Vol. 4: 309–331, 2008 ENDANGERED SPECIES RESEARCH Printed June 2008 doi: 10.3354/esr00073 Endang Species Res Published online May 27, 2008 OPENPEN ACCESSCCESS Abundance and habitat preferences of the short-beaked common dolphin Delphinus delphis in the southwestern Mediterranean: implications for conservation A. Cañadas1,*, P. S. Hammond2 1ALNITAK & SEC, Nalón 16, 28240 Hoyo de Manzanares, Madrid, Spain 2Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife KY16 8LB, UK ABSTRACT: The Mediterranean sub-population of short-beaked common dolphin is believed to have suffered a steep decline in the Mediterranean in recent years, and in 2003 it was listed as endangered in the IUCN Red List of Threatened Species. Effective conservation will depend critically on our understanding of the relationship between the species and its habitats. The Alborán Sea is believed to be the most important remaining Mediterranean habitat for this species, and thus consti- tutes a vital source of information for the development of conservation measures. We used spatial modelling to estimate the abundance and explore the habitat use of common dolphins in this area, examining regional, seasonal and interannual variations, as well as the influence of biological factors such as presence of calves, interspecific relationships and behaviour. From 1992 to 2004, 37 385 km of non-systematic line transects generated 738 sightings in a 19 189 km2 study area. The point esti- mate of abundance was 19 428 (95% CI = 15 277 to 22 804) dolphins. Seasonal and geographical vari- ations in abundance were detected, with higher average density in summer than in winter, and in the Western Alborán Sea than in the east Gulf of Vera, which has different physical/environmental char- acteristics. No overall trend in abundance was observed in the Alborán area. However, a decline was observed in the Gulf of Vera, with a summer density 3-fold lower in the period from 1996 to 2004 than in 1992 to 1995. A potential link of this decline with prey depletion due to the exponential growth of aquaculture in the area is discussed. Clear differences in habitat use were also found when examin- ing the influence of biological factors. In particular, groups with calves and groups that were feeding preferred more coastal waters. This result could have important implications for the development of conservation measures for this species in the Mediterranean. KEY WORDS: Common dolphin · Mediterranean · Generalized Additive Models · GAMs · Abundance · Trends · Habitat use · Alborán Sea Resale or republication not permitted without written consent of the publisher INTRODUCTION the IUCN, was listed as endangered in the Interna- tional Union for the Conservation of Nature and Nat- Short-beaked common dolphins Delphinus delphis ural Resources, World Conservation Union (IUCN) Red (hereafter referred to as ‘common dolphins’) in the List of Threatened Species, based on criterion A2, Mediterranean appear to have suffered a steep decline which refers to a 50% decline in abundance over the over recent decades (UNEP/IUCN 1994, Aguilar et al. last 3 generations (35 to 45 yr), the causes of which 1995, Bearzi et al. 2003), although no reliable time ‘may not have ceased or may not be understood or series of abundance data exist. In 2003, the Mediter- may not be reversible’ (www.iucnredlist.org/search/ ranean common dolphin subpopulation, as defined by details.php/41762/summ). Possible causes of this de- *Email: [email protected] © Inter-Research 2008 · www.int-res.com 310 Endang Species Res 4: 309–331, 2008 cline include: by-catch, depletion of food resources (Bearzi et al. 2004). Genetic studies of Mediterranean due to overfishing, toxic pollution or a combination of common dolphins found that animals in the Alborán these factors (Bearzi et al. 2003). Sea are genetically more closely related to those in The IUCN/Species Survival Commission (SSC) the northeast Atlantic than to those in the eastern Cetacean Specialist Group has recommended that Mediterranean and may constitute a different popula- studies to ‘investigate the distribution, abundance, tion from the latter (Natoli 2005). This structuring of population structure, and factors threatening the con- the population may have important implications for servation of short-beaked common dolphins in the conservation. Mediterranean and Black Seas’ be intensified (Reeves The only large-scale cetacean survey in the western et al. 2003). ACCOBAMS (the Agreement for the Con- part of the Mediterranean Sea was carried out in 1991 servation of Cetaceans in the Black Sea, Mediter- to 1992 (Forcada & Hammond 1998). The latter study ranean Sea and contiguous Atlantic Area under the estimated abundance of common dolphins in the UN Convention for Migratory Species) commissioned Alborán Sea as follows: 14 736 common dolphins (95% the development of a Conservation Plan for the com- CI = 6923 to 31 366; coefficient of variation [CV] = 0.40) mon dolphin in the Mediterranean Sea (Bearzi et al. in an area of 90 670 km2. The only other estimate of 2004). In its Implementation Priorities (ACCOBAMS abundance in the Mediterranean is for a small inshore 2002) proposed that areas containing critical habitat area in the Ionian Sea, based on photo-identification of for priority species should be identified, in which pilot around 100 animals (Bearzi et al. 2003). conservation and management projects should be Previous studies in the northeastern Alborán Sea developed and implemented immediately. In the Con- examined the influence of depth and slope on the dis- servation Plan, areas considered to be of special tribution of common dolphins (Cañadas et al. 2002) but conservation value for common dolphins (Areas of without any attempt to predict distribution based on Conservation Importance, ACIs), and therefore poten- the described relationships. In the present study we tial marine protected areas (MPAs) were selected analyse a much more extensive dataset on this species based on the — admittedly limited — knowledge of the in the whole northern Alborán Sea to model regional, distribution and frequency of occurrence of common seasonal and inter-annual variation in abundance and dolphins in the Mediterranean. habitat preferences, and to map model predictions to This knowledge — as well as information on the inform spatial aspects of conservation and manage- scale and relative importance of actual or potential ment. To estimate abundance we use model-based anthropogenic threats to dolphins and priorities of pos- line-transect sampling methods (Hedley et al. 1999, sible mitigation measures — is lacking, but must be Marques 2001), recently applied to common bottlenose increased quickly if effective conservation actions are dolphins Tursiops truncatus in this area (Cañadas & to be implemented (Bearzi et al. 2004). Most impor- Hammond 2006). tantly, baseline population abundance and information Distribution and habitat use of a species with com- on distribution is needed to put any identified threats plex ecology, social structure and behaviour are very to common dolphins into context and to establish prior- likely influenced not only by ‘extrinsic’ (i.e. abiotic and ity conservation and mitigation measures. Monitoring biotic environmental) factors but also by ‘intrinsic’ distribution and abundance is necessary to determine factors (e.g. reproductive status, feeding strategies, whether these measures are fulfilling established behaviour, inter-specific relationships). Understanding conservation objectives. these influences and the inter-relationships among In general, MPAs or marine reserves may offer a them will greatly improve our understanding and solution to some of the challenges involved in manage- interpretation of the species’ ecology as well as ment of the marine environment. They allow conserva- improving our ability to develop targeted and more tion efforts or management actions to focus at a geo- effective mitigation and conservation measures. In the graphical scale that is more tangible for the general present paper, we begin to explore the habitat prefer- public. In this way, marine reserves can allow the ences of common dolphins in the study area, examin- organisation of human activities through zoning (e.g. ing ‘intrinsic’ factors such as presence of calves, inter- ‘no take’ zones and buffer zones), at the same time as specific relationships and behaviour, thereby building preserving the natural attributes of the region and its on the work of Cañadas et al. (2002, 2005). This is the value as a tool for public awareness and education. first time these methods have been used in such an One of the ACIs selected in the ACCOBAMS Con- investigation of a cetacean species. servation Plan for common dolphins in the Mediter- We discuss the implications of the results for the con- ranean Sea is the Alborán Sea (see Fig. 1), largely servation of common dolphins in the Mediterranean because the encounter rates for this species in this Sea, especially with respect to bycatch in fishing nets area are the highest in the whole Mediterranean basin and the establishment of MPAs. Cañadas & Hammond: Abundance and habitat preferences of Delphinus delphis 311 MATERIALS AND METHODS surveyed the region of Murcia (northern Gulf of Vera). From 1992 to 1998 surveys were made during March Data collection. Survey area, survey design and and April and from June to September. The study area searching effort: Surveys were conducted during the was sampled in January,