Common bottlenose dolphin
Updated: October 2018
The common bottlenose dolphin is perhaps one of the best-known Cetaceans. It has appeared in numerous films and TV shows, and is frequently displayed in zoos and aquariums. This dolphin is a widespread and abundant species, occurring in tropical and temperate coastal and pelagic marine waters worldwide. The name comes from the distinct rostrum (the “beak”) found on the front of the head, and the gently curved mouth gives the appearance of smiling. This is a relatively large, robust dolphin, with light grey to black colouration dorsally, and white or cream colouration ventrally. In some areas, distinct inshore and offshore populations can be found, with the inshore dolphins being generally smaller in size than the offshore.
It has been estimated that there is a minimum of 600,000 bottlenose dolphins world-wide (Hammond et al. 2012).
Common bottlenose dolphins have a worldwide distribution in temperate and tropical waters
Harvests for human consumption or bait in fisheries in certain parts of the world.
The common bottlenose dolphin is a widespread and abundant species, and is listed on the IUCN Red List in the category of Least Concern (Hammond et al. 2012).
Latin: Tursiops truncatus
Icelandic: Stökkull, höfrungar
Danish: Øresvin, døgling delfin
English: Common bottlenose dolphin, bottlenose dolphin
More than 40 years for males and 50 years for females
2.6 m and 190-260 kg (coastal) / 3.7-3.8 m and 450 kg (offshore). Males are typically larger. Calves are between 84-140 cm and 20 kg at birth.
MIGRATION AND MOVEMENTS
Some coastal dolphins maintain long-term, multigenerational home ranges, others have more complex movements. Offshore dolphins generally range over much greater areas and in larger groups, showing seasonal migration.
Common bottlenose dolphins show a varied diet depending on season and location, adapting both their foraging tactics and prey selection to local conditions. Pelagic and demersal fish are the main components, but they also feed on cephalopods and crustaceans.
The common bottlenose dolphin (Tursiops truncatus) is perhaps one of the best-known Cetaceans, and likely what most people picture when they think of a dolphin. It is a widespread and abundant species, occurring in tropical and temperate coastal and pelagic marine waters worldwide. Bottlenose dolphins are familiar from appearances in films and TV shows, as well as from being exhibited in numerous zoos and aquariums.
Their name comes from the distinct rostrum found on the front of their head (often referred to as a beak), which is typically 7 to 8 cm long. Their mouths are gently curved, dipping slightly downward from the tip of the beak and upwards at the base, giving them the appearance of smiling. The beak contains from 76 to 98 conically-shaped teeth.
The bottlenose is a relatively large, robust dolphin. The body is streamlined with a tall, falcate dorsal fin located in the middle of their back. There are considerable regional variations in body size, to the extent that different “morphotypes” or “ecotypes” are recognized throughout its range (Oudejans et al. 2015). In some areas, distinct inshore and offshore populations can be found, with the inshore dolphins being generally smaller in size than the offshore. The differences between the two types are large enough that they may eventually be classified as separate species (Hammond et al. 2012).
Average body length for the coastal ecotype of this dolphin is around 2.6 m, with weights from 190 to 260 kg. Offshore dolphins can reach 3.8 m in length for males and 3.7 for females. An average body weight offshore is around 450 kg, although weights up to 650 kg have been recorded (www.WDC.org). Males of both ecotypes are typically larger than females, although considerable variation in size‐at‐age for both sexes at all ages has been noted (Read et al. 1993). Calves are from 84 to 140 cm long at birth and weigh approximately 20 kg (Wells and Scott 1999).
The bottlenose dolphin is a light grey to black colour dorsally, with white or cream colour underneath. Some individuals may also have lighter colour on the sides.
Bottlenose dolphins are a long-lived species. Dolphins found stranded along the Mississippi coast in the Gulf of Mexico were found to be up to 30 years old (Mattson et al. 2006). In a long-term study of dolphins in Florida, males were observed to live for more than 40 yrs and females for more than 50 (Connor et al. 2000).
All bottlenose dolphins around the world were previously recognized as Tursiops truncatus, but recently the genus has been split into two species: T. truncatus and T. aduncus. The latter species is called the Indo-Pacific bottlenose dolphin. There is also a separate subspecies of bottlenose dolphin recognized in the Black Sea, T. t. ponticus (Hammond et al. 2012).
Life History and Ecology
Bottlenose dolphins are very active at the surface, and can often be observed breaching or spyhopping. They may ride the bow waves of ships and in some locations are well known for following fishing vessels (Byrd and Hohn 2017, Jones and Sayigh 2002, Wells and Scott 1999). They are often observed associating with other species of dolphins and whales.
Although active at the surface, bottlenose dolphins spend most of their time underwater. One tagged female in Florida was found to spend about 87% of her time submerged (Mate et al. 1995). Bottlenose dolphins tend to make numerous short dives from the surface, rather than long deep dives. Dolphins in the Moray Firth, Scotland, were observed to spend most of their time within the surface layers of the water column. Relatively little time was spent below 10 m, although dolphins were occasionally seen diving to depths of around 50 m (Hastie et al. 2006).
Bottlenose dolphins are also known for their aggressive behaviour. Physical violence (body slamming, tail hitting, charging, jawing, and biting), hostile head-to-head posturing and acoustic threats have all been observed between dolphins (Robinson 2014), and many dolphins carry scars and rake marks from such encounters. Adult male dolphins have also been seen attempting to kill dolphin calves. Robinson (2014) describes one event in which an adult male chased, repeatedly rammed, tossed out of the water and attempted to asphyxiate a newborn calf. If the mother loses her calf in this way, she may become available for mating.
Dolphins may also be aggressive towards other species, particularly harbour porpoises. Violent and sometimes fatal attacks on harbour porpoises by bottlenose dolphins have been reported in several locations (Santos and Peirce 2003, Spitz et al. 2006). Around the Moray Firth, Scotland, 63% of the harbour porpoises found stranded appeared to have been attacked by bottlenose dolphins and died from trauma, specifically bone fractures and internal injuries (Ross and Wilson 1996).
Bottlenose dolphins produce a variety of sounds that are typically separated into three categories: whistles, burst-pulse sounds, and echolocation clicks (Jones and Sayigh 2002).
Whistles are used in communication, in social interactions and in maintaining contact among each other. Individual dolphins produce a “signature whistle”, a unique frequency modulation pattern which identifies the individual and is likely used to maintain social bonds. The burst-pulse vocalizations may also be used to communicate (Jones and Sayigh 2002). Dolphin communication is complex, and dolphins have been observed to modify their sounds through learning. One study observed dolphins using their whistles to “address” each other, as a whistle produced by one dolphin would be matched by a dolphin emitting the same whistle type (Janik 2000a).
Echolocation clicks are high-frequency sounds produced by the dolphins in order to locate prey and to navigate (Nuuttila et al. 2017). They are often emitted in “click trains”, a long sequence of sound that usually increases in speed as the dolphin gets closer to its prey. Another type of sound that has been observed associated with feeding is termed a “bray”. Dolphins in the Moray Firth were observed to emit these low-frequency sounds when feeding on salmonid fish (Janik 2000b). It is thought that the sounds are used to help the dolphins capture this particular prey.
Bottlenose dolphins are a highly social species, and are commonly found in small groups of from 2 to 15 individuals (Wells and Scott 1999), although larger groups are sometimes observed in offshore areas. The social structure is often described as “fission – fusion”, in which individuals associate in small groups that may change on a daily or even hourly basis (Connor et al. 2000). In some locations, associations between animals of a similar age and reproductive state are commonly observed, with adult dolphins associating with dolphins that they grew up with or spent time with as juveniles (Robinson et al. 2017). Such long-term alliances may be shaped by kin selection, mating strategies, ecological factors such as feeding strategies, or perhaps some combination of those (Louis et al. 2017). Dolphins in the Shannon estuary, however, do not appear to associate in such groups, but rather have strong and persistent bonds between individuals (Baker et al. 2017).
Cooperative foraging behaviour has been observed in bottlenose dolphins, in which groups work together to herd schools of prey fish (Wells and Scott 1999), sometimes encircling them in open water, and at other times driving them up onto a sloping shore.
Bottlenose dolphins are found stranded on coastlines throughout their range, and at various times of the year. Along the eastern coast of the US, from Florida to Maine, an average of 150 dolphins stranded each year in the period 1972 to 1997 (McLellan et al. 2002).
In some locations, strandings are more frequent at certain times of the year. Along the coast of North Carolina in the US, adult dolphins stranded most frequently during the spring and fall, and less frequently from June through August (Thayer et al. 2003). Neonates in this area stranded more often in April and May, with lower levels in the fall and winter. Along the north‐central Gulf of Mexico coast, strandings of both adult and neonate dolphins peaked in March and April (Mattson et al. 2006), and along the coast of South Carolina about half of the annual dolphin strandings occurred between April and July (McFee and Hopkins-Murphy 2002).
There are various reasons why dolphins may become stranded. Strandings may occur when dolphins follow a high tide in to coastal waters and become trapped when the tide recedes. Dolphins have also been seen to intentionally beach in order to catch fish that they have chased onto shore. This foraging strategy has been observed throughout their range (Silber and Fertl 1995).
Other stranded animals show signs of interactions with fishing gear, which may have contributed to their death prior to being found on shore. Between 1997 and 2008, 17% of all dolphins found stranded along the coast of North Carolina showed evidence of contact with some type of fishing gear (Byrd and Hohn 2017). In some cases gear is still attached to the stranded dolphin, while in others the dolphin shows lesions which have been caused by gear. On the coast of France, 21% of stranded bottlenose dolphins were found to have lesions caused by fishing gear (ICES 2018).
One study of strandings of common dolphins, bottlenose dolphins and harbour porpoises in north-western Spain looked at various meteorological, oceanographic, prey abundance and fishing-related variables to see if any pattern was apparent. They found that the local ocean meteorology (particularly the strength and direction of the winds) along with the winter North Atlantic Oscillation Index best explained the stranding pattern, and that there was no association with fishing effort or landings occurring in the same area (Saavedra et al. 2017).
Sexual maturity and breeding
The age at sexual maturity for bottlenose dolphins varies by both sex and location. In Florida, females were found to be sexually mature at from 5 to 12 years old, and males at from 10 to 13 years, while females along the Atlantic coast central US Atlantic coast were found to mature at from 7 – 13 years old (Wells and Scott 1999). In the Moray Firth, Scotland, females were observed to be mature at from 6 to 13 years old, and produced their first calf at a mean age of 8 (Robinson et al. 2017).
In the western north Atlantic, calves are born mainly in the late spring and early summer. In Sarasota Bay Florida, the calving peak occurred between May and July (Wells and Scott 1990), and similar timing was observed off the coast of North Carolina (Thayer et al. 2003). In the Moray Firth, calves were produced throughout the summer, from May to October, but with a seasonal pulse which corresponded to the peak in summer water temperatures (Robinson et al. 2017).
Gestation lasts for about 12 months, and a single calf is produced. At birth, calf lengths range from 84 to 104 cm (Wells and Scott 1999). The calf usually remains with its mother for several years. Milk is the primary food for the first year, but nursing has been observed to continue for up to 5 years (Connor et al. 2000). Females generally produce a calf every 2 or 3 years, but the interval may in some cases be longer. Dolphins in the Moray Firth were observed to have a mean interbirth interval closer to 4 years (Robinson et al. 2017). Factors such as maternal age and size, breeding experience, dominance, individual associations, group size and other social factors are all felt to influence reproductive success.
Food and feeding
Bottlenose dolphins have a varied diet, which changes by habitat and season. Studies in various parts of their range have shown that they take a wide variety of locally abundant prey items, adapting both their foraging tactics and prey selection to local conditions.
In North Carolina, for example, at least 31 different species of prey items from 21 different families were found in the stomachs of coastal bottlenose dolphins (Gannon and Waples 2004). Pelagic and demersal fish are the main components of the diet, but they are also seen to take cephalopods and crustaceans.
Dolphins in Cardigan Bay, Wales were found to prey on sandeels (Ammodytidae sp.), cod (Gadus morhua), whiting (Merlangius merlangius), sprat (Sprattus sprattus), Atlantic herring (Clupea harengus), mackerel (Scomber sp.) and a variety of flatfish (Nuuttila et al. 2017). Dolphins found stranded on the Scottish coast were found to have fed mainly on cod, saithe (Pollachius virens) and whiting, and to a lesser extent on salmon (Salmo salar), haddock (Melanogrammus aeglefinus), and cephalopods (Santos et al. 2001). Crustacean and polychaete remains were also found in a couple of stomachs in this study. Dolphins feeding offshore in the northeastern Atlantic appear to concentrate on shoaling fish such as Atlantic herring and whiting (Nuuttila et al. 2017). Whiting and hake (Merluccius merluccius) were also found to be important prey items for dolphins feeding off the northwest coast of Spain (Santos et al. 2007).
Differences in inshore and offshore feeding have also been noted on the US coast. Dolphins found stranded along the North Carolina coast had fed primarily on weakfish (Cynoscion regalis), Atlantic croaker (Micropogonias undulatus), and spot (Leiostomus xanthurus), as well as on inshore squid (Loligo sp.) and striped anchovy (Anchoa hepsetus) (Gannon and Waples 2004). Dolphins that were found stranded inside estuaries had a diet dominated by Atlantic croaker, while dolphins found stranded elsewhere had diets dominated by weakfish. Inshore squid was eaten commonly by dolphins in the ocean, but not by those in the estuaries. This study also found that there was considerable variation in the diet by season.
Variation in the diet between the sexes has also been noted in some locations. Dolphins in Sarasota Bay, Florida, take small fish largely associated with seagrass habitat such as pigfish (Orthopristis chrysoptera), pinfish (Lagodon rhomboids), mojarra (Gerreid sp.) but also larger species such as striped mullet (Mugil cephalus) (Rossman et al. 2015). This study found that while the males appeared to specialize on the seagrass- associated prey, the females in this area had broader foraging habits and took prey from a variety of habitats.
Sharks appear to be the main predators of bottlenose dolphins, though they may also be taken by killer whales (Orcinus orca). Dolphins in many locations show scarring from shark bites, and dolphin remains have been found in shark stomachs. Off the southeastern coast of the United States bull (Carcharhinus leucas), tiger (Galeocerdo cuvier) and dusky sharks (Carcharhinus obscures) have all been observed to prey on bottlenose dolphins (Wells and Scott 1999).
Health – diseases and parasites
Like all other Cetaceans, bottlenose dolphins can be affected by various diseases and may be infected by parasites. They are commonly found to have external lesions or scars on their bodies, either from shark attacks or attacks by other dolphins, or sometimes from interactions with fishing gear.
One disease that has been apparent in bottlenose dolphins is morbillivirus, which can rapidly affect many animals in an area. This virus has been responsible for several “Unusual Mortality Events” along the east coast of the United States. In a 10 month period from June 1987 to March 1988, a ten-fold increase in stranding rates was seen along the coast, which was later determined to be caused by morbillivirus (McLellan et al. 2002). Another outbreak of morbillivirus occurred along the coast between July 2013 and March 2015, killing some 1650 dolphins (Hayes et al. 2016).
A more complete discussion of some of the other diseases and conditions which affect bottlenose dolphins may be found in Eissa and Abu-Seida (2015).
Parasites may be found in or on various parts of the bottlenose dolphin’s body. Whale “lice”, a type of Cyamid amphipod, have been found on the skin, and in one case were associated with lesions which penetrated the skin into the underlying adipose tissue (Woodard et al. 1969). These amphipods do not appear to harm the dolphin.
Internally, dolphins have been found to be host to various types of helminth parasites. The nematode Anisakis sp. is a common parasite in the guts of various Cetaceans, and is present in bottlenose dolphins as well. In one case, this parasite was also found infecting the skin of a dolphin (Van Beurden et al. 2015). Other nematodes are present in the lungs. The lungworm Halocercus lagenorhynchi is commonly found in dolphins, and was present in 77% of stranded dolphins examined in Florida (Fauquier et al. 2009). Infection with these lungworms leads to mucopurulent bronchiolitis and pneumonia (Woodard et al. 1969). Parts of the lungs containing the nematodes eventually become walled off, sclerotic, and calcified. In most of the dolphins examined in Florida these calcified lesions were mild, chronic, and did not cause the death of the dolphins.
The digestive tracts of dolphins may harbour various species of nematodes, cestodes and digeneans or trematodes. Quiñones et al. (2013) found 7 different helminth species in 15 bottlenose dolphins from the western Mediterranean: 3 digeneans: Pholeter gastrophilis, Synthesium tursionis, and Brachycladium atlanticum; 1 nematode Anisakis sp.; and 3 cestodes: Tetrabothrius forsteri, Diphyllobothrium sp., and Strobilocephalus triangularis. Synthesium tursionis was the most prevalent of these parasites, found in over 70% of the dolphins examined (Quiñones et al. 2013). Another trematode which has been found in dolphins is Campula palliataI, which may infect the liver (Woodard et al. 1969).
Distribution and habitat
Bottlenose dolphins have a worldwide distribution in temperate and tropical waters. They are found in waters across the Atlantic Ocean, in both the northwest and northeast. Off the coast of North America, they are generally found in waters with surface temperatures ranging from about 10°C to 32°C (Wells and Scott 1999).
In the northwest Atlantic, they are seen inshore during the summer months off New England and along the US coast south to Florida. Offshore they range as far north as Nova Scotia (Wells and Scott 1999).
In the northeast Atlantic, they are regularly seen off the coasts of Portugal, Spain, France, the UK and Ireland (Hammond et al. 2013). Around the UK and Ireland, they have a patchy distribution with resident populations in Cardigan Bay (Wales) the Moray Firth (north-east Scotland) and the Shannon estuary (Ireland), but are also often sighted in waters to the north and west of Scotland, up to around the Faroe Islands. They are also found in the North Sea, and have been recorded along the Norwegian coast as far north as the Lofoten Islands.
Bottlenose dolphins inhabit a wide range of habitats throughout their worldwide distribution. Some groups of coastal dolphins may be found in estuaries, bays, lagoons and other shallow inshore regions, occasionally ranging far up into rivers. Offshore, some dolphins are resident around islands or archipelagos, for example around the Azores (Silva et al. 2008) or the Madeira Islands (Dinis et al. 2016). Other offshore bottlenose dolphins exhibit low site fidelity, ranging instead over large areas.
Migrations and movements
Some coastal dolphins maintain long-term multi-generational home ranges in certain bays or estuaries. In such locations, females generally range less than males, and have more site-fidelity (Robinson et al. 2017, Wells and Scott 1990). Other coastal dolphins have more complex movements, and may alternate between residing in one location and making occasional long-range travels (Wells and Scott 1999). Dolphins from around northern Scotland, for example, were seen to travel distances up to 1,277 km (Robinson et al. 2012), while a dolphin tagged off the Florida coast in the Gulf of Mexico covered a distance of 2,050 km in 43 days (Wells et al. 1999). Coastal dolphins around Ireland have also been documented making large-scale movements, around Ireland and between Atlantic coastal waters and the North Sea (Oudejans et al. 2015).
Offshore dolphins generally show low site fidelity and range over much greater areas in larger groups (Oudejans et al. 2015). At the northern edge of their range in the western north Atlantic they seasonally migrate, moving south in the winter months and north in the summer (Hammond et al. 2012). Seasonal patterns of movement have also been observed in bottlenose dolphins off Wales, with dolphins moving inshore and becoming more commonly seen in late summer (Nuuttila et al. 2017). This may correspond to seasonal concentrations of migratory fish that the dolphins are preying on. During the winter, dolphins in this area were observed moving further north and dispersing over a wider area. In the waters off Ireland, surveys carried out in 2015 and 2016 found an eightfold increase in bottlenose dolphin abundance during winter compared to summer (ICES 2018).
North Atlantic stocks
Bottlenose dolphins are a wide-ranging species and for the most part do not have clearly defined stocks in the north Atlantic. There is however some evidence of population structure in certain estuaries or around island groupings where they may be a resident population of dolphins, but even in those areas there are transient dolphins and much gene flow.
In the western north Atlantic, along the eastern coast of the United States, there are a number of stocks recognized. There are 5 coastal stocks of bottlenose dolphins: Northern Migratory and Southern Migratory Coastal Stocks, a South Carolina/Georgia Coastal Stock, a Northern Florida Coastal Stock and a Central Florida Coastal Stock, as well as a Western North Atlantic offshore stock (Hayes et al. 2016). These stocks are differentiated by their abundance, migratory patterns and stable isotope ratios, and show varying degrees of genetic differentiation (Toth et al. 2012). There are also a number of smaller populations recognized along the southern US coast which inhabit the estuarine systems of some of the larger rivers.
In the northeastern Atlantic, bottlenose dolphin stock structure is not well understood. As in the northwestern Atlantic, differences between inshore and offshore dolphins have been noted. In the waters around Ireland three genetically distinct populations have been identified: a resident population in the Shannon estuary, a coastal population in the waters off western Ireland and a likely offshore population (Oudejans et al. 2015). These appear to be separate breeding populations. Other resident populations around the UK are found in Cardigan Bay (Wales) and the Moray Firth (north-east Scotland).
On the northwest coast of Spain, stable isotope analysis of skin and muscle tissue from stranded dolphins was used to assess dietary variation, habitat segregation and population substructure. It revealed the presence of two clear groups of Galician bottlenose dolphins, a northern and southern group (Fernández et al. 2011).
Further investigation throughout the northeastern Atlantic is needed in order to more fully understand bottlenose dolphin population structure in the region.
Current abundance and trends
Determining population numbers for bottlenose dolphins is difficult, as they are a highly mobile species which can sometimes occur in very large groups. This makes them hard to survey, although there has been some work done in various parts of their range in order to produce population estimates. It is estimated that there is a minimum of 600,000 bottlenose dolphins world-wide (Hammond et al. 2012).
In the northeastern Atlantic, several surveys have been carried out for different small cetacean species, including bottlenose dolphin. One example is the multinational Small Cetacean Abundance in the North Sea and Adjacent waters (SCANS) surveys, a series of surveys which took place in 1994, 2005 and 2016.
The 1994 survey was not able to generate a population estimate for bottlenose dolphins. The 2005 survey gave an estimated abundance of dolphins in the survey area of 16,641 (CV = 0.42, 95% CI 7,618 – 36,351) (Hammond et al. 2017). The SCANS III survey in 2016 gave a population estimate for the same area of 27,697 (CV = 0.23, 95% CI 17,662 – 43,432) (Hammond et al. 2017).
Another survey, the Cetacean Offshore Distribution and Abundance in the European Atlantic (CODA) survey, investigated European Atlantic waters beyond the continental shelf in 2007 and estimated a population of 19,295 CV = 0.25) bottlenose dolphins for the survey area (CODA 2009).
In the north Atlantic, a series of surveys called the North Atlantic Sighting Surveys (NASS) have been carried out since the late 1980s for various species of cetaceans. Between 1987 and 2015, a total of 91 dolphins were seen on these surveys. The number seen is too low to be able to make a population estimate for bottlenose dolphins in the north Atlantic (D. Pike, personal communication).
Some of the inshore populations around Ireland and the UK have been surveyed. In Cardigan Bay, Wales, the most recent population estimate was 258 (95% CI 161–508) for the bay area (ICES 2018). Around 400 – 500 are found in Scottish and Irish coastal waters particularly the Shannon River estuary and the Moray Firth (Berrow et al. 2010 and Thompson et al. 2011 in Robinson et al. 2012).
In the western Atlantic, surveys have been done primarily along the eastern coast of the United States. In the summers of 2010 and 2011, aerial surveys were flown off the coast covering waters from Florida to the lower Bay of Fundy. The best population estimate for the offshore stock of bottlenose dolphins in the western north Atlantic is 77,532 (CV=0.40) (Hayes et al. 2016). The same surveys also provided population estimates for the inshore stocks along the US east coast. The inshore northern migratory coastal stock is estimated at 11,548 (CV=0.36), the southern migratory coastal stock is estimated at 9,173 (CV=0.46), the South Carolina/Georgia coastal stock at 4,377 (CV=0.43), the northern Florida coastal stock at 1,219 (CV=0.67), and the central Florida coastal stock at 4,895 (CV=0.71) (Waring et al. 2015). This makes the total inshore population some 31,000 dolphins. In addition, there are various smaller populations of several hundred dolphins living in estuarine systems along the southern part of the US east coast. Depending on the size of the estuary, there may be from 100 to 400 dolphins in each (Waring et al. 2015).
It is difficult to establish population trends for bottlenose dolphin populations, as most parts of their range have not been systematically surveyed. Even in areas where repeated surveys have taken place, the methods used are not always comparable, and the time-series is short relative to the life cycle of the dolphins. Another problem with detecting trends is that the precision of the population estimates that are obtained is generally low, making it difficult to infer with any confidence whether populations are decreasing, stable or increasing.
In the eastern north Atlantic, the multinational Small Cetacean Abundance in the North Sea and Adjacent waters (SCANS) surveys, are one of the few systematic surveys which have included estimates for bottlenose dolphins. These surveys took place in 1994, 2005 and 2016, although an estimate could not be made for 1994. The 2016 survey found a larger population of dolphins in the survey area than the 2005 survey did (Hammond et al. 2107). It is not clear whether this reflects an increase in the population or a change in distribution.
In many locations around the northeastern Atlantic Ocean, coastal bottlenose dolphin populations declined or disappeared during the late 1800s or early 1900s, but currently most of the populations in this region seem to be stable (NAMMCO 2016).
On a smaller scale, the population in Cardigan Bay, Wales has been monitored for the past 17 years. Over that time, the numbers of dolphins has fluctuated, but the population estimate obtained in 2017 is not significantly different from the estimate made in 2001 (ICES 2018).
Stranding rate can also be used as an indirect measure of population, although other factors can and do influence strandings. In northwestern Spain, the stranding rate did not appear to reflect any trend in dolphin abundance over the period 2000 to 2013 (Saavedra et al. 2017).
In the western north Atlantic, three different abundance estimates are available for the western north Atlantic offshore stock. The summer 1998 surveys estimated 29,774 (CV=0.25), summer 2002/2004 surveys estimated 81,588 (CV=0.17) and the summer 2011 surveys 77,532 (CV=0.40) (Hayes et al. 2016). A trend analysis, however, has not been done using these estimates due to methodological differences between the surveys.
The common bottlenose dolphin is a widespread and abundant species, and is listed on the IUCN Red List in the category of Least Concern (Hammond et al. 2012). While there may be threats acting on local populations, none of these is considered to be causing a global population decline.
Although not considered endangered, bottlenose dolphins are listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES), as well as in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). These conventions provide for international cooperation in the management of species like the bottlenose dolphin that are widely distributed.
The bottlenose dolphin is also recorded in the European Union (EU) Habitats Directive as a Species of Special Interest. As such, it requires the designation of Special Areas of Conservation (SAC) by EU Member States for its protection. One example of an SAC is Cardigan Bay, Wales, where there is a resident population of around 200 dolphins.
Hunting and utilisation
Bottlenose dolphins are harvested in certain parts of the world for human consumption or for use as bait in fisheries. These harvests currently occur in Peru, Sri Lanka, Taiwan and Japan (Hammond et al. 2012). In the Faroe Islands, small numbers of bottlenose dolphins have been harvested occasionally in combination with the pilot whale drives. Since 2000, a total of 45 have been harvested there, with none taken since 2009 (https://www.whaling.fo/).
Other human impacts
Bottlenose dolphins are vulnerable to by-catch in a variety of fishing gears throughout their range: in gillnets, driftnets, purse seines, trawls, long-lines, and on hook-and-line gear, although levels of by-catch are low relative to other cetaceans such as common dolphins or harbour porpoises. Dolphins may get caught by not seeing nets, by chasing fish into them, or by taking fish from nets (depredation). In some areas they are known to follow fishing vessels out to where they set or retrieve gear (Byrd and Hohn 2017), perhaps increasing their chances of becoming caught or entangled in gear.
Levels of by-catch are not well known in most parts of the bottlenose dolphin’s range due to a lack of reporting. One estimate of some fishery-related mortality has been made for the western north Atlantic offshore stock. A mean annual death of 39.4 (CV=0.29) dolphins due to interactions with the northeast sink gillnet, northeast bottom trawl, mid-Atlantic bottom trawl, and pelagic longline fisheries was determined (Hayes et al. 2016). The total mortality for this stock is unknown, however, as there are other fisheries occurring in this region with unreported by-catch.
Age-class and season appeared to play an important role in levels of by-catch observed in dolphins off North Carolina. A study found that the by-catch risk for older calves and subadults was 1.5 (in the summer) to 3.5 (in the spring) times greater than that for an adult or young-of-year dolphin (Byrd and Hohn 2017).
No by-catch of bottlenose dolphins has recently been reported for any of the fisheries taking place in Norway, Iceland, Greenland or the Faroe Islands (NAMMCO 2017).
Coastal populations of bottlenose dolphins may be exposed to various types of human activity occurring in their habitat: vessel traffic, construction, oil and gas exploration or other industrial activity. Dolphins may be directly or indirectly disturbed or harassed by boat traffic, including tourist ships on “dolphin watching” excursions.
Noise is of particular concern for bottlenose dolphins, as they use sound extensively for both foraging and communication. Dolphins in the Sado estuary, Portugal, changed both their call rate and whistle characteristics in the presence of certain vessels (Luis et al. 2014). The overall call rate decreased in the presence of operating vessels, especially ferry boats. In Florida, dolphins were observed to increase their rate of whistle production at the onset of a vessel approach (Buckstaff 2004). These changes in vocalizations could potentially disturb important social interactions such as mating or mother-calf communication.
Dolphins also react to the presence of vessel traffic. Dolphins in Sarasota Bay, Florida, swam closer together, changed heading, and increased swimming speed significantly in response to an approaching vessel (Nowacek et al. 2001).
Behavioural changes such as these could have long-term effects on the species if animals that are feeding, resting, or breeding are disturbed by noise or vessel traffic. In Shark Bay, Australia, the presence of tourist vessels, which often approach quite close to the dolphins, was found to contribute to declining dolphin numbers in the area (Bejder et al. 2006). In this and other cases, the effect of noise and disturbance could be that dolphins get displaced and are forced into areas of poorer habitat. This is of particular concern for small coastal populations inhabiting a single bay or estuary.
Like other marine mammals, bottlenose dolphins ingest persistent organic pollutants (POPs) in their diet, consuming fish which have been exposed to these compounds from coastal runoff and human industrial activities. Coastal bottlenose dolphins in particular may be exposed to contaminants as their habitats may be adjacent to areas of high human population and industrial activity. Since they are a relatively long-lived species, contaminant levels may build up over time to quite high levels.
Polychlorinated biphenyls (PCBs) are of particular concern for bottlenose dolphins. In a large Europe-wide analysis of stranded (n = 929) or biopsied (n = 152) cetaceans, it was found that bottlenose dolphins in the northeast Atlantic and Mediterranean had mean PCB levels in their blubber that were among the highest recorded in cetaceans globally (Jepson et al. 2016). These concentrations are higher than the level at which severe toxic effects are known to occur. High levels of PCB contamination can, for example, cause immunosuppression which could have been a significant contributing factor in the death of many of the stranded individuals in the European study that showed fatal infectious diseases on necropsy (Jepson et al. 2016).
High PCB levels in female dolphins may also be transferred to their young. A disproportionately high maternal transfer of accumulated PCBs was noted in primiparous females in the Moray Firth, Scotland (Robinson et al. 2017), which could result in brain cell damage, thyroid inhibition and immunosuppression in the fetus. This might be a contributing factor to the low survival rate of first-born calves seen in this area (Robinson et al. 2017).
Elevated PCB levels are also seen in coastal bottlenose dolphin populations in the US. Male dolphins in some parts of Florida have been found to have levels of PCBs more than an order of magnitude greater than the threshold for adverse health effects (Hammond et al. 2012). High levels of PCBs in the blubber have also been noted in North and South Carolina (Waring et al. 2015). As in the Moray Firth, these high levels of PCBs are correlated with higher levels of mortality in first-born offspring, and so could be affecting the reproductive success of these populations.
Bottlenose dolphins in Florida were also found to have high concentrations of total Mercury in their blood and skin, among the highest levels reported worldwide (Titcomb et al. 2017). Such high levels of Mercury may have adverse health effects for the dolphins, particularly in hepatic, renal, endocrine, hematological, and immune parameters (Titcomb et al. 2017).
The exposure of bottlenose dolphins to these environmental pollutants and the subsequent effects on their population health is an area of concern and ongoing research.
Climate change could have a number of impacts on bottlenose dolphins either directly or indirectly. For example, if water temperatures increase, this could have a direct impact on the habitat available for dolphins and consequently their distribution. Changes in water temperature could also affect the distribution of the dolphins’ prey species and so indirectly affect dolphin distribution.
Research in NAMMCO member countries
The common bottlenose dolphin is included in the NAMMCO-coordinated Trans North Atlantic Sighting Surveys (T-NASS). While bottlenose dolphins are not the primary target species, sightings of them are recorded during the surveys. So far, however, there have not been enough sightings made to be able to produce a population estimate.
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