea and Rodríguez 1995; Ross and Wilson 1996; Jepson and conditions were up to 3 on the Douglas sea force scale Baker 1998). However, the reasons for bottlenose dolphins’ (approximately equivalent to the Beaufort wind force scale) attacks,
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