|Last modified: Thursday September 24th, 1999|
Bottlenose dolphinTursiops truncatus (Montagu, 1821)
The taxonomy of the genus Tursiops has been somewhat confused. Currently, most researchers agree that there a single species, Tursiops truncatus, with a number of regional forms. A number of species have been proposed earlier, including T. gilli (California and Mexican Pacific coast), T. nuuanu (offshore Pacific, including the Eastern Tropical Pacific (ETP)) and T. aduncus (South Africa, Australia) (Klinowksa, 1991). There may be grounds for considering the aduncus form a separate subspecies of T. truncatus (Ross and Cockcroft, 1990).
The body of the bottlenose dolphin is long and somewhat stocky. The melon is well-defined. The lower jaw extends beyond the upper jaw, curving slightly upwards at the tip. This dolphin is usually dark grey dorsally and white or pinkish ventrally (Minasian et al, 1984). In some areas, bottlenose dolphins develop ventral spotting with age. This has been seen in several Australian coastal populations (Ross and Cockcroft, 1990). The dorsal fin is large and falcate. The flippers are medium-sized and slightly rounded. The flukes are broad, thin and somewhat rounded at the tips and have a well-defined median notch. This dolphin can reach 4 m in length and weigh up to 650 kg (Minasian et al, 1984). There are regional variations in maximum length and weight. In the Northwest Atlantic, they are on average 250-260 cm, with a maximum of 309 cm (Mead and Potter, 1990). In this area, there is a clear distinction in body measurements, including lengths, between coastal (warm water) and offshore (cold water) populations, the latter being the largest (247 vs. 289 cm) (Hersh and Duffield, 1990). Ross and Cockcroft (1990) found a correlation between average water temperature and body size in Australian waters as well. There are 18-26 pairs of sharp conical teeth in each jaw.
The bottlenose dolphin is found worldwide in temperate and tropical waters, both offshore and coastal. It does not occur in polar waters. In non-tropical waters, it is found mostly in the coastal zone, sometimes to the edge of the continental shelf and beyond (Klinowska, 1991). They can also be found in river estuaries (dos Santos and Lacerda, 1987). Bottlenose dolphins usually occur in small groups. In Tampa Bay, the average school size is 5 (Weigle, 1990). In the Sado estuary, Portugal, the mean group size is 13.7 (dos Santos and Lacerda, 1987). Group sizes can vary from 1 to 100 or more, but in general a group consists of 2-15 animals. Two types of groups can be distinguished: pods (small units of dolphins that associate closely and engage in similar activities) and herds (which are temporary aggregations of pods) (Shane et al, 1986). In South Africa, group size are larger (average 140, range 3-1,000).
At birth, the bottlenose dolphin is 90-130 cm long and weighs about 30 kg. Gestation is 11-12 months. Calves usually start weaning at 4-12 months (Verwey and Wolff, 1981). They are fully weaned at 18-20 months. The oldest calf still nursing was 38 months old (Perring and Reilly, 1984) On the US Atlantic coast, the average size at birth is 117 cm and the average weight 20.4 kg. They are physically mature at 245 cm (age 13 years). Females become reproductively mature at about 10 years, which is when they are ovulating regularly. The first ovulation (sexual maturity) occurs at 5-7 years (Perrin and Reilly, 1984). Males become sexually mature at 10-15 years (Mead and Potter, 1990).
Few studies have been done on bottlenose dolphin survival rates. Hersh et al (1990) determined an annual mortality rate of 0.069-0.092 (survival rate (ASR) 0.931-0.908) for the Indian-Banana River system in Florida. For Sarasota Bay, the annual loss rate was 0.039, which includes known deaths, assumed deaths and emigrations, although to date no evidence for emigration has been found (no known individuals lost from the Sarasota Bay population have been resighted in other populations), so the annual loss rate is probably the annual mortality rate (associated ASR: 0.961) (Wells and Scott, 1990). The ASR for bottlenose dolphins in oceanaria was calculated to be 0.951 for the period 1988-1992. There was no statistically significant difference between this ASR and the one calculated by Wells and Scott (1990) for the Sarasota population (Small and DeMaster, 1995).
In Tampa Bay, dolphin schools consisted for 9.7% of calves (Weigl, 1990). In Sarasota Bay, the annual recruitement rate is 0.048. The crude birth rate is 0.055. There is a high variability in birth rate, resulting in varying numbers of calves in the population. The maximum calf ratio was 20% in 1986 (Wells and Scott, 1990). The mean annual immigration rate was 0.025.
Very few population assessments are available for this species. Most estimates are for local populations only. The total population on the US Northeastern coast is estimated at 10,000-13,000 animals. The population in the Eastern Tropical Pacific (ETP) was estimated at 588,000 in 1978. There may be as much as 40,000 in the Gulf of Mexico (including about 100 animals in Sarasota Bay, a well-studied population), 35,000 in the North Pacific and Japan, 250 on the South African Indian Ocean coast near Natal and less than 1,000 north of Natal. There are no reliable estimates for the Black Sea population (Klinowska, 1991). The population in the Western North Pacific was estimated to be 168,792 (102,101-279,045) in 1993 (Miyashita, 1993). Kenney (1990) noted seasonal changes in bottlenose dolphin abundance in the Continental Shelf waters off the Northeastern US coast, with the highest numbers of dolphins in summer (400-700 inshore and 9,300-12,100 offshore dolphins).
Population assessment is further complicated by shifts in populations due to changing environmental conditions. Through the 1960s there were bottlenose dolphins in San Diego Bay. Then they disappeared, possibly due to increased pollution of the Bay. When the water quality improved in the 1970s, the dolphins returned (Klinowska, 1991). In the Netherlands, the small resident population (30-40 animals) disappeared from the Wadden Sea, when the Zuyderzee was closed and the annual herring run disappeared (Verwey and Wolff, 1981).
Bottlenose dolphins are opportunistic feeders, usually going after the most abundant prey species. In the North Sea, they feed mainly on herring (Clupea harengus), spur-dog (a small shark species, Squalus acanthias), haddock, dory (Zeus faber) and sole (Solea solea) (Verwey and Wolff, 1981). In the Indian ocean population off Natal, South Africa, the most important food species are Pomadasys olivaceum (a grunt species), cuttlefish (Sepia officinalis), Scomber japonicus (a mackerel species) and Pagellus bellotti (Cockcroft and Ross, 1990). Barros and Odell (1990) found that in the Southeastern US, the most important prey species were sciaenids (drums, croakers and sea trouts) and haemulids (grunts). There were significant regional differences. They could find no evidence for the supposed preference for mullet (Mugil spp.) in this area. Depending on the caloric value of the fish, an adult bottlenose dolphin eats 5-15 kg/day, lactating females up to 20 kg/day.
|Bottlenose dolphins have been exploited in a number of ways. A number of animals have been taken for captive display purposes (from 1860 through 1983, more than 2,700 have been taken for display purposes, 1,595-1,635 of those were taken in US waters, 580 in Japanese waters). The earliest record of a bottlenose dolphin on display in Europe dates from 1883, taken from UK waters and displayed at the Brighton Aquarium (Klinowska, 1991). There still is a drive fishery for this species for its meat in Japan, in which a few hundred animals are taken every year. See the table below for recent capture statistics. Most of the directed kill numbers are from Japanese drive fisheries. The live captures from 1986-1989 are mostly from US waters (and 2 from Argentina in 1986) and were for display and research purposes. Since 1989, no permits have been issued for live capture of marine mammals for display purposes by the US NMFS (Lawson and Buck, 1997). The live captures from 1991 onwards are all from Japan. The purpose of these captures is unclear. Some appear to have been released as part of population and behavior studies, other may have been captured for public display in Japan.|
|Year||Directed kill||Live capture||Bycatches|
Increasingly, bottlenose and other dolphins have become the target of dolphin watching and wild dolphin interaction programs. Currently there are 295 communities in 65 countries that now have commercial whale and dolphin based tourist operations, generating in excess of US$ 500 million in yearly revenues (Orams, 1997). There is increased concern that the pressure generated by these activities may result in altered behavioral patterns, especially where people enter the water with dolphins and where the dolphins are provisioned. Especially in the case of provisioned dolphins, behavioral patterns alter significantly and increased aggression may occur (Orams (1995), Bryant (1994)). In Monkey Mia, a significant increase in calf mortality in provisioned dolphins has been noted (calf survival rate 0.36 for provisioned mothers, 0.67 for non-provisioned mothers) (Orams, 1997).
The bottlenose dolphin is a host to a number of parasites. The most common ones are the trematodes Braunina cordiformis, which can be found in the digestive system, especially in the pyloric stomach and Nasitrema spp, which can be found in the nasal sinuses (Mead and Potter, 1990).
Dolphins have been seen chasing various species of stingrays, but they probably don't eat them. Such interactions may result in puncture wounds from stingray spines. These wounds can be fatal, if the spine punctures vital organs, such as the lungs or liver (Walsh et al, 1988). In Portugal, bottlenose dolphins have been observed tossing jellyfish (Catostylus tagi) with vigorous flicks of the tail flukes. This seems to be a favorite passtime in summer (June to October) (dos Santos and Lacerda, 1987).
The most important predators of bottlenose dolphins are probably sharks, although in some areas also killer whales take bottlenose dolphins (Shane et al, 1986). Reactions of dolphins towards sharks vary from avoidance, through tolerance to active aggression. This is probably species related. The strongest reactions are elicited by known predators such as tiger and bull sharks.
A number of interactions between bottlenose dolphins and other species have been documented. Both affiliative and aggressive behavior has been seen between bottlenose dolphins and Atlantic spotted dolphins in the Bahamas (Herzing and Johnson, 1997). In the Moray Firth, Scotland, groups of bottlenose dolphins have been observed harassing and killing harbor porpoises (Ross and Wilson, 1996). In the Galápagos Islands, false killer whales and bottlenose dolphins have been seen jointly harassing a group of sperm whales (Palacios and Mate, 1996). Bottlenose dolphins in the Pacific are often seen in mixed-species groups, most commonly with pilot whales (Globicephala macrorhynchus), pantropical spotted dolphins (Stenella attenuata) and spinner dolphins (Stenella longirostris) (Scott and Chivers, 1990).
There are a number of records of prolonged interactions between bottlenose dolphins and people. There have been (and still are) a number of sociable individuals, such as Percy (England), Donald (Man, Wales), Jean-Louis (Brittany), Opo (New Zealand), Dolly (Key West, Florida) and Flipper (Norway). There is also a sociable group in Monkey Mia, Shark Bay, Australia, which started out as a free format interaction. Currently the dolphins are fed in a regulated program. The Imragen in Mauretania are aided in their fishery for mullet by a group of bottlenose dolphins (Lockyer, 1990).
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