Owing to their occurrence in coastal waters, common bottlenose dolphins are regularly exposed to a wide variety of human activities. Those in the Mediterranean were subjected to hunting until the 1960s (Bearzi et al. 2004). The main threats in recent times include: 1) reduced availability of prey caused by overfishing and environmental degradation; 2) incidental mortality in fishing gear; and 3) toxic effects of xenobiotic chemicals.
Excessive fishing pressure is a growing concern worldwide and is having profound direct and indirect impacts on Mediterranean ecosystems (Sala 2004). In the Mediterranean Sea, it is acknowledged that unsustainable fishing has led to dramatic ecological changes and caused the decline of many fish stocks, including key common bottlenose dolphin prey (Caddy 1997; FAO 1997, 2000). Nutritional stress may be a factor in the low density of bottlenose dolphins in several Mediterranean areas (Bearzi et al. 1999, 2005, 2006). Conversely, density is high where prey is abundant (e.g. in the Amvrakikòs Gulf, Greece; Bearzi et al. 2007).
Incidental mortality in fishing gear - particularly trammel and set gillnets, but also drift gillnets - is a frequent occurrence, and in some Mediterranean areas the available studies raise serious concern, suggesting that annual fishery-induced mortality is locally unsustainable (Silvani et al. 1992; Brotons and Grau 2005; Fortuna 2006).
Interference with coastal fisheries ("depredation") can result in animals being shot, harpooned or harassed (Bearzi 2002; Gazo et al. 2004) although such retaliation probably occurs less frequently now than in the past when dolphins were regarded as vermin and systematically persecuted (Bearzi et al. 2004). Intentional killing may still be a serious problem in areas where acute conflict exists. However, depredation or damage to fishing gear by the dolphins does not necessarily always lead to open hostility towards them. Attitudes towards dolphins along the Mediterranean coasts vary greatly according to cultural, religious or other factors.
Contaminant levels, particularly of organochlorine compounds, in Mediterranean bottlenose dolphins are very high compared to levels reported from other areas (Corsolini et al. 1995; Aguilar et al. 2002; Fossi and Marsili 2003) and are a concern due to their potential effects on reproduction and health (Fossi and Marsili 2003). At concentrations similar to or lower than those detected in Mediterranean animals, compounds such as PCBs have been associated with reproductive disorders and immune-system suppression in bottlenose dolphins from other populations (Lahvis et al. 1995; Schwacke et al. 2002; Hall et al. 2005). Although organochlorine contamination is decreasing in some areas (Tolosa et al. 1997), levels in Mediterranean cetaceans remain exceptionally high (Aguilar and Borrell 2004).
In addition to the main threats listed above, mass mortality (die-offs), direct disturbance from boating activities, and noise represent potential threats at local scales. Die-offs appear to have affected common bottlenose dolphins to a lesser extent than other Mediterranean species such as the striped dolphin (Aguilar and Raga 1993). However, bottlenose dolphins elsewhere have experienced mass mortality (Lipscomb et al. 1994; Duignan et al. 1996; Birkun et al. 1998). As mass mortality may be partly related to the animals' weakened immune systems induced by exposure to xenobiotics or by stress from poor nutrition (Aguilar and Borrell 1994; Calzada et al. 1996; O'Shea and Aguilar 2001), the risk to common bottlenose dolphins in the Mediterranean is considered high. Direct disturbance by recreational boating is another potential threat (Lusseau 2003; Constantine et al. 2004) that has been poorly investigated in the Mediterranean. The number of recreational boats was correlated with avoidance of certain areas by dolphins in the north-eastern Adriatic during the summer (Fortuna 2006).