Scientific name: Tursiops truncatus
Distribution within the Mediterranean:
Conservation status in the Mediterranean:
Conservation status in the world:
Between 1,9 and 4,1 meters in adults
Can be confused with:
Individual identification characteristics:
The bottlenose dolphin (Tursiops truncatus) is a cetacean of the family Delphinidae in the suborder odontoceti. It’s probably one of the best known cetaceans, primarily because it’s one of the cetaceans most commonly seen in dolphinariums worldwide, on TV and even in movies. It is a large-sized dolphin, relatively robust and with a rather short and stocky beak, clearly differentiated from the melon. The dorsal fin, which is large, tall, wide-based, falcate and pointed at the tip, is located in the middle of the back. The pectoral fins are moderately long, thin, dark and pointed at the tips.
Its pigmentation is rather monochromatic, being mostly dark at the back (although it varies from pale gray to almost black) and white, and sometimes pinkish, at the belly. Some individuals may show a paler blaze that elongates from the upper part of the pectoral fins to the dorsal cape.
Bottlenose dolphins have between 36 and 54 pointed teeth in each jaw. In adult animals many of the teeth may be worn down or missing.
It is one of the biggest members of the family Delphinidae. Its maximum body size varies depending on the region, in a way inversely proportional to the water temperature. Bottlenose dolphins usually reach 1,9 – 4,1 metres, males being slightly bigger than females. The bigger animals live in the northwestern Atlantic, in UK waters. Its length at birth oscillates between 85 centimeters and 1,4 meter. Its maximum weight is 650 kg, although most individuals are much smaller.
DISTRIBUTION AND MIGRATORY PATTERNS WITHIN THE MEDITERRANEAN SEA
The bottlenose dolphin is a widely distributed species that usually lives in shallow waters of the continental shelf from temperate to tropical regions. Some authors believe that it was continuously distributed in coastal waters in the Mediterranean Sea. It is still more abundant in coastal waters above the continental shelf, but its distribution is more fragmented (Reeves & Notarbartolo di Sciara, 2006). It has been spotted in all areas of the Mediterranean basin, from the Strait of Gibraltar to the Dardanelles and although it has also been seen off the northern coast of Africa, the research effort on the species has been more concentrated on the northern part of the basin. It has also been found in some archipelagos such as the Balearic Islands or around islands like Corsica, Sardinia or Sicily (Forcada et al., 2004 & Bearzi et al., 2008a). It is abundant in the Black and Adriatic Seas, in some areas of the Aegean Sea, off the northern coasts of Corsica and Sardinia, off the coast of Algeria and in some regions of the Catalan coast and south of the Iberian Peninsula.
Some studies based on DNA microsatellites and mitochondrial DNA suggest that there are 3 different populations in the Mediterranean and Black Seas area. These populations are separated by physical boundaries (green lines on the map) (Natoli et al., 2005), although these boundaries would not restrict the movement of the species along the basin, but would separate regions with different habitats. The three populations living in the Mediterranean area would be genetically distinct from the ones living in the Atlantic ocean. Thus the boundaries located in the strait of Marmara, would separate the Black Sea and the eastern Mediterranean populations. The Italian peninsula and Sicily would form another barrier which would clearly separate the western and eastern Mediterranean populations. The last boundary, the Strait of Gibraltar, separates the Atlantic population from the western Mediterranean population. The latter boundary could be influenced by the Almeria-Oran front which would also affect the prey distribution of the Bottlenose dolphin.
HABITAT AND FEEDING
The bottlenose dolphin is an opportunistic feeder and its diet is strongly influenced by the habitat where the animals live (Bearzi et al., 2008). In fact, the feeding behaviour as well as the diet, changes depending on the area, the season, the habitat, the ecological niche and even the demography of the group (Gannon & Waples 2004). Some studies show that there are differences between coastal and offshore ecotypes (Walker et al., 1999). It generally eats demersal fish species such as the European hake (Merluccius merluccius), the European conger (Conger conger), the red mullet (Mullus barbatus), the striped red mullet (Mullus surmuletus), although they can also eat cephalopods such as the common cuttlefish (Sepia officinalis) or the common octopus (Octopus vulgaris) (Bearzi et al., 2008).
The diet is highly variable, even groups living close to another prey on different species. In the Mediterranean there are some differences between the different regions. In the Ionian Sea, bottlenose dolphins feed on demersal fish species (Bearzi et al., 2005a), in the Amvrakikos Gulf (Greece) it mainly feeds on fishes of the family Clupeidae such as the European pilchard (Sardina pilchardus) or the round sardinella (Sardinella aurita) (Bearzi et al., 2008b), in the northwestern Mediterranean its main prey is the European hake (Merluccius merluccius) and some cephalopods (Blanco et al., 2001) and off the northeastern coast of Sardinia the main prey are some fish species such as the common grey mullet (Mugil cephalus), the salema (Sarpa salpa) and the sardinella aurita (Sardina pilchardus) that are attracted to floating fish farms (Díaz López, 2006a). In fact, the latter are also a feeding source for the bottlenose dolphin, that feeds on the fish that grow there such as the European seabass (Dicentrarchus labrax), the gilthead seabream (Sparus aurata) and corb (Sciaena umbra) (Díaz López, 2006a).
The bottlenose dolphin is considered a rather coastal species, although it has also been seen in pelagic waters around oceanic islands or in waters below the continental shelf, especially above the continental slope. In the Mediterranean Sea, its habitat changes depending on the region. In this sense, bottlenose dolphin can inhabit shallow waters (less than 50 meters) close to the coast such as the gulf of Trieste (Genov et al., 2008), in the mouth of the river Po, (Triossi et al., 2012) or in the semi-enclosed gulf of Amvrakikos in Greece (Bearzi et al., 2008b). Bottlenose dolphins can also be seen in waters around archipelagos or island of the Mediterranean Sea, and in waters above the continental shelf and slope (Forcada et al., 2004; Bearzi et al., 1997). In areas with a high abundance of cetaceans such as the Ligurian sea, the bottlenose dolphin is associated with waters of the continental shelf (Azzelino et al., 2008), while in very productive waters, such as the Alboran sea, it usually inhabits waters between 200 and 400 meters deep, avoiding waters deeper than 600 meters (Cañadas et al., 2002; Cañadas & Hammond 2006).
REPRODUCTION AND LIFE HISTORY
Pregnancy lasts for 12 months and births show a certain degree of seasonality with a peak in spring and summer. Newborns will be totally mother-dependent for at least 1 year, during which they mainly feed on milk. In many cases, lactation continues for another year. Weaning occurs when the calves are 18 – 20 months old, although they can remain with their mothers until they are 6 years old. Calves usually leave their mothers at the moment the next calf is born.
Females reach sexual maturity at the age of 5 – 13 years, usually before males, which attain sexual maturity at the age of 9 – 14 years. At this moment, female measure between 220 and 235 cm and males measure between 245 and 260 cm. During the first stage of their lives the female growth rate is faster, but at the age of 5 to 6 years, the trend reverses and from that moment, males growth rate is faster (McFee et al., 2010). Females are the first to reach the maximum size, when they are 10 years old, while males reach their maximum seize at the age of 10 – 20 years. (Wells, 2003). Maximum age for females is 57 years and for males is 48.
ECOLOGY, BEHAVIOUR AND IDENTIFICATION
Group composition changes depending on the region, the habitat, the prey availability or the activity of the animals among other factors. Groups inhabiting coastal waters are usually small in size, from 2 to 20 individuals, while groups living in pelagic areas are larger sometimes numbering more than 1.000 animals (Shane et al., 1986; Perrin et al., 2009; Jefferson et al., 2008). The Mediterranean sea is not an exception and many studies conducted in the area have spotted small groups. Thus studies conducted in Greece, Sardinia, the Adriatic Sea, the Balearic Islands, the Alboran Sea show that groups are usually formed by 1 to 15 individuals with a mean of 7 animals. There are between 43 and 69 individuals in the biggest groups (Bearzi et al., 1997; Díaz López & Bernal Shirai, 2008; Díaz López et al., 2013; Forcada et al., 2004; Genov et al., 2008; Cañadas & Sagarminaga, 1994; Gannier, 2005; Gomez de Segura et al., 2006). Some studies have shown that the groups containing a calf are larger that those formed only by adults (Bearzi et al., 1997). Bottlenose dolphins do not usually associate with other species but there are some areas where such associations can be seen (Cañadas et al., 2002).
Group composition is rather dynamic and there are different social units such as nursery groups, mixed groups of juveniles or adult males as individuals or strongly bonded pairs (Perrin et al., 2009; Shane et al., 1986). Nursery groups are usually formed by reproductive females and their calves, which can stay with their mothers for 3 to 6 years. Mixed groups of juveniles are rather large, formed by sub-adult males or individuals of both sexes and usually move within some portions of the distribution range. Females can stay in the group or return to their mothers for short periods of time, but males stay together and they start developing close associations with other males of the same age that will last over time. (Wells, 2003). Male groups are small, usually formed by one or two individuals. These are strong bonds that can last for more than 20 years. They interact with female groups.
The bottlenose dolphin is an active species during the day but also during the night and it shows different kinds of aerial behaviour. They are usually seen surfing the waves caused by vessels or bigger whales, fluke slapping or performing acrobatic breaches. It is a very good swimmer and its dives usually last for 3 to 4 minutes in shallow waters and a little bit more in deeper waters. It show a great variety of feeding behaviours such as different speeds chases, fishwhacking, in which fish are struck with the dolphin’s flukes, pushing fish onto shore or kerplunking herding fish with consecutive leaps and fluke slaps.
Together with the killer whale (Orcinus orca), the bottlenose dolphin has one of the best developed communication systems among all cetaceans. It produces three types of acoustic signals including ecolocalization clicks, which are used in navigation, feeding and detection of predators; pulses that are used in some social interactions and whistles (Perrin et al., 2009). The latter are specific for each individual and are used to recognize every animal individually (Janik et al., 2006). Whistles are mainly used when two groups meet at sea (Quick et al., 2012).
It is rather easy to distinguish the species at surface thanks to its tall, wide-base dorsal fin; its size, it is bigger and more robust than the other dolphin species of the Mediterranean Sea, the striped dolphin (Stenella coeruleoalba) and the short-beaked common dolphin (Delphinus delphis); and the grey coloration of its body. It can be confused with the Risso’s dolphin (Grampus griseus), although the latter species does not have a differentiated beak, has many scars on the body of the older individuals and it is a little bit bigger than the bottlenose dolphin.
INDIVIDUAL IDENTIFICATION CHARACTERISTICS
As with other cetacean species, the bottlenose dolphin has unique and stable features, that help in the identification of every individual of a certain population. In this case, researchers mainly use 3 traits, all of them located in the dorsal fin (Würsig & Jefferson 1990):
- Shape of the dorsal fin and notches in its posterior margin: The shape of the dorsal fin doesn’t change over time and therefore it is a useful trait in order to do a first selection between different groups of individuals. In addition notches or nicks in its posterior margin also show stability over time (Würsig & Jefferson 1990; Wells & Scott 1990) and allow individual identification. In that case, some authors note down the shape, the position, the size and the number of marks and they classify the different animals following these parameters. Other authors developed a technique based on the Dorsal Ratio which can be only used in fins with two or more notches (Defran et al., 1990), and it’s calculated as follows. First we have to select two notches, then divide the distance from the top point of the upper notch (A) to the top point of the lower notch (B) by the distance from the top of the dorsal fin (T) and the top point of the lower notch (B). The advantage of this technique is that the ratio does not change if the size of the photo changes.
- Scars on the dorsal fin and on the back: The scars on the dorsal fin and back are used as secondary identification traits because of their tendency to change over time, especially the most superficial ones (Lockyer & Morris 1990). This type of scar is really useful if they are combined with other identification traits such as the notches of the dorsal fin. Some authors don’t catalog individuals that are only identified using such marks. (Berghan et al., 2008).
- Pigmentation spots on the dorsal fin: Some authors believe that this feature changes over time (Weir et al., 2008), but it is sometimes used as a complement to other permanent traits detailed above. These are abnormal pigmentation that are unique to each individual.
All these traits are used in many photo-identification studies, but in order to be fulfilled successfully, there have to be new sightings of previously spotted and photographed individuals. It is also important to have photographs of both sides of the dorsal fin. If not, the same individual could be classified as two different animals when considering the two sides of the fin as two different dolphins. It is also recommended to identify more than one of the identification traits detailed above for each individual (Würsig & Jeferson 1990).
CONSERVATION STATUS AND MAIN THREATS
The IUCN (International Union for Conservation of Nature) listed the species in the world as least concern, although the trend of its population is unknown. The world’s population of the species is estimated at around 600.000 individuals (Perrin et al., 2009). The Pacific Ocean is the area with the largest populations. Around 226.200 animals live in the Eastern Tropical Pacific (Wade, 1993), about 168.000 in the northwestern Pacific (Miyashita, 1993), 3.215 individuals in waters around Hawaii (Barlow, 2006) and about 2.273 in the waters off California, Oregon and Washington (Forney 2007). In the Atlantic Ocean, numbers aren’t as big as in the Pacific Ocean. Around 123.762 individuals live off the eastern coast of North America, 52.000 in the Gulf of Mexico, about 12.6999 in European Atlantic waters and 1.000 individuals around the Faroe Islands. Some thousands of bottlenose dolphins live in the Black Sea, although the exact number remains unknown. There are not exact estimations of the number of individuals living in the Mediterranean Sea, but some studies conducted in the northwestern part of the basin estimated that there are about 7.654 individuals around the Balearic Islands (Forcada et al., 2004) and about 584 in the Alboran Sea (Cañadas & Hammond 2006). The whole population of the Mediterranean is believed to be composed of 10.000 individuals (Reeves et al., 2006). These numbers and the fact that the population has been decreasing during the last decade, had led the IUCN to list the species as vulnerable in the Mediterranean Sea. The main threats of the species in this area are:
- Intentional kills: This was probably the most important cause of the decline of the population until the 1960s. Thanks to the governments of the different countries, intentional killing is now illegal and has almost disappeared. The culling campaigns have their origin in the eighteenth century, as dolphins started to be seen as competitors by fishermen. These campaigns took place in countries like Spain, Italy or Croatia. They were really important in the Adriatic Sea during the second part of the twentieth century (Bearzi et al., 2004), when different culling campaigns where organized and fishermen were rewarded by the different governments for every dolphin killed. Although the numbers don’t differentiate between species, 335 dolphins (mainly short-beaked common dolphins Delphinus delphis and bottlenose dolphins) where killed in the Adriatic Sea between 1933 and 1935, and 600 individuals where killed between 1955 and 1960.
Towards the end of the twentieth century, as the concern over marine mammals grew, new laws where made that forbade the indiscriminate killing of dolphins (1979 in Italy and 1995 in Croatia). Although these new laws and several international conventions are in place, the intentional killing of marine mammals still exists and it is an issue of concern throughout the world (Bearzi et al., 2004).
- Overfishing: Although the overlapping between the prey species of the bottlenose dolphin and the main target species of the world’s fisheries may not suppose a direct competition, the overfishing in waters above the continental shelf, main feeding habitat of the bottlenose dolphin, may affect the species in the future (Bearzi et al., 2008). In the last decades the improvement of different fishing gear and methods entailed a rise in the number of fish caught. This fact is specially obvious in the Mediterranean Sea, where species like the European hake (Merluccius merluccius), the red mullet (Mullus barbatus), the striped red mullet (Mullus surmuletus), the European pilchard (Sardina pilchardus) or the common octopus (Octopus vulgaris), all of which are considered as part of the bottlenose dolphin’s diet, are currently being overfished in the Mediterranean (Lleonart, 2005).
Although an opportunistic species like the bottlenose dolphin may have more chances of surviving than more specialized species, different authors have seen that the abundance of the species is higher in those areas where the fishing pressure is less (Bearzi et al., 2008).
- Fisheries interaction: The association of the bottlenose dolphin with some fishing methods such as trawling, gill-nets and even marine fish farms is enhanced by the fact that dolphins expend less energy to catch the same amount of fish. This relationship may cause harm and even the death of the dolphins. There are other fishing gear that may affect the species, especially gill-nets (used in countries like France, Greece, Italy, Spain or Tunisia), drift-nets (used in countries like Italy, France, Spain or Morocco) and tuna trap nets (Di Natale & Notarbartolo Di Sciara 1994; Díaz López, 2006b). Trawling fisheries may be significant locally, as seen off Israel, where 67 individuals where found dead between 1993 and 2004. Furthermore, anti-preadator nets located around fish farms, may also cause the death of some dolphins (Díaz López & Bernal Shirai 2007).
Some fishing gear have been forbidden by the authorities, but they could have caused a large amount of damage in the past. A good example are drift-nets. Considering just the Italian fleet, these fishing method could be responsible for the death of more than 8.000 dolphins, especially striped dolphins (Stenella coeruleoalba) and short-beaked common dolphins (Delphinus delphis) (Di Natale & Notarbartolo Di Sciara 1994). This is one of the major issues of concern of this species for the future. It is therefore necessary that more exhaustive studies on the effects of accidental catch are carried out, because it could be locally unsustainable. (Bearzi et al., 2008).
- Pollution: The pollution of the ocean is a important problem, that may affect all the species that inhabit them. Cetaceans, as predators located at the top of the food web, are especially vulnerable to pollution due to a process known as bioaccumulation, which causes the concentration of a pollutant in an organism to be higher than the concentration of this pollutant in the environment. Compounds such as DDTs or PCBs, which are resistant to degradation and have a lipophilic nature, tend to accumulate in the cetacean’s blubber, reaching extremely high levels (Borrell & Aguilar, 2007). Although the complete effects of these compound on the marine mammals are still unknown, it is known that they may cause disorders in the concentration of hormones, reproductive disorders (Reijnders, 1998), and they may affect the growth of the individuals and the immune system (Aguilar, 2000).
The Mediterranean Sea is one of the most polluted seas of the world and the DDTs and PCBs concentrations found in some cetacean species are the highest of the world (Aguilar, 2000). Although the concentrations of these compounds may be decreasing in some species (Borrell & Aguilar, 2007), it still is an important issue of concern, just because the levels in the dolphins are still too high. (Bearzi et al., 2008; Fossi et al., 2003). Furthermore, the effects of high levels of heavy metals, such as mercury, cadmium, arsenic or lead are still unknown, and these have been found in different species of cetaceans (Bilandzic et al., 2012; Lahaye et al., 2006; Bellante et al., 2012).
The presence of these pollutants, which cause the depression of the immune system of cetaceans, may facilitate the dispersal of infectious agents, such as Morbillivirus, that has caused more than one outbreak in the Mediterranean sea, especially affecting the striped dolphin (Stenella coeruleoalba) population (Aguilar, 2000 & Bearzi et al., 2008).
- Live capture: Capturing alive cetaceans and removing them from their population whether for their exhibition in dolphinariums or for scientific purposes, has the same effect as killing them, because the captive animal can not help in maintaining their natural population (Reeves et al., 2003). Although this has been, and still is an usual practice in some regions of Japan (Katsuya et al., 1984), there are only a few cases reported in the past in the Mediterranean Sea (Collet, 1984). The live capture of individuals still occurs sporadically in the Mediterranean. For example, 30 individuals were caught in Turkish waters in 2006 (Bearzi et al., 2008). Although live capture of dolphins may not have a global effect, it can have a bigger effect on local populations (Bearzi et al., 2008).
- Other threats: There are other threats, such as maritime traffic or submarine noise, affecting the population of bottlenose dolphin in the Mediterranean sea. Different studies have shown that both examples may alter the cetaceans behaviour and new studies should be developed in order to get a better understanding of the specific effects on cetaceans of these threats. In fact, human-made noise (seismic surveys, drilling or underwater explosions), may be an issue of concern in the future.
Climate change may also alter the distribution and abundance of cetaceans in the coming years. Although it is not possible to predict the extent of its effects, it is known that they moderate in an indirect way the different species. For example creating changes in the distribution and abundance of the cetacean’s prey species.
There are several international conventions protecting the bottlenose dolphin, amongst them the appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the appendix II of the Convention on Migratory Species (CMS) or the Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic Area (ACCOBAMS). They are also protected by the laws of many Mediterranean countries. Such protection has brought the creation of different Marine Protected Areas in different countries, although, there is only one specifically developed to ensure the protection of cetaceans. This area is the Pelagos Sanctuary, in the Ligurian Sea, created in 1999 and located between the northern coast of Corsica and Italy.
In order to ensure the conservation of the species in the Mediterranean sea, more studies that allow the scientific community to expand the knowledge on the species should be developed. The lack of knowledge may by harmful because it can delay the approval of new important conservation measures or affect the efficiency of the existing ones (Bearzi et al., 2008). It is therefore necessary that different countries have the political will to establish new measures in order to mitigate the threats caused by human activities (Bearzi et al., 2008).
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