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Tursiops aduncus (Ehrenberg,
1833)
English: Indo-Pacific bottlenose dolphin, Indian Ocean Bottlenose
Dolphin
German: Grosser Tümmler des Indischen Ozeans
Spanish: Delfín mular del Oceano Indico
French: Grand dauphin de l'Océan Indien
Family Delphinidae
Tursiops aduncus © Würtz-Artescienza (see "links")
1. Description
T. aduncus resembles T.
truncatus, having a relatively robust body, moderately long
beak, and a falcate dorsal fin (Jefferson et al. 2008). However,
the species tends to be smaller than T. truncatus, has a proportionately
longer rostrum and, most distinctivly, develops ventral spotting
at about the time of sexual maturity (Wells and Scott, 2002). Maximum
size is geographically variable, with length and weight for males
of 238 cm and 160 kg respectively off Zanzibar, East Africa (Amir
et al. 2007) and maximum reported values of 2,7 m and 230 kg, respectively
(Jefferson et al. 2008).
The taxonomic status of Tursiops aduncus was uncertain until
1999, when Wang et al. (1999) confirmed that in Chinese waters two
genetically distinct morphotypes of bottlenose dolphins, which he
referred to T. truncatus and T. aduncus, existed in
sympatry.
Genetic evidence suggests that T. aduncus is more closely
related to pelagic Stenella and Delphinus species,
and in particular to S.
frontalis, than to T. truncatus (Wells and Scott,
2002). However, this seems to be inconsistent with osteological
characters (Wang and Yang, 2009), and further studies using multiple
independent characters will be required to resolve these issues.
2. Distribution
Information on the distribution of T. aduncus is patchy,
but the species seems to be widespread along the entire eastern
coast of Africa, through the Red Sea and Persian Gulf, eastwards
as far as Taiwan and south-eastward to the coastal waters of Australia
(Curry ,1997; Möller and Beheregaray, 2001; Wells and Scott,
2002). However, the level of continuity in the distributional range
is unknown (Wang and Yang, 2009).
Distribution of Tursiops aduncus in coastal tropical
to warm temperate waters of the
Indian and western Pacific Oceans (Hammond et al. 2008;
Wang and Yang, 2009; © IUCN; enlarge
map).
3. Population size
In some regions where these dolphins have been studied, the populations
have been found to be small compared to nearby open-ocean populations
of common bottlenose dolphins (T. truncatus) and other species.
Off the south coast of Zanzibar East Africa, Stensland (2004) estimated
population size at 161 (95% CI 144-177). High frequency of re-sightings
indicated that the species was resident in the area.
For the Arabian Gulf (Preen, 2004) report population sizes to reach
1200. In the region between Kuwait and Oman, the Indian Ocean bottlenose
dolphin is the most common cetacean (71% of groups and individuals).
However, the estimates of cetacean abundance in the United Arab
Emirates differed significantly between 1986 and 1999 and indicated
a population decline of 71%. At least two die-offs of marine mammals
occurred between these surveys.
Known large concentrations are in regions with large shallow-water
areas. There are 2,700 in Shark Bay on the western coast of Australia
(Preen et al. 1997) and 700-1000 around North Stradbroke Island
on the eastern coast of Australia (Chilvers and Corkeron, 2003).
Cribb (2006) counted 191 individuals in the Adelaide Dolphin Sanctuary,
Gulf St Vincent, South Australia. Lukoschek and Chilvers (2008)
used different methods to estimate abundances of bottlenose dolphins
in central eastern Moreton Bay in 2000 and found 407 (+/- 113.5)
animals in that area with a line-transect study. As opposed to this,
they determined 818 (+/- 152) animals for 1998 with a photo-identification
study. The reasons for the difference seem to lie in the different
methodologies employed.
In the coastal waters of Amakusa-Shimoshima Island, western Kyushu,
Japan, Shirakihara et al. (2002) estimated population size in 1995-1997
at 218 individuals (CV = 5.41%). In Kagoshima Bay there were 50
individuals in 1999, 40 individuals in 2000, and 50 individuals
in 2001. The slight change in the number of identified dolphins
indicated the possibility of their residency inside the bay (Nanbu
et al. 2006).
Given the restricted areas of potentially suitable habitat, populations
of T. aduncus in the South Pacific islands are likely small,
i.e. in the hundreds (Reeves and Brownell, 2009).
4. Biology and Behaviour
Habitat: Boat based and photo-identification
surveys conducted in the Adelaide Dolphin Sanctuary, Gulf St Vincent,
South Australia, yielded a high occurrence of bottlenose dolphins
over the bare sand habitat. Mean dolphin group size was largest
over the bare sand habitat for all behavioural activities observed
(Cribb, 2006). However, in other regions, T. aduncus prefers areas
with rocky and coral reefs, or sea grass. They can be found over
waters 200m deep but are encountered most frequently in waters shallower
than 100 m. They seem to prefer water temperatures of 20-30°C,
with a minimum of 12°C (Wang and Yang, 2009).
Photo of Tursiops aduncus
by courtesy of marine wave 95.com (see "links")
Schooling: Lukoschek and Chilvers (2008) estimated
group size of bottlenose dolphins in central eastern Moreton Bay
using line-transect surveys in 2000, obtaining a mean of 2.85 animals
per group Mark-recapture surveys in 1998 yielded 5.78 animals per
group. Differences in methodologies employed seem to be responsible
for the discrepancies observed. Connor et al. (2006) reported that
synchronous surfacing in male bottlenose dolphins is associated
with alliance membership and that synchrony between members of cooperating
alliances is more common during social behaviour with female consorts.
Males within the alliance form temporary trios and occasionally
pairs in order to consort with individual females (e.g. mounting,
displays, chasing) (Connor et al. 2001).
In Kagoshima Bay (Japan), Indo-Pacific bottlenose dolphins were
observed near the shore of the innermost area of the bay. The mean
school size was 30 individuals (Nanbu et al. 2006). In the coastal
waters of Amakusa-Shimoshima Island, western Kyushu, Japan, the
group commonly consisted of more than 100 individuals. The large
group size was thought to be a response to feeding on schooling
fishes (Shirakihara et al. 2002).
Mixed-species groups of Indo-Pacific bottlenose and humpback dolphins
frequently occur off the south coast of Zanzibar. Defense against
predators coupled with social advantages may offer an explanation
for the formation of these groups. However, mixed-species groups
may have different functions depending on the individuals that participate
(Stensland, 2004). Mixed-species groups include T. truncatus,
Pseudorca
crassidens, Delphinus sp., Sousa
chinensis, and Stenella
longirostris (Wang and Yang, 2009).
Reproduction: In the Port River estuary, Adelaide, South
Australia, Steiner and Bossley (2008) found the calving season to
last from December to March, which coincides with the maximum surface
water temperature of the estuary. Inter-birth intervals was 3.8
y when the previous calf was weaned and 1.7 y when the previous
calf died. Average crude birth rate was 0.064, which is similar
to that found for other bottlenose dolphin populations. Average
gestation time is 12 months (Wang and Yang, 2009). Around Zanzibar
(Unguja) Island (Amir et al. 2007) calves were born at a length
of around 104.5 cm and weight of 15.5 kg. Males became sexually
mature at about 13 years, which is late for dolphins, at a length
about 202 cm and weight of 112 kg. They may live more than 36 years,
based on counts of Growth Layer Groups (GLGs) in teeth.
Food: There is great geographical variety in the species'
diet. The primary prey species seem to be benthic and reef-dwelling
fish and cephalopods, less than 30 cm in length, of continental
shelf waters (Wang and Yang, 2009). Off the coast of Zanzibar, Indo-Pacific
bottlenose dolphins forage on a relatively large number of prey
species. However, only a few small- and medium-sized neritic fish
and cephalopods contribute substantially to the diet. While stomach
contents comprised 50 species of bony fish and three species of
squid, only 5 species of fish, Uroconger lepturus, Synaphobranchus
kaupii, Apogon apogonides, Lethrinus crocineus, Lutjanus fulvus,
and 3 species of squid, Sepioteuthis lessoniana, Sepia latimanus
and Loligo duvauceli, were the most important prey species.
Uroconger lepturus proved to be the most important prey
species of mature dolphins, whereas Apogon apogonides was
the preferred prey of immature dolphins. The ecology and behavior
of the preferred fish prey species indicate that the dolphins in
that area forage over reefs or soft bottom substrata and near the
shore (Amir et al. 2005a). Around oceanic islands, epi- and mesopelagic
fish and cephalopods as well as benthic crustaceans dominate the
diet (Wang and Yang, 2009).
5. Migration
Dolphins of different regions appear to exhibit strong year-round
residency and natal philopatry in both sexes, with males being more
dispersive than females. Although generally considered a coastal
species, movements across deep oceanic waters have been reported
(Wang and Yang, 2009).
Fury and Harrison (2008) investigated habitat utilisation in two
Australian subtropical estuaries and found that 60% and 37% of identified
dolphins were residents, 26% and 21% occasional visitors and 14%
and 42% transients in the Clarence and Richmond Rivers, respectively.
In the coastal waters of Amakusa-Shimoshima Island, western Kyushu,
Japan, individuals identified in one season were frequently resighted,
with percentages of mostly over 60% during the subsequent seasons
Most of the dolphins off Amakusa were year-round residents, although
the total extent of their habitat was unknown (Shirakihara et al.
2002).
6. Threats
Indo-Pacific bottlenose dolphins in coastal areas are exposed to
reduced prey availability caused by environmental degradation and
overfishing to and habitat degradation due to marine construction
and demolition. The cumulative impact of these threats and those
listed below is likely to result in longitudinal population declines
(Hammond et al. 2008). E.g. in the Port River estuary, Adelaide,
South Australia, Steiner and Bossley (2008) reported high calf mortality
rates caused by direct impacts by entanglements, boat strikes, deliberate
attacks, or exposure to toxic pollution: First-year calf mortality
(30%) and mortality rate for calves prior to weaning (46%) were
higher than mortality rates described for other locations.
Direct catch: In many regions of the world, e.g. Philippines,
Taiwan and East Africa, the species is taken directly for human
consumption or as bait for fishing operations. Through past drive
hunting, hundreds may have been captured per season in the Penghu
Islands, although only 20 were captured in 1993, possibly an indication
of local depletion (Wang and Yang, 2009).
Incidental catch: Shark nets placed along the coast of KwaZulu-Natal,
South Africa, to protect bathers from shark attacks result in an
incidental by-catch of dolphins at twice the level suggested by
the IWC as the maximum sustainable capture rate for a cetacean population
(Natoli et al. 2008).
Incidental catches are reported from drift- and bottom set gillnet
fisheries off Zanzibar (Unguja Island). Amir et al. (2002) expressed
concern over exceedingly high by-catch rates for T. aduncus
at this location; between 1995 and 1999, 43 Indo-Pacific bottlenose
dolphins were reported taken, 50% of these in 1999. However, the
numbers were extrapolated to about twice that value. In a later
study (Amir et al. 2005b), conducted between 2000 and 2003, 68 Indo-Pacific
bottlenose dolphins were recorded. Most of the bycatches (71%) were
in nets set off the north coast of Unguja Island. These figures
suggests that the incidental capture of dolphins in Zanzibar's artisanal
gillnet fisheries is an ongoing problem and is high enough to have
a significant negative impact on local populations.
A large number of possibly over 2000 per year was incidentally taken
in northern Australian waters by a Taiwanese drift gillnet fishery
n the 1980s, which after establishment of strict regulations moved
into neighbouring waters off Indonesia (Wang and Yang, 2009).
Live capture:
The government of the Solomon Islands issued a permit for export
of up to 100 live dolphins per year in 2008, most likely to be only
T. aduncus. If an international standard rule allowing 1%
or 2% of a population to be removed annually were applied in this
instance, the local T. aduncus population would have to be
at least 5,000 or 10,000 to sustain the permitted level of exports.
However, an ongoing photo-identification study around Guadalcanal
Island has catalogued only something more than 100 individuals (Reeves
and Brownell, 2009).
Pollution: Lavery et al. (2008) report on trace metal concentrations
in stranded or by-caught T. truncatus in South Australia
from 1988 to 2004. This species had higher mean tissue burdens of
liver Pb (0.45 g/kg), Cd (6.45 mg/kg), Hg (475.78 mg/kg), Se (178.85
mg/kg) and Zn (93.88 mg/kg) and bone Pb (2.78 mg/kg), as opposed
to D. delphis and T. truncatus, probably reflecting
their coastal habitat and benthic prey. Lavery et al. (2009) found
that two dolphins had high metal burdens, high metallothionein concentrations,
renal damage, and evidence of bone malformations, indicating possible
severe and prolonged metal toxicity.
Tourism: Stensland and Berggren (2007) found behavioural
changes in Indo-Pacific bottlenose dolphins in response to boat-based
tourism off the south coast of Zanzibar. While movement patterns
of dolphin groups were not affected by the presence of 1 to 2 tourist
boats without swimmers, groups displayed a significantly larger
proportion of erratic (non-directional) movements as tourist activities
increased and when swimmers were present. Females travelled more
frequently, with potential negative effects on the time available
to nurse their calves. Intense non-regulated dolphin tourism may
lead to a shift in habitat use by nursing females, and the apparent
changes in dolphin behaviour due to the increased levels of tourism
may ultimately reduce fitness at both individual and population
levels.
Natural threats: Both scar frequencies and attack rate suggest
that dolphins in Shark Bay, Western Australia face a greater risk
of predation than bottlenose dolphins in other locations. Bite scars
from sharks were found on 74.2% (95 of 128) of non-calves; most
of these were inflicted by tiger sharks (Galeocerdo cuvier).
Large sharks (>3 m) are responsible for a disproportionate number
of attacks. However, bites from small carcharhinid sharks on 6.2%
of dolphins suggest that some of these small sharks may be dolphin
ectoparasites (Heithaus, 2001).
7. Remarks
Range states (Hammond et al. 2008):
Australia; Bangladesh; Brunei Darussalam; Cambodia; China; Djibouti;
Egypt; Eritrea; Guam; Hong Kong; India; Indonesia; Iran, Islamic
Republic of; Japan; Kenya; Madagascar; Malaysia; Mozambique; Myanmar;
Northern Mariana Islands; Oman; Pakistan; Papua New Guinea; Philippines;
Saudi Arabia; Singapore; Solomon Islands; Somalia; South Africa;
Sri Lanka; Taiwan, Province of China; Tanzania, United Republic
of; Thailand; Timor-Leste; United Arab Emirates; Viet Nam; Yemen.
T. aduncus is categorized as "Data Deficient"
by the IUCN (Hammond et al. 2008). However, the variety and intensity
of threats the species is exposed to may well lead to a re-consideration
of this status. There are serious concerns about the depletion of
local populations, particularly as the species shows strong residency
and limited population sizes in many regions (Wang and Yang, 2009).
Populations in the Arafua/Timor Sea are listed in Appendix II of
CMS. For general remarks on south-east Asian species, see also Perrin
et al. (1996). The species is listed in Appendix II of CITES.
8. Sources
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© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.
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