Ecology and conservation of common bottlenose dolphins Tursiops truncatus in the Mediterranean Sea

INTRODUCTION

The common bottlenose dolphin Tursiops truncatus (hereafter ‘bottlenose dolphin’; Fig. 1) has been studied intensively in numerous locations around the world, and today is one of the best known of the 85+ living cetacean species (Leatherwood & Reeves, 1990; Wells & Scott, 1999; Reynolds, Wells & Eide, 2000). In the Mediterranean Sea (Fig. 2), modern cetacean field studies started only in the late 1980s (Notarbartolo di Sciara & Bearzi, 2005) and relatively little is yet known about bottlenose dolphins in the region. Most of the research to date has been conducted in northern portions of the basin, confined to coastal areas.

A first partial attempt to assess the status of bottlenose dolphins in the Mediterranean was made in the context of a regional Red List workshop in March 2006 (Reeves & Notarbartolo di Sciara, 2006). Participants agreed that the Mediterranean ‘subpopulation’ of bottlenose dolphins qualified as ‘Vulnerable’ according to the International Union for Conservation of Nature (IUCN) Red List criteria (Bearzi & Fortuna, 2006). This proposed classification was based primarily on suspected population declines caused by (i) directed kills and extermination campaigns conducted until at least the early 1960s in portions of the basin and (ii) recent and ongoing incidental mortality in fishing gear (generally known as ‘by-catch’). The assessment also took into consideration ongoing threats from overfishing of prey populations (overlap between prey species of bottlenose dolphins and target species of commercial fisheries has been demonstrated in several cases) and from more generalized habitat degradation, understood to include environmental contamination by chemical pollutants and the disturbance caused by marine traffic (Bearzi & Fortuna, 2006). Although not easily quantified or linked to the survival or fitness of individual animals, these latter factors were suspected of contributing to an overall decline in numbers of bottlenose dolphins in the region.

The scattered and uneven character of the literature on bottlenose dolphins in the Mediterranean complicates understanding of their ecology and population trends, and further delays the already slow process of implementation and enforcement of regional conservation actions. Although some studies of bottlenose dolphins are long-running and of high scientific quality, only a small proportion of the data has been published in peer-reviewed scientific journals. Here, we summarize the published information with the intention of facilitating further studies and stimulating and helping to guide conservation efforts. The available information also includes a number of unpublished abstracts and short contributions from conference proceedings, reports and academic theses or dissertations. Such material is not formally peer-reviewed, often presents preliminary information and is of variable quality, making it difficult or impossible to evaluate its reliability. Most of the ‘grey literature’ has been excluded from this review, although in a few instances, unpublished sources of essential information have been cited.

This article (i) reviews and synthesizes information on the distribution, ecology, behaviour, interactions with fisheries and status of Mediterranean bottlenose dolphins; (ii) identifies threats likely to have affected the dolphin population in both historic and present times; (iii) calls for the enforcement of existing obligations on the part of Mediterranean States with regard to the protection of the marine environment; and (iv) describes other actions needed to protect bottlenose dolphins in the region.

DISTRIBUTION

The bottlenose dolphin is one of the most frequently observed cetaceans in the Mediterranean (Notarbartolo di Sciara & Demma, 2004; Reeves & Notarbartolo di Sciara, 2006). Bottlenose dolphins occur in most coastal waters of the basin and have been reliably reported in the waters of Albania, Algeria, Croatia, Cyprus, France, Gibraltar (United Kingdom), Greece, Israel, Italy, Montenegro, Morocco, Slovenia, Spain, Tunisia and Turkey. They occur regularly around many of the region's offshore islands and archipelagos.

Many of the Mediterranean areas inhabited by these dolphins are subject to intensive human use, e.g. the straits of Gibraltar, Bonifacio and Messina, and the gulfs of Lion, Genoa and Trieste. Within their overall range, gaps with very low densities of animals have been documented, e.g. in the north-western Ligurian Sea (France and Italy) and in the north-western Gulf of Vera (Spain). Variation in density is likely related to several factors, including (i) habitat characteristics, (ii) local availability of suitable prey and (iii) the generally gregarious nature of bottlenose dolphin communities. Moreover, the effects of past extermination campaigns (Bearzi, Holcer & Notarbartolo di Sciara, 2004) and a variety of ongoing threats (see ‘Present threats’) have probably contributed to the patchiness of the current distribution of bottlenose dolphins across the region.

POPULATION STRUCTURE

Of the two species recognized within the genus Tursiops, the common bottlenose dolphin T. truncatus and the Indo-Pacific bottlenose dolphin T. aduncus, only the former is known to occur in the Mediterranean Sea and North Atlantic Ocean (Rice, 1998).

Mediterranean bottlenose dolphins are genetically differentiated from those inhabiting the contiguous eastern North Atlantic Ocean and Scottish waters. Based on nuclear and mitochondrial DNA analyses, distinct populations have been identified across the Black Sea and the Mediterranean Sea (Natoli et al., 2005). The genetic analysis of 145 samples along a continuous distributional range from the Black Sea to the eastern North Atlantic (16 samples from the Black Sea, 74 from the Mediterranean Sea, 35 from the eastern North Atlantic and 20 from Scotland) found population structure with boundaries that coincided with the transitions between different types of habitat. The different zones can be characterized by ocean floor topography and by features such as surface salinity, productivity and temperature. Five populations were identified: Black Sea, eastern Mediterranean, western Mediterranean, eastern North Atlantic and Scottish. The Black Sea population showed the highest differentiation from other populations. Significant genetic differentiation was observed between populations from the eastern and the western Mediterranean. The boundary between the western Mediterranean and the eastern North Atlantic was the weakest observed, although the two populations still showed significant genetic differentiation. Despite no obvious physical barriers, the eastern North Atlantic and the Scottish populations also showed genetic differences. There was genetic evidence of directional emigration of females at the extreme of the range, although neither sex showed a strong bias for greater dispersal (Natoli et al., 2005).

Population structure of bottlenose dolphins around the Iberian Peninsula was investigated through isotopic signatures and organochlorine pollutant loads in tissues of stranded animals from Catalonia, Valencia and the Balearic Islands and adjacent Atlantic waters (Huelva and Portugal; Borrell et al., 2006). Significant differences in stable isotopes of carbon (¹³C/¹²C) and in PCB congener profiles indicated that dolphins from the Atlantic and the Mediterranean do not intermingle. In the Mediterranean, dolphins from Catalonia and Valencia were indistinguishable, suggesting a common distribution area. However, dolphins from the Balearic Islands differed from those of mainland Spain in their DDT/PCB ratio and from all the other sample groups in their PCB congener profiles, suggesting that the deep waters between the Balearic Islands and the Iberian Peninsula represent an effective barrier for the species (Borrell et al., 2006).

Evidence of population structure has also been found in other Mediterranean delphinids, including short-beaked common dolphins Delphinus delphis (Natoli et al., 2008), striped dolphins Stenella coeruleoalba (Fossi et al., 2004; Gaspari et al., 2007b) and Risso's dolphins Grampus griseus (Gaspari, Airoldi & Hoelzel, 2007a). These findings suggest not only that the more obvious physical boundaries such as the Strait of Gibraltar (minimum width about 45 km and sill depths less than 145 m) and the Turkish Straits system (Dardanelles minimum width about 450 m and sill depths less than 55 m) represent barriers to the movement of individuals, but also that the much wider Sicily Channel (143 km and sill depths less than 200 m), oceanic features such as the Almería-Orán front (Tintoréet al., 1988), and more generally, differences in habitat characteristics restrict the movements of bottlenose dolphins (Natoli et al., 2005). Further evidence of genetic discontinuities, especially in relation to the Almería-Orán front, comes from mtDNA studies on other highly mobile top predators such as bluefin tuna Thunnus thynnus (Carlsson et al., 2004) and Atlantic bonito Sarda sarda (Viñas, Alvarado Bremer & Pla, 2004) as well as from a variety of bottlenose dolphin prey species (Chikhi, Agnese & Bonhomme, 1997; Rolda'n et al., 1998; Naciri et al., 1999; Pérez-Losada et al., 2002; Zardoya et al., 2004; Bargelloni et al., 2005; Atarhouch et al., 2006; Magoulas et al., 2006).

NUMBERS AND POPULATION TRENDS

Little is known about the numbers of bottlenose dolphins in the Mediterranean Sea. There is no basin-wide estimate and the most reliable information comes from a few local studies (Table 1). Numbers in the published and unpublished literature are based on different methodological approaches, including estimates of absolute abundance obtained through mark-recapture methods or Generalized Additive Models (GAMs), maximum number of photo-identified individuals, and discovery curves (i.e. curves showing the cumulative number of individuals identified as a function of photo-identification effort, usually expressed as survey days with photos). Useful information on past and present occurrence also comes from stranding records from Algeria, Croatia, France, Greece, Italy, Malta, Morocco, Spain, and Tunisia (however, a stranded carcass is not necessarily indicative of the dead animal having lived nearby). Virtually nothing is known for large portions of the south-eastern part of the basin.

Most studies in coastal waters are limited to relatively small areas of 400–1000 km² and probably do not cover the entire range of the groups under study. Relatively recent, broad-scale shipboard surveys (4000–80 000 km²) showed that in some Mediterranean areas bottlenose dolphins are present both near shore and offshore, and densities can range between 4 and 20 animals per 100 km² (Ben Naceur et al., 2004; Forcada et al., 2004; Cañadas & Hammond, 2006; Gomez de Segura et al., 2006; and see Table 1). Studies tend to focus on areas of relatively high dolphin density, although study site preferences may also depend on logistical and other considerations. In this situation, the total population size in the Mediterranean remains uncertain, but it is unlikely to exceed the low 10 000 s (Bearzi & Fortuna, 2006).

Indirect but convincing evidence of dolphin abundance in historical times can be found in early accounts describing interactions with fisheries and systematic attempts to exterminate dolphins (including bottlenose dolphins) in Mediterranean coastal waters (De Marchesetti, 1882; Gourret, 1894; Del Rosso, 1905; Cuculic, 1960; Stoka, 1962; and see ‘Past culling campaigns’ below).

The only Mediterranean area with quantitative historical information that can be used to infer population trends is the northern Adriatic Sea. There, bottlenose dolphin numbers probably declined by at least 50% in the second half of the 20th century, largely as a consequence of deliberate killing initially, followed by habitat degradation and overfishing of prey species (Bearzi et al., 2004; Bearzi & Fortuna, 2006). For other parts of the northern Mediterranean, e.g. Spain, Italy and southern France, the available information is less precise but suggests similar trends (Blanco & González, 1992; Borrell et al., 2000). The nature and extent of threats vary with region, but the overall pattern has led to the suspicion that the Mediterranean population has been reduced by at least 30% over the last 60 years (Reeves & Notarbartolo di Sciara, 2006).

ECOLOGY AND BEHAVIOUR

Bottlenose dolphins in the Mediterranean are often regarded as predominantly ‘coastal’ or ‘inshore’ animals but this designation may be misleading as they can be encountered in continental shelf and shallow plateaux waters at any distance from the coast (Notarbartolo di Sciara et al., 1993; Bearzi et al., 2004; Ben Naceur et al., 2004; Gomez de Segura, Tomas & Raga, 2004; Gannier, 2005; Gnone et al., 2006; Azzellino et al., 2008b). They inhabit a wide variety of habitats including continental shelf waters (Gomez de Segura et al., 2006; Azzellino et al., 2008b), lagoons and enclosed seas (Bearzi et al., 2008), and the waters surrounding islands and archipelagos (Bearzi, Notarbartolo di Sciara & Politi, 1997; Mussi & Miragliuolo, 2003; Forcada et al., 2004; Fortuna et al., 2007). In the Alborán and Balearic Seas they occur across the entire shelf, along the shelf edge and in deep waters of the continental slope, and in productive waters 200–500 m deep (Cañadas, Sagarminaga & García-Tiscar 2002; Forcada et al., 2004; Cañadas et al., 2005). In the Strait of Gibraltar, at or near the western limits of what is defined here as the Mediterranean region, bottlenose dolphins occur mainly in water 200–600 m deep, over steep slopes (de Stephanis et al., 2008). In Greek waters they are found in coastal areas, straits, gulfs, semi-closed eutrophic waters, and steep coasts with no continental shelf (Frantzis et al., 2003; Bearzi et al., 2005, 2008a).

Genetic or other evidence of coastal and offshore ‘ecotypes’, as found in other areas (e.g. Duffield, Ridgway & Cornell, 1983; Van Waerebeek et al., 1990; Mead & Potter, 1995), has not been reported in the Mediterranean (Natoli et al., 2005). However, there has not been an appropriate effort to investigate this possibility using a large sample of specimens, as has been attempted, e.g. in striped dolphins (Gaspari et al., 2007b). Movements into and across pelagic waters may occur, and bottlenose dolphin groups are sighted occasionally in offshore waters deeper than 2000 m. At the other extreme, bottlenose dolphins sometimes enter sea enclosures, estuaries, lagoons and rivers (Bearzi et al., 2004; Sackl, Smole & Saveljic, 2007). In the semi-closed Amvrakikos Gulf, a 400 km² lagoon in western Greece, bottlenose dolphins show high levels of site fidelity and have adapted to murky eutrophic waters about 30 m deep (Bearzi et al., 2008a).

The size of bottlenose dolphin groups varies according to biogeographic region, prey availability, activity and other factors. Most encounters have been with groups of fewer than 10 individuals (Table 2). As the definition of ‘group’ can vary among studies and sampling methods are often undefined, group sizes in Table 2 (and those reported in a number of unpublished contributions) represent relative values that cannot be directly compared.

Associations with other cetacean species are uncommon, although in some areas mixed aggregations with short-beaked common dolphins (Notarbartolo di Sciara et al., 1993; Bearzi & Notarbartolo di Sciara, 1995; Cañadas et al., 2002; Bearzi et al., 2003) and long-finned pilot whales Globicephala melas (Cañadas et al., 2002; de Stephanis et al., 2008) have been observed.

The only available information on the behaviour and social organization of bottlenose dolphins in the Mediterranean comes from groups living in coastal waters, and little is known about offshore groups. In two coastal studies, groups were highly fluid, and a change in group size and/or composition occurred on average every hour (Bearzi et al., 1997, 2005). Little is known about social organization. In at least one study, sexual segregation did not appear to be the rule, as groups included animals from both sexes (Bearzi et al., 1997).

Lone ‘sociable’ animals (Lockyer, 1990) that leave their affiliates and establish residency in ports or bays, where they may interact regularly with humans, have been reported fairly often in the Mediterranean. Although these occurrences often end prematurely when the animal is killed, some individuals have survived for several years under unusual circumstances (Müller et al., 1998; Müller & Bossley, 2002).

The behaviour of bottlenose dolphins can differ considerably depending on the habitat in which the animals are living (Shane, Wells & Würsig, 1986). Feeding habits, in particular, seem to shape the behaviour of these dolphins, whose diet has been described as catholic or opportunistic (e.g. Leatherwood, 1975; Barros & Odell, 1990). In Mediterranean coastal waters, bottlenose dolphins target primarily demersal prey (e.g. Miokovic, Kovacic & Pribanic, 1999; Blanco, Salomón & Raga, 2001) during feeding sessions characterized by dives lasting 3–5 min and occasionally up to 8 min, depending on water depth (Bearzi, Politi & Notarbartolo di Sciara, 1999; Bearzi et al., 2005). Reported prey items include demersal species such as European hake Merluccius merluccius, European conger Conger conger, red mullet Mullus barbatus, striped red mullet Mullus surmuletus, common cuttlefish Sepia officinalis, common octopus Octopus vulgaris and a variety of other bony fishes and molluscs. As most studies have relied on stomach contents from stranded animals, inferences may be subject to bias (Barros & Clarke, 2002). In some Mediterranean areas, bottlenose dolphins also feed on Clupeidae or other epipelagic prey (Díaz López, 2006a; Bearzi et al., 2008a). Diet and foraging behaviour appear to vary widely depending on area, season or trophic niche occupied by the local dolphins. Even geographically contiguous groups may show dramatically different foraging behaviour and prey preferences. For instance, epipelagic species including European pilchard Sardina pilchardus and round sardinella Sardinella aurita are important prey for bottlenose dolphins living in the Amvrakikos Gulf (Bearzi et al., 2008a), whilst those around the Greek island of Kalamos – less than 100 km distant – predominantly target demersal prey (Bearzi et al., 2005). In north-eastern Sardinia, Italy, bottlenose dolphins were observed feeding on wild fish attracted by a fish farm facility, including flathead mullet Mugil cephalus, salema Salpa sarpa, European pilchard and garpike Belone belone, and occasionally on farmed fish escaping from cages or being discarded at sea, including European seabass Dicentrarchus labrax, gilthead seabream Sparus aurata and brown meagre Sciaena umbra (Díaz López, 2006a).

Predation by sharks is a significant cause of mortality of bottlenose dolphins (Tursiops spp.) and other small cetaceans in several areas around the world (e.g. in Australia: Corkeron, Morris & Bryden, 1987; Mann & Barnett, 1999; Heithaus, 2001; in the south-eastern United States: Wood, Caldwell & Caldwell, 1970; Wells, Scott & Irvine, 1987). In the Mediterranean, however, predation is regarded as a minor cause of mortality. Bottlenose dolphins, as well as other small Mediterranean delphinids, are preyed upon occasionally by sharks, principally great white sharks Carcharodon carcharias but also several other species (Fergusson, 1994; Morey et al., 2003). The numbers of large sharks have declined drastically in Mediterranean waters over the past century (Ferretti et al., 2008) and this may have reduced predation pressure considerably. Other potential dolphin predators, such as the killer whale Orcinus orca and the false killer whale Pseudorca crassidens, are rare in the Mediterranean (Notarbartolo di Sciara, 1987; Reeves & Notarbartolo di Sciara, 2006).

IMPACT ON FISHERIES

In this section, we consider a range of activities by bottlenose dolphins that may result in real or perceived economic losses for Mediterranean fishermen. Targeted kills of dolphins and incidental mortality of dolphins in fishing gear, i.e. ‘interactions’ with fisheries that cause damage to dolphins, are treated separately under ‘Past culling campaigns’ and ‘Present threats’.

PAST CULLING CAMPAIGNS

Numerous early reports and artefacts from the Mediterranean refer to dolphins interacting with fishermen and people. What has been portrayed as a rather idyllic relationship appears to have changed dramatically as fisheries developed in the region. In 1587, a Papal Decree was issued ‘anathematizing the vermin’ in response to concerns in France about the effects of dolphins on fisheries. Starting in the late 18th century, dolphins came to be regarded as competitors and a major concern of fishery managers. Hence, new means were developed and deployed to kill the largest possible number of dolphins (Smith, 1995). Nineteenth century reports describe attempts by fishermen to keep dolphins away from their nets using, e.g. loud noises, weapons, modifications of fishing techniques and schedules, and large-mesh nets surrounding the fishing nets to protect them from dolphin incursions (Bearzi, 2002). Direct killing (culls) and bounties were supported by several governments for at least a century (Smith, 1995). Conflicts with fisheries reportedly were acute in several areas off Spain (A. Aguilar, personal communication), France (Gourret, 1894; Duguy et al., 1983; Bompar, 2000), Italy (Barone, 1895; Del Rosso, 1905; Bearzi et al., 2004), and particularly the former Yugoslavia (today's Slovenia, Croatia, Serbia and Montenegro) where thousands of bottlenose dolphins and short-beaked common dolphins were killed and landed for state bounties (Bearzi et al., 2004).

For most areas, the historical record is far from complete. The northern Adriatic Sea is an exception (Bearzi et al., 2004). Dolphins there were long viewed as pests deserving systematic extermination. Although most of the records fail to specify the species involved, in a few cases there is clear reference to both short-beaked common dolphins and bottlenose dolphins (Faber, 1883; Brusina, 1889; Ninni, 1901; Bearzi et al., 2004). The first record of a monetary reward offered for a killed dolphin dates back to 1872 (Crnkovic, 1958). No information is available to indicate the scale and duration of the first culling campaigns, but at least 335 dolphins were killed between 1933–35 (Crnkovic, 1958). The main campaign was launched in 1949, with the intent of eradicating dolphins from the region. Reported landings include 788 dolphins killed between 1955–60. Of these, 153 were killed in the district of Rijeka (at that time a relatively small city) between 1956–57 (Crnkovic, 1958; Marelic, 1961). There is no record of bounties after 1960. However, even after that time, fishermen in the eastern Adriatic often carried guns on board and shot dolphins frequently (Bearzi et al., 2004). At least until the early 1960s the animosity towards dolphins was tremendous, and every opportunity was taken to harm them (Peksider-Srica, 1931; Cuculic, 1960; Marelic, 1961). Although it is true that the perception of dolphins as mere competitors and game trophies progressively changed in subsequent years, deliberate killing remained legal until 1995 when all marine mammals became protected under Croatian law. Bounty hunting also occurred in Italy from the early 1930s (Brunelli, 1932). In the late 1950s and early 1960s, the reward for killing a dolphin was said to be equivalent to an entire week's pay for an Italian fisherman (Stanzani & Piermarocchi, 1992). An unknown number of dolphins were caught in the northern Adriatic between 1964–78 for live display in captive facilities (Greenwood & Taylor, 1978; Duguy et al., 1983). Also, killing dolphins for food or sport was a relatively common practice in Italy until a few decades ago, and it was only in 1979 that the government prohibited unauthorized killing (Bearzi et al., 2004).

Although the available numbers refer only to a few years in a series of culling campaigns that encompassed almost a century, there is no doubt that a great number of dolphins were killed along the eastern Adriatic coast alone between 1930–60, especially when wounded animals that sank or escaped and died later are taken into account (Mitchell, 1975; Bearzi et al., 2004). In the 1950s, dolphins (including bottlenose dolphins and short-beaked common dolphins) in the eastern portion of the Adriatic were said to number in the thousands (Crnkovic, 1958; Marelic, 1961). Although such claims have no scientific basis, they are nevertheless consistent with the high numbers reportedly killed and they support the general perception of substantial abundance in the recent past. Today, the available evidence suggests that the number of bottlenose dolphins in the entire Adriatic is unlikely to exceed a few hundred.

PRESENT THREATS

Owing to their occurrence in coastal waters, bottlenose dolphins in the Mediterranean are exposed to a wide variety of human activities. Whilst intentional killing was likely the most important cause of mortality until the 1960s (see previous section), important ongoing threats include incidental mortality in fishing gear and the reduced availability of key prey caused by region-wide overfishing and environmental degradation. Additional potential or likely threats include the toxic effects of xenobiotic chemicals, epizootic outbreaks, direct disturbance from boating and shipping, noise, and the consequences of climate change. It is worth noting that this same array of known and potential threats applies to riverine, estuarine and coastal cetaceans in many other parts of the world as well (e.g. Reeves et al., 2003).

CONSERVATION FRAMEWORK

Large-scale campaigns to cull dolphins have stopped, cetaceans are protected by law in most Mediterranean countries and deliberate killing has become far less frequent than in the past. Several attempts have been made to ban some fishing gears or methods, such as driftnets, that cause high dolphin mortality. In addition, the existing laws and treaties in force today provide a potentially solid framework for the conservation of bottlenose dolphins and other cetaceans, and for the protection of their habitat in the Mediterranean region. Unfortunately, implementation and enforcement of those instruments have not been consistently effective. Actions taken to date have not been adequate to maintain a ‘favourable’ status of bottlenose dolphin populations (as advocated by the EU Habitats Directive) or to prevent further population decline. Tangible actions to protect bottlenose dolphins and other Mediterranean cetaceans have been surprisingly few, especially if one considers the large number of existing laws, regulations and agreements that have been in place for years and even decades (Table 3).

About a hundred national Marine Protected Areas (MPAs) of different types, sizes and purposes have been established in Algeria, Croatia, Cyprus, France, Greece, Israel, Italy, Lebanon, Malta, Monaco, Morocco, Slovenia, Spain, Syria, Tunisia and Turkey, but specific measures for cetacean conservation are rarely included in their management plans. A notable exception is the 87 000 km² cetacean sanctuary created in 1999 by France, Italy, and the Monaco Principality in the Corso-Ligurian-Provençal Basin (the ‘Pelagos Sanctuary’; Notarbartolo di Sciara et al., 2008). In addition, 3 years of provisional protection (from July 26th, 2006) were granted to the waters east of the islands of Losinj and Cres (Croatia) following a proposal for the creation of a special zoological reserve for dolphins. After that period, the protection is expected to become permanent. If appropriately managed, MPAs could contribute to bottlenose dolphin conservation by preserving their prey and habitat, reducing the risks of mortality in fishing gear, providing refuge from noise and other types of disturbance, raising awareness, stimulating research and facilitating exchange of information (Hoyt, 2005). However, levels of enforcement are often low and many of the existing Mediterranean MPAs merely represent ‘paper parks’ (Bearzi, 2007; Guidetti et al., 2008). Other types of action that can provide direct or indirect benefits to bottlenose dolphins include area-, season-, or fishery-specific reductions in fishing effort, changes to fishing gear or fishing practices to reduce incidental mortality, curtailment of inputs of toxic pollutants, and boating regulations.

CONCLUSIONS

Bottlenose dolphins have apparently been better able than short-beaked common dolphins (the other predominantly coastal cetaceans in the Mediterranean) to persist in spite of attempts at eradication and the habitat degradation caused by constantly increasing coastal development (Airoldi & Beck, 2007) and use of destructive fishing practices. The flexible social organization and opportunistic diet and behaviour of bottlenose dolphins probably make them relatively resilient and able to adapt to changing environmental conditions. Still, bottlenose dolphins were probably much more abundant in the Mediterranean in historic times (see ‘Past culling campaigns’) and our present perception of the species' conservation status in the region may be influenced by the ‘shifting baselines syndrome’ (Pauly, 1995; Sáenz-Arroyo et al., 2005).

Bottlenose dolphins have been studied only in relatively small portions of the Mediterranean basin; wide areas remain largely unexplored. Research must play a major role in filling the gaps in knowledge. Insufficient knowledge can delay the adoption of meaningful conservation measures and limit the efficacy of those in place. However, the risks of simply perpetuating calls for more research must also be considered. Inaction is too often justified by the inadequacy of information. Waiting for more and better data can delay the management process indefinitely, and unwillingness to act based on what is known can allow the status of cetacean populations to deteriorate further.

The fate of Mediterranean bottlenose (and other) dolphins depends on range States having the political will to take action to mitigate known anthropogenic threats. Management measures that could benefit bottlenose dolphins are already embedded in a large number of existing laws and treaties. If all such measures were to be fully implemented and enforced, and the range States were doing everything to which they were committed based on multiple obligations under agreements that are already in force with regard to fishing, pollution and other forms of habitat degradation, many of the problems would be addressed and the status of dolphin populations would likely improve. Compliance with existing obligations such as those listed in Table 3 therefore represents the highest management priority. National monitoring of population status and anthropogenic threats is also needed, and should result in conservation plans followed quickly by concrete action.

Mediterranean biodiversity is undergoing rapid alteration due to human activities (Bianchi & Morri, 2000) and attempts to slow this alarming, widespread trend are encountering substantial resistance. At least in part, this is because progress in conservation requires actions with socio-economic consequences. The existing legal commitments of countries in the region might go a long way towards mitigating or even solving some of the problems facing Mediterranean bottlenose dolphins. However, provisions that run counter to what is perceived as being in the self-interest of States, institutions, companies and individuals will continue to be difficult to implement and enforce. No less than a fundamental shift in public values may be required. In this context, efforts by committed scientists and individuals are extremely important, particularly when unambiguous science-based conservation messages are conveyed directly to managers and the general public.