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 (Bearzi et al. 2004, 2008), 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).
Fishery-related mortality
Due to their opportunistic behaviour and predominantly coastal occurrence, Bottlenose Dolphins in the Mediterranean are at risk of entanglement in many types of fishing gear. In addition to incidental mortality, depredation and damage caused by dolphins to fishing gear may result in animals being shot or harassed in retaliation (Di Natale and Notarbartolo di Sciara 1994). Incidental mortality of Bottlenose Dolphins has been reported from Algeria, Croatia, France, Greece, Israel, Italy, Malta, Morocco, Spain, Tunisia, and Turkey (Bearzi et al. 2008). Bycatch in set nets reportedly is frequent in coastal waters throughout the basin (Di Natale and Notarbartolo di Sciara 1994, Díaz López 2006, Brotons et al. 2008). Significant mortality also was reported in pelagic driftnets off Morocco, Spain, Malta, Italy and Turkey (Di Natale and Notarbartolo di Sciara 1994, Di Natale 1995). Few attempts have been made to assess the impact of fishery-related mortality on local populations of Bottlenose Dolphins, and the actual magnitude of bycatch and retaliation events is unknown in most cases (Bearzi et al. 2008). However, the available studies and circumstantial evidence for local populations raise serious concern, suggesting that annual fishery-induced mortality is locally unsustainable in at least some cases (e.g. Brotons et al. 2008). Data from strandings can be informative with regard to the occurrence and relative scale of bycatch. Along the Italian coasts in the years 1986–2005, Bottlenose Dolphins were the second most numerous species classified as bycaught after the Striped Dolphin (301 and 71 records, respectively; Podestà 2007). Of 694 Bottlenose Dolphins stranded in Italy during the same period, 71 (11%) showed signs of bycatch (Podestà 2007). Of a total of 21 Bottlenose Dolphins stranded in France in 2003, eight reportedly had been bycaught (Dhermain 2003). Signs included specimens gutted, missing peduncles or fins, obvious net scars, and/or ropes tied to the tails. These percentages are likely underestimates, considering that the majority of the remaining carcasses were in an advanced state of decomposition and therefore signs, if present, would not necessarily have been observable. Bycatch in trawl nets appears to be infrequent in most Mediterranean areas, but may be locally significant. For instance, of 67 Bottlenose Dolphins found dead stranded or adrift along the coasts of Israel between 1993 and 2004, 26 (39%) were judged to have been taken in trawl nets (Feingold et al. 2005). Bottlenose Dolphins have not been reported entangled in fish farm gear in the Mediterranean, except when anti-predator nets (mesh size 15 cm) are deployed (Díaz López and Bernal Shirai 2007). The overall frequency of intentional killing (see ‘Past culling campaigns’) has drastically declined over the years, due in part to protective legislation in most Mediterranean countries. However, targeted kills still occur in certain areas (e.g. Tudela 2004, Gazo et al. 2008). In addition to killing in retaliation for damage to fisheries, killing with harpoons or guns for local consumption of meat was reported as recently as the early 1990s in the Ligurian and Tyrrhenian seas, notwithstanding legal protection (Di Natale 1991, Di Natale and Notarbartolo di Sciara 1994). Such occurrences seem to have become rare in more recent times. The illegal use of dynamite for fishing in several Mediterranean areas (e.g. Reynolds et al. 1994, Tudela 2004) represents another fishery-related threat to Bottlenose Dolphins. Though impact at the basin level is probably low, it may be significant locally and a few Bottlenose Dolphin deaths suspected to have been caused by explosives have been reported.
Prey depletion
Overfishing is having profound direct and indirect impacts on Mediterranean ecosystems (Sala 2004) and it has contributed significantly to dramatic ecological changes, including decline of dolphins and other megafauna (Bearzi et al. 2008) and caused the decline of many fish stocks (Caddy and Griffiths 1990, De Walle et al. 1993, Stanners and Bourdeau 1995, Garcia et al. 2005). Some of the Mediterranean fish stocks that have been either ‘overexploited’ or ‘fully exploited’ include important bottlenose dolphin prey (Bearzi et al. 2008, 2010). Reduced carrying capacity (i.e., fewer prey available) due to overfishing was proposed as one explanation for the low densities of Bottlenose Dolphins in the Adriatic and Ionian Seas. Conversely, Bottlenose Dolphin densities tend to be high in areas where prey is still abundant (Bearzi et al. 2008).
Contamination by xenobiotics
Contaminant levels, particularly of organochlorine compounds, in Mediterranean Bottlenose Dolphins are very high compared to the levels reported for Bottlenose Dolphins in some other areas (Corsolini et al. 1995, Marsili and Focardi 1997, Aguilar et al. 2002, Fossi and Marsili 2003, Wafo et al. 2005, Borrell et al. 2006, Borrell and Aguilar 2007, Storelli et al. 2007). At concentrations similar to or lower than those documented for Mediterranean Bottlenose Dolphins, compounds such as PCBs or PAHs have been associated with reproductive disorders, immune-system suppression and neoplasia (Lahvis et al. 1995, Reddy et al. 2001, Schwacke et al. 2002, Jaber et al. 2005, Hall et al. 2006). Although organochlorine contamination is decreasing in some areas, levels in Mediterranean Bottlenose Dolphins remain high (Tolosa et al. 1997, Aguilar and Borrell 2004, Borrell and Aguilar 2007, Storelli et al. 2007). Monitoring of toxic chemicals, risk assessment and intervention protocols therefore represent important precautionary measures (Schwacke et al. 2002, Fossi and Marsili 2003, Jaber et al. 2005, Porte et al. 2006). Various and sometimes high levels of heavy metals have been found in stranded bottlenose dolphins from the Mediterranean (e.g. Leonzio et al. 1992, Frodello et al. 2002, Roditi-Elasar et al. 2003, Lahaye et al. 2006). The effects of these metals at the population level are unknown.
Epizootic outbreaks
Epizootic outbreaks appear to have affected Bottlenose Dolphins to a lesser extent than other Mediterranean delphinids, such as the Striped Dolphin (Aguilar and Raga 1993, Van Bressem et al. 1993). Morbillivirus infections have been reported in a Bottlenose Dolphin stranded on the Mediterranean coast of Israel in 1994 (Tsur et al. 1997) and another stranded in Mauritania (Atlantic coast of West Africa) in 1988 (Van de Bildt et al. 2001). Bottlenose Dolphins elsewhere have experienced mass mortality from such outbreaks, e.g. in Black Sea waters (Birkun et al. 1998, Birkun, 2006) and on the Atlantic coast of the United States, where more than half of one local population may have died (Lipscomb et al. 1994, Duignan et al. 1996, Schulman et al. 1997). As epizootic phenomena may be related to immune-system compromise induced by exposure to xenobiotics and/or by stress from poor nutrition (Aguilar and Borrell 1994, Calzada et al. 1996, O’Shea and Aguilar 2001), the risk of disease outbreaks in Bottlenose Dolphins in the Mediterranean may be considerable.
Boat traffic and acoustic disturbance
There has been a great expansion of recreational boat traffic and shipping in the Mediterranean in recent decades (Dobler 2002) but the potential for resultant behavioural disruption and habitat loss has been investigated only to a limited extent. Permanent or temporary avoidance of one Mediterranean area by Bottlenose Dolphins as a consequence of a large seasonal increase in boat traffic was reported in coastal waters of Croatia (Fortuna 2006). Similar negative effects have been reported from other places around the world (Allen and Read 2000; Lusseau 2004, 2005; Bejder et al. 2006). The noise from various human activities in addition to boating/shipping - e.g. seismic surveys, dredging, drilling, underwater explosions, and the use of military or other sonars – is also a cause for concern (Richardson et al. 1995, Nowacek et al. 2007).
Climate change
Some of the effects of global warming have become dramatically apparent in recent years (IPCC 2007). Climate change has the potential to affect a range of biological processes and cause significant shifts in marine and other biota (e.g., Peñuelas et al. 2002, Parmesan and Yohe 2003, Diaz-Almela et al. 2007). Increased seawater temperature has been observed in Mediterranean deep (Bethoux et al. 1990) and surface waters (Metaxas et al. 1991, Bethoux and Gentili 1996) and there is increasing evidence of biological responses to such warming (e.g., Francour et al. 1994, Diaz-Almela et al. 2007). Effects of climate change on cetaceans, often mediated via changes in prey abundance and distribution, have become apparent in several non-Mediterranean areas (Learmonth et al. 2006, Simmonds and Isaac 2007) and similar effects may be occurring in Mediterranean waters (Azzellino et al. 2008, Cañadas and Hammond 2006). At present, however, it is impossible to predict the net effect of climate change on Bottlenose Dolphins – either in the Mediterranean Sea or elsewhere.
Live capture
The removal of live cetaceans from the wild – whether for captive display, for ‘dolphin-assisted therapy’ (Marino and Lilienfeld 2007) or for research purposes – is equivalent to incidental or deliberate killing, as the animals brought into captivity or killed during capture operations are no longer available to help maintain their wild populations (Reeves et al. 2003). ‘Takes’ of Bottlenose Dolphins are prohibited in most Mediterranean riverine States by national legislation or international agreements (Bearzi et al. 2008). However, live captures (and any mortality and social disruption associated with capture operations) still occur occasionally (Bearzi et al. 2008).
