Tursiops truncatus (Montagu,
English: Common bottlenose dolphin
German: Grosser Tümmler
Spanish: Delfín mular
French: Grand dauphin
Tursiops truncatus © Würtz-Artescienza (see "links")
The common bottlenose dolphin is presumably the most familiar of
the small cetaceans because of its coastal occurrence around the
world, its prevalence in dolphinaria and zoos and its frequent appearance
in the media (Jefferson et al. 2008). This species is recognized
by its medium-sized, robust body, with a sharp demarcation between
the melon and the short rostrum, and the moderately curved dorsal
fin. Pigmentation is light grey to black dorsally, with a light
belly. Adult length ranges from 2-3.8 m and body mass from 220-500
kg, varying geographically. Body size seems to vary inversely with
water temperature in many parts of the world (Bloch and Mikkelsen,
2000; Wells and Scott, 2009).
Geographical variation in bottlenose dolphins is only vaguely comprehended,
and in most parts of the world subspecific designations are best
avoided. The name T. t. truncatus (type locality: Great Britain)
may be applied to the offshore populations on both sides of the
North Atlantic, and some authors have used it for similar animals
that live in the temperate waters of the western North Pacific,
South Africa, Walters Shoal, southern Australia, and New Zealand
(Rice, 1998 and refs. therein).
Often, there are size differences between neighbouring populations:
The dolphins that live in the Black Sea (named T. t. ponticus
Barabash-Nikiforov, 1940) are smaller than those in the North Atlantic
and possess a uniquely shaped skull, while those in the Mediterranean
are intermediate in size. The Black Sea population was found to
be genetically distinct from these two populations, with relatively
low levels of mtDNA diversity (Viaud-Martinez et al. 2008).
In some parts of the world, sharply differentiated inshore and
offshore populations live in close proximity. Results of mtDNA analyses
do not indicate genetic isolation among offshore populations from
different ocean basins, but do show that there are differing coastal
or inshore populations which are genetically isolated from offshore
populations (Rice, 1998 and refs. therein). Thus bottlenose dolphins
occurring in the pelagic waters of the North Atlantic, including
the Azores and Madeira archipelagos, were shown to belong to a large
oceanic population, which must be regarded as a single conservation
unit (Querouil et al. 2007).
In the eastern South Pacific, there is also genetic evidence for
a single, wide-ranging Peru-Chile offshore stock, whereas a separate
cluster is formed by the Peruvian inshore ecotype and a single resident
inshore community (pod-R) in central-north Chile differing from
both (Sanino et al. 2005).
Oceanic bottlenose dolphins seem to maintain high levels of gene
flow, unlike coastal populations. E.g. in the Gulf of Mexico, a
significant genetic population structure was found among four resident,
inshore bottlenose dolphin stocks (Sarasota Bay, FL, Tampa Bay,
FL, Charlotte Harbor, FL and Matagorda Bay, TX) and one coastal
stock (1-12 km offshore). This is surprising given the short geographical
distance between many of these areas and the lack of obvious geographic
barriers to prevent gene flow (Sellas et al. 2005).
Finally, genetic work (Le Duc et al. 1999; Wells and Scott, 2009),
osteological comparisons (Wang et al. 2000; Wang and Yang 2009)
and morphological analyses by Hale et al. (2000) support the view
that some bottlenose dolphins of the tropical Indian Ocean and western
aduncus, are reproductively isolated from the widespread
Bottlenose dolphins are found primarily in coastal and inshore
regions of tropical and temperate waters of the world, and population
density seems to be higher near-shore. There are also pelagic populations,
such as those in the eastern tropical Pacific and around the Faroe
Islands. The bottlenose dolphins occurring around the Faroe Islands
(62°N 7°W) seem to be the most northerly of the North Atlantic
offshore populations (Bloch and Mikkelsen, 2000).
Distribution of Tursiops truncatus: widely
distributed in cold temperate to tropical seas
worldwide (map mod. from Hammond et al. 2008; © IUCN; enlarge
In the Atlantic T. truncatus occurs north
to Massachusetts, the southern coast of Iceland, northern Norway
(Lofoten Islands), the Mediterranean and Black seas. In the Pacific
it ranges north to the Okhotsk Sea, the Skuril Islands and Central
California. In the Southern Hemisphere T. truncatus occurs
south to Tierra del Fuego, South Africa, Australia and New Zealand
(Wells and Scott, 2009). The species is rare in the Baltic Sea,
and there is some question as to its occurrence in the Barents Sea
(Wells and Scott, 1999 and refs. therein).
3. Population size
Summing available estimates, a minimum world-wide estimate is 600,000
(Wells and Scott, 2009; Hammond et al. 2008). There are recent abundance
estimates for several parts of the species' range, but the there
is generally insufficient data to estimate population trends:
From central Florida to Canada, the abundance estimate of the western
North Atlantic offshore stock from aerial and vessel surveys conducted
between 2002 and 2004 provides complete coverage of the offshore
habitat during summer months. The combined abundance estimate from
these surveys is 81,588 (CV=0.17) (Waring et al. 2009).
From Florida to New Jersey, the primary habitat of the western
North Atlantic coastal morphotype during summer months is in waters
less than 20 m deep, including estuarine and inshore waters. Re-analysis
of stranding data (McLellan et al. 2003) and extensive analysis
of genetic, photo-identification, satellite telemetry, and stable
isotope studies demonstrate a complex mosaic, with seven prospective
stocks of coastal morphotype bottlenose dolphins inhabiting nearshore
coastal waters along the Atlantic coast. Best estimates are from
summer aerial surveys conducted in 2002 and/or 2004: Northern Migratory
Stock: 7,489 (CV = 0,36), Southern Migratory Stock: 10,341 (CV =
0,33), Southern North Carolina stock: 4,818 (CV = 0,50), South Carolina
stock: 1,952 (CV = 0,28), Georgia stock: 5,996 (CV = 0,37), Northern
Florida stock: 3,064 (CV =0,24) and Southern Florida stock: 6,317
(CV = 0,26) (Waring et al. 2009).
Estimates of the northern Gulf of Mexico coastal stocks date from
1991-1994 surveys with 9,912 (CV = 0,12) in the eastern, 4,191 (CV
= 0,21) in the northern and 3,499 (CV = 0,21) in the western parts
of the northern Gulf of Mexico (Waring et al. 2009). For the northern
Gulf of Mexico continental shelf and slope stock, the best abundance
estimate is based on data pooled from 2000 through 2001 for continental
shelf vessel surveys and is 17,777 (CV=0.32) (Waring et al. 2009).
The northern Gulf of Mexico Oceanic (outer continental shelf) stock
estimate is pooled from 2003 to 2004 data and is 3,708 (CV=0.42)
From the North Atlantic Sightings Surveys in 1987 and 1987 (NASS-87
and NASS-89) a very cautious estimate of the bottlenose dolphins
around the Faroe Islands comes to about 1,000 animals (Sigurjónsson
et al. 1989; Sigurjónsson and Gunnlaugsson, 1990; Bloch and
A wide-scale survey in 2005 of western European continental shelf
waters including the western Baltic, North Sea and Atlantic margin
as far as southern Spain estimated that there were 12,600 bottlenose
dolphins in this area (CV=27%, Hammond et al. 2006).
Estimates for the eastern tropical Pacific stem from data gathered
in the early 1990's, yielding 243,500 (CV=29%) (Wade and Gerrodette
1993), and the same can be said for Japanese surveys in the Northwestern
Pacific west of 180ºE where 168,000 (CV=26%) were found, including
36,791 (CV=25%) in Japanese coastal waters (Miyashita 1993). More
recently, the California/Oregon/Washington offshore stocks were
estimated on the basis of ship surveys conducted between 2001-2005
at 3,257 (CV= 0.43) offshore bottlenose dolphins (Barlow 2003, Forney
2007). The California coastal stock abundance, estimated from photographic
mark-recapture surveys in 2004 and 2005 and including an additional
35% of animals (lacking identifiable dorsal fin marks) was ca. 450-500
animals (Carretta et al. 2009).
In north-eastern Mediterranean waters, including a putative subpopulation
in the Balearic Islands, total abundance was estimated as 7,654
(CV = 0.47). Abundance in inshore waters of the Balearic Islands
varied from 727 (CV = 0.47) dolphins in spring 2002 to 1,333 (CV
= 0.44) dolphins in autumn 2002, with an average estimate of 1,030
(CV = 0.35) (Forcada et al. 2004). In the central Spanish Mediterranean,
aerial surveys conducted between 2001 and yielded a mean abundance
of 1,333 dolphins (95% CI = 739-2,407) (Gomez de Segura et al. 2006).
Off southern Spain, surveys conducted in the Alboran Sea between
2000 to 2003 yielded an estimate of 584 dolphins (95% CI=278-744)
(Cañadas and Hammond, 2006).
In eastern Ionian Sea coastal waters boat surveys conducted between
1993 and 2003 yielded 235 bottlenose dolphin sightings. Individual
photo-identification showed a relatively stable presence, some individuals
showing high levels of site fidelity and others using the area only
occasionally (Bearzi et al. 2005). In the 400-km² Amvrakikos
Gulf, western Greece, boat surveys conducted between 2002 and 2005
yielded a total population estimate of 148 individuals (95% CI=132-180).
Mean dolphin density in the Gulf was 0.37 animals km² (Bearzi
et al. 2008).
The total population size in the Black Sea is unknown. However,
there are recent abundance estimates for parts of the range suggesting
that population size is at least several thousand (Birkun 2006).
Finally, there are only a few estimates from other parts of the
world. Approximately 900 bottlenose dolphins inhabit the 400km stretch
of coastal waters off Natal, south-east of southern Africa (Wells
and Scott, 1999 and refs. therein; Reyes, 1991 and refs. therein).
In the eastern Sulu Sea, Dolar et al. (2006) estimated the population
size at 2,630.
4. Biology and Behaviour
Habitat: As a result of increased pelagic survey efforts,
researchers have come to recognise T. truncatus as a truly
cosmopolitan species. Although it tends to be primarily coastal,
it can also be found in pelagic waters (Wells and Scott, 1999).
Bottlenose dolphins exploit a wide variety of habitats. The inshore
form frequents river mouths, bays, lagoons and other shallow coastal
regions (between 0.5-20m). Occasionally they may travel far up into
rivers. In Santa Monica Bay, California, e.g. they occur year-round
and are found 80% of the time in waters within 0.5 km of shore (Bearzi,
T. truncatus off Tenerife, Spain, North Atlantic
© Boris Culik
The offshore form is apparently less restricted in range and movement,
and can be found in many productive areas, particularly in the tropics.
Some offshore populations are residents around oceanic islands.
A coastal habitat seems to be preferred in the Black Sea, with limited
movements into offshore waters (Reyes, 1991 and refs. therein).
Limits to the species' range appear to be temperature related, either
directly, or indirectly through distribution of prey.
Off the coasts of North America, they tend to inhabit waters with
surface temperatures ranging from about 10°C to 32°C (Wells
and Scott, 1999 and refs. therein). In the Gulf of Mexico off the
coast of Louisiana, their distribution is correlated with depth,
distance to shore, bottom oxygen, distance to the edge of a hypoxic
zone and density of sciaenid fishes, particularly Atlantic croaker,
Micropogonias undulatus (Good et al. 2006), and in Sarasota
Bay, Florida, seagrass areas are particularly important to foraging
dolphins (Weiss, 2006).
In Moray Firth, NE Scotland, there are clear relationships between
feeding events and submarine habitat characteristics; during June
and July certain forms of feeding occur primarily over steep sea
bed gradients and in deeper waters (Hastie et al. 2004). Along the
Dorset coast of England chlorophyll a and fish distribution (brill,
cuttlefish, plaice, Pollack, red and grey mullet, sole, sprat and
spurdog) were the main factors influencing distribution and could
explain 13.5% and 88% of the frequency of dolphin sightings, respectively
(Sykes et al. 2003).
Food: The differences between inshore and offshore Tursiops
are also reflected in their feeding habits. The inshore form feeds
primarily on a variety of fish and invertebrates from both the littoral
and sub-littoral zones, whereas mesopelagic fish and oceanic squids
are commonly reported as the diet of animals of the offshore form
(Reyes, 1991 and refs. therein). Diet varies with local prey availability
including benthic-reef and sandy-bottom prey and their associated
predators, pelagic schooling fish and cephalopods, and deeper-water
fish (Wells and Scott, 1999 and refs. therein).
In North Carolina, USA, sciaenid fishes were the most common prey,
with Atlantic croaker (Micropogonias undulatus) dominating
the diet of dolphins that stranded inside estuaries, whereas weakfish
(Cynoscion regalis) was most important for dolphins in the
ocean. Inshore squid (Loligo sp.) was eaten commonly by dolphins
in the ocean, but not in the estuaries (Gannon and Waples 2004).
The overwhelming majority of prey were soniferous species, which
are detected through passive listening. Gannon et al. (2005) showed
that by listening passively, dolphins obtain useful information
on identity, number, size and location of soniferous prey. Once
prey is detected, dolphins then use echolocation for tracking during
pursuit and capture.
In South Carolina, stomach contents included fish (89% of stomachs)
and cephalopods (sole dietary component in 11% of stomachs). Dolphins
preferred smaller-sized benthic and demersal fish species of the
family Sciaenidae, with star drum (Stellifer lanceolatus)
the most abundant species. Brief squid (Lolliguncula brevis)
was the most frequently observed cephalopod (Pate, 2008).
Off the coast of Normandy, France, the diet was dominated by gadoid
fish (Trisopterus sp.), gobies and mackerel (Scomber scombrus)
(de Pierrepont et al. 2005).
Off Galicia, north-western Spain, the most important prey species
between 1990 and 2005 were blue whiting (Micromesistius poutassou)
and hake (Merluccius merluccius), both of high commercial importance.
Although bottlenose dolphins are often seen close inshore, their
diet suggests that they feed at the shelf edge. The amount of hake
in the diet remained stable against a background of falling local
abundance, while the amount of blue whiting declined despite an
increase in spawning stock size (Santos et al. 2007).
Off Peru, both coastal and offshore dolphins consumed Pacific sardines,
anchovetas, and hake, but demersal species such as sciaenids and
toadfish were found only in coastal dolphins. By contrast, the offshore
animals were the only ones with mesopelagic fish and squid in their
stomachs (Wells and Scott, 1999 and refs. therein).
Although individual feeding is perhaps most prevalent, co-operative
herding of schools of prey fish has been reported from a number
of regions. During the hunt, dolphins are very agile and were observed
to rapidly manoeuvre during chases of fish in open water or around
patches of rooted vegetation. Video analysis of chase sequences
indicates that mean rate of turn was 561.6 degrees /sec with a maximum
rate measured at 1,372.0 degrees /sec (or 3.8 turns per sec). High
turning rates with small turning radii were primarily the result
of maneuvers in which the dolphin rolled 90 degrees and rapidly
flexed its body ventrally (Maresh et al. 2004). In the deep waters
surrounding the Bermuda Pedestal, satellite-tracked dolphins travel
a mean distance of 28.3 km/day. Dive behaviour correlates with the
reported nightly vertical migrations of mesopelagic prey. At night,
dive depths are greater than 450 m and last longer than 5 min. whereas
during daytime dives are restricted to 50 m of the surface, lasting
less than 1 min (Klatsky et al. 2007).
In Mauritania and Brazil, dolphins regularly drive schools of mullet
towards fishermen wading with nets in shallow water, and in other
regions they have been observed feeding behind shrimp trawlers and
in the vicinity of small purse seiners, collecting discarded fish
from these operations after the nets are retrieved, and stealing
fish from a variety of fishing gear (Wells and Scott, 1999 and refs.
therein). E.g. in Florida, bottlenose dolphins prey on king mackerel
(Scomberomorus cavalla) taken by the troll fishery. Dolphins
took 6% of king mackerel caught by charter fishermen and 20% of
fish caught by commercial fishermen, causing substantial losses.
A modification to the outrigger planer was suggested to deter bottlenose
dolphins from engaging in depredation without causing a reduction
in catch (Zollett and Read, 2005).
Schooling: Group size is commonly around 2-15 animals, but
large herds of several hundred to a thousand are regularly seen
offshore (Bloch, 1998; Wells and Scott, 2009). In order to maintain
group cohesion, bottlenose dolphins developed individually distinctive
signature whistles to transmit identity information, which was found
to be independent of the caller's voice or location (Janik et al.
The plasticity of bottlenose dolphin behaviour is shown, e.g. in
the San Luis Pass area near Galveston, Texas, where there are two
populations of bottlenose dolphins using adjacent habitats in different
ways. Resident dolphins forage predominantly in the bays and pass
and display group foraging behaviour. In contrast, Gulf dolphins
are only observed foraging in coastal waters, and do so individually.
These behavioural differences may reflect strategies based on habitat
variation but may also be indicative of distinct social structures.
There is also a seasonal component to behaviour and group size,
with larger mixed groups and more social behaviour occurring in
summer (Henderson and Würsig, 2007).
Bottlenose dolphins are commonly associated with other cetaceans,
such as pilot whales, white-sided, spotted, rough-toothed and Risso's
dolphins, and humpback whales. Hybrids with other species are known
from both captivity and in the wild (Jefferson et al. 1993; Bloch,
1998; Wells and Scott, 1999). However, interspecific interactions
may be aggressive, and in Baia Norte, southern Brazil, e.g. attacts
of bottlenose dolphins on estuarine dolphins (Sotalia
guianensis) were observed (Wedekin et al. 2004) Aggressive
and lethal interactions with harbour porpoises (Phocoena
phocoena) were frequently reported (e.g. Read, 1999).
Reproduction: Longevity in females is more than 57 years
and in males up to 48 years (Wells and Scott, 1999). Females reach
sexual maturity at 5 - 13 years and males at 9-14 years. Spring
and summer or spring and autumn calving peaks are known for most
populations, and gestation lasts about 12 months (Jefferson et al.
1993; Wells and Scott, 2009).
According to Wells and Scott (1999; 2009), coastal dolphins exhibit
a full spectrum of movements, including 1) seasonal migrations,
2) year-round home ranges, 3) periodic residency, and 4) a combination
of occasional long range movements and repeated local residency.
Long-term residency may take the form of a relatively permanent
home range or repeated occurrence in a given area over many years.
For example, the residents of several dolphin communities along
Florida's west coast have maintained relatively stable home ranges
during more than 28 years of observations. In other areas, residency
is long-term but more variable. Dolphins seen frequently during
1974-1976 in Golfo San Jose, Argentina, showed a subsequent decline
in frequency of occurrence, but were still occasionally identified
in the area 8-12 years later.
Along the central west coast of Florida, communities of resident
dolphins appear to inhabit a mosaic of overlapping home ranges.
Most of the activities of the residents are concentrated within
the home ranges, but occasional movement between ranges occurs also.
The same applies to bottlenose dolphins off San Luis Pass, Texas
(Maze and Würsig, 1999). Within the home range, habitat use
varies with season, with shallow estuarine waters frequented during
the summer and coastal waters and passes between barrier islands
used during the winter (Wells and Scott, 1999 and refs. therein).
However, behaviour may also vary among animals within the same area:
Simões-Lopez and Fabian (1999) found that in Laguna, southern
Brazil 88.5% of the individuals were resident and the rest were
In Southern California, a high proportion of dolphins photographed
off Santa Barbara, Orange County, and Ensenada, Mexico, were also
photographed off San Diego. The majority of these dolphins exhibited
back-and-forth movements between study areas, with no evidence of
site fidelity to any particular region. Minimum range estimates
were between 50 and 470 km. Minimum travel-speed estimates were
11-47 km/d, and all dolphin schools sighted during the study were
within 1 km of the shore (Defran et al. 1999). Long-distance movements
have also been reported in conjunction with an El Niño warm
water event, expanding the species' range more than 500 km northward
(Wells and Scott, 1999 and refs. therein). Following the El Niño,
some dolphins remained in northern waters, while others returned
to their previous range to the south. Würsig (1978, in Wells
and Scott, 1999) reported a 600-km round-trip for several identifiable
dolphins in Argentina. Tanaka (1987) reported that a satellite-tracked
dolphin off Japan apparently travelled 604 km in 18 days along the
Dolphins living at the high latitude or cold water extremes of the
species' range may migrate seasonally. It has been suggested that
some dolphins may use seasonal home ranges joined by a travelling
range: a 4-month cycle of occurrence of dolphins was observed in
Golfo San Jose, Argentina (Wells and Scott, 1999 and refs. therein).
In Moray Firth, northeastern Scotland, bottlenose dolphins were
seen in all months of the year, but numbers were low in winter and
spring and peaked in summer and autumn. Individuals exhibited rapid
movements across the population's range, and one individual was
sighted at locations 190 km apart within a 5-day period (Wilson
et al. 1997). Similarly in the Faroes, bottlenose dolphins are observed
all year round but with peaks in March and July-October (Bloch,
1998). In the coastal waters of Cornwall, UK, dolphins demonstrated
a seasonal residency pattern, spending the winter in southern Cornwall
and moving farther north-eastward during spring and summer. The
dolphins occupied a linear coastal range of 650 km. Within this
range they repeatedly made long-distance journeys covering up to
1,076 km and lasting up to 20 days (Wood, 1998).
Wells et al. (1999) tracked two rehabilitated adult male bottlenose
dolphins with satellite- linked transmitters in 1997. "Rudy"
was equipped in the Gulf of Mexico off central west Florida. He
moved around Florida and northward to Cape Hatteras, NC, covering
2,050 km in 43 d. "Gulliver" was released off Cape Canaveral,
FL. He moved 4,200 km in 47d to a location north-east of the Virgin
Islands. Gulliver swam through 5,000-m-deep waters 300 km offshore
of the northern Caribbean islands, against the North Equatorial
Long-distance migrations are presumably regularly undertaken by
offshore bottlenose dolphins, whose diet is comprised of highly
migratory species of fish and squids. E.g. in the Azores, North
Atlantic bottlenose dolphins carry out extensive movements and have
large home ranges in response to the lower density and patchy distribution
of prey compared to other areas. The extensive ranging behaviour
and the lack of territoriality provide an opportunity for interbreeding
between different island groups, thus preventing genetic differentiation
within the population (Silva et al. 2008).
Direct catch: Directed fisheries taking bottlenose dolphins
have previously occurred around the Black Sea as well as in Mexico,
Guatemala, Costa Rica, the West Indies, Venezuela, Sri Lanka, and
off southern Africa, India and Peru. Drive fisheries for bottlenose
and other dolphins were also reported from the Republic of China
(Taiwan), but the numbers are not known. The species was taken in
a drive fishery in the Faroe Islands which dates back to 1803, annual
takes numbering from 1-308, often in mixed schools with long finned
pilot whales (Globicephala
melas) (Reyes, 1991 and refs. therein; Bloch, 1998). However,
there are no reports on catches in recent years (NAMMCO, 2008).
In Peru, coastal fisheries still take Tursiops and other
cetaceans for human consumption, using gill nets, purse seines,
and harpoons. A similar fishery occurs in Sri Lanka (Wells and Scott,
1999 and refs. therein; Wells and Scott, 2009). Although direct
killing has noticeably decreased since dolphin hunting was banned
by law in 1996, around a thousand dolphins and other small whales
are still falling victim annually to fishermen to supply bait meat
for the shark fishery there. The most significant take probably
occurs off Japan, where bottle-nose dolphins are killed for human
consumption, bait and because of perceived competition with fisheries
(Wells and Scott, 2009). Reported catches in 2007 are 300 in the
drive fisheries and 101 in the hand-harpoon fishery (Iwasaki, 2008).
Live captures: More than 530 Tursiops have been taken
from US waters since the passage of the Marine Mammal Protection
Act of 1972 (MMPA), particularly from the southeastern USA. Present
federal regulations limit the annual allowable take to less than
2% of the minimum estimated population in designated management
areas, but no bottlenose dolphins have been collected in US waters
since 1989. Some live-capture removals continue in other countries,
including Cuba where at least 238 were captured in 1986-2004 (Van
Waerebeek et al. 2006), the Solomon Islands, Japan, and Russia (Wells
and Scott 1999; 2009).
Incidental catch: Fisheries around the world account for
incidental takes of bottlenose dolphins, in gillnets, driftnets,
purse seines, trawls, long-lines, and on hook-and-line gear used
in commercial and recreational fisheries, but the present level
of take remains unknown (Hammond et al. 2008).
Along the east coast of the United States, by-catches of bottlenose
dolphins in gillnet fisheries exceed removal levels set under the
US Marine Mammal Protection Act (Cox et al. 2004). The use of acoustic
deterrents, so-called "pingers" to deter dolphins from
nets, however, did not show clear results and was not recommended.
In North Caroliona interactions between dolphins and gill nets are
common, and many of these interactions are food-based. Surprisingly,
however, dolphins engaging in depredation do not appear to become
entangled; instead it seems more likely entanglement occurs as a
result of dolphins failing to change course around the net (Read
et al. 2003). A new type of net material made with barium sulphate
was found to be acoustically more reflective than the standard nylon
net and should increase dolphin detection range sufficiently to
reduce entanglement (Mooney et al. 2004).
The best protective measure, however, seems to be a decrease in
fishing effort. A marked decrease in fishing effort for spiny dogfish
in North Carolina corresponded with a marked decrease in winter
stranding rates of bottlenose dolphins with entanglement lesions
(Byrd et al. 2008). In South Carolina, analysis of historical strandings
showed that approximately 24% of the 42 entanglement cases from
1992-2003 resulted from the blue crab fishery. The average number
of entanglements per year exceeded 1% of PBR across a five-year
period (1999-2003) (Burdett and McFee, 2004). Longlines seem to
be far less dangerous: in the Hawaii-based longline fishery targeting
primarily tunas and swordfish. In 159,572 sets during 1994-2005,
only 1 bottlenose dolphin was caught (Forney and Kobayashi, 2007).
A high proportion of the common dolphins that strand on the south
coast of England in winter months bear evidence of fishery interactions.
Many animals were recorded from trawl tows targeted at bass. Preliminary
mitigation trials using pingers, however, were not effective, and
current work is focussed on using exclusion grids to allow dolphins
to escape from the sleeve of the trawl, as the number of stranded
by-caught dolphins has raised concerns for their conservation status
Around the Balearic Islands, the artisanal gillnet fishery is experiencing
a growing problem with bottlenose dolphins depredating bottom-set
nets. The resulting catch loss engenders hostility from fishers,
and interactions between dolphins and nets can result in bycatch
mortality. A large-scale experimental trial using pingers to deter
dolphins from the nets suggests that some pinger types are effective
in reducing net interactions (Brotons et al. 2008). Whereas cetacean
interactions with fishing gear are reported regularly and most frequently
involve incidental capture, another cause of mortality is the ingestion
of gill-net parts and subsequent larynx strangulation documented
e.g. in 10% of dolphins stranded along the Croatian coast of the
Adriatic Sea (Gomeric et al. 2009).
The use of shark nets to protect bathing beaches in South Africa
and Australia has caused mortality as well. Dolphins were found
with full stomachs, indicating recent feeding in the vicinity of
the nets, and there was a correlation of mortality rates with the
direction of the prevailing current. Attempts to prevent the animals
from entangling by incorporating active and passive devices in the
net were not successful. The relatively high incidental catches
of coastal dolphins off South Africa has prompted concerns that
the take is not sustainable (Wells and Scott, 1999 and refs. therein).
Incidental catches in Chinese fisheries reach several hundred per
year (Yang et al. 1999), and a large incidental take of Tursiops
has apparently occurred in the Taiwanese gill net fishery off Australia,
with an annual mortality perhaps exceeding 2000 animals, although
these may be of the other species, Tursiops aduncus. Molecular
monitoring of 'whalemeat' markets in the Republic of (South) Korea
revealed that at least some T. truncatus by-catch is sold
and used in human consumption (Baker et al. 2006).
Peddemors (1999) summarises for the coast of Africa south of 17°S
that more research emphasis should in future be placed on possible
detrimental interactions due to overfishing of delphinid prey stocks.
Increased commercial fishing pressure will inevitably increase interactions
between the fishery and the affected delphinids. One of the inshore
species considered to be vulnerable is the bottlenose dolphin in
KwaZulu-Natal and Namibia.
Killing: Tursiops have been intentionally killed
by fishermen in Japan and Hawaii, and presumably such practices
are found elsewhere in their range (Reyes, 1991). The Japanese drive
fishery off Iki Island and the Kii Peninsula took several hundred
Tursiops truncates annually to reduce the perceived competition
with the commercial fishery for yellowtail, Seriola sp. (Wells
and Scott, 1999 and refs. therein).
Pollution: Its worldwide distribution and great adaptability
to diverse habitats make this species a good indicator of the quality
of inshore marine ecosystems. Concentrations of many contaminants
in common dolphin tissues are magnified through bio-accumulation
and often are the highest recorded in any mammal.
In the blubber of South African specimens, concentrations of polychlorinated
biphenyls and dieldrin in tissues of males reached levels that theoretically
could impair testosterone production and thus reduce reproductive
ability. First-born calves received 80% of their mother's body burden
of contaminant residues, possibly leading to increased neonatal
mortality (Wells and Scott, 1999, and refs. therein), as even relatively
low levels of PCBs and DDT can result in a decline in immune system
function (Lahvis et al. 1995). This was confirmed by subsequent
results. In primiparous females from Sarasota Bay, Florida, PCB
concentrations in blubber and plasma and the rates of first-born
calf mortality were both high. Subsequent calves of similar age
had lower concentrations than first-born calves (Wells et al. 2005).
Persistent organic pollutants in coastal ecosystems have half-lives
of decades or more, and their signatures can be used to confirm
site-specificity of local populations. In Biscayne Bay (Miami, FL),
male dolphins in the northern, metropolitan area had PCB concentrations
that were 5 times higher than in their congeners from the southern,
more rural area (43 mg/g vs. 8.6 mg/g wet mass), demonstrating local
and persistent differences in habitat use. PCB concentrations in
northern bay dolphins are high compared to other estuarine dolphin
populations and may place these animals at risk of reproductive
failure and decreased immune function (Litz et al. 2007). Similarly,
along the coast of Georgia, southeastern USA, blubber PCB concentrations
from free-ranging animals from the Turtle/Brunswick River estuary
were ten times higher (77µg/g lipid) and showed different
Aroclor 1268 signatures compared with strandings samples from Savannah
area estuaries 90 km to the north, confirming that inshore T.
truncatus populations exhibit long-term fidelity to specific
estuaries (Pulster and Maruya, 2008) and highlighting the necessity
to control local pollution sources.
Contamination of bottlenose dolphin is a world-wide problem, and
blubber samples from the Bay of Bengal (southeast coast of India)
contained considerable concentrations of the organochlorine pesticides
hexachlorocyclohexane (HCHs), dichlorodiphenyl trichloroethane (DDTs),
and polychlorinated biphenyls (PCBs) (Karuppiah et a. 2005). Dolphins
stranded on the coasts of the Mediterranean sea between 2000 and
2003 showed very high concentrations of PCBs and DDT in all tissues
and organs analysed. Their values were still comparable to those
obtained during the 1990's in the Mediterranean environment (Wafo
et al. 2005).
In the Charleston Harbor estuary, South Carolina, USA, the levels
of another bioaccumulative chemical, polybrominated diphenyl ethers
(PBDE) in dolphins represent some of the highest measured in marine
mammals (5,860 ng/g lipid) and warrant further investigation of
potential deleterious effects (Fair et al. 2007).
Almost all bottlenose dolphins found stranded along the western
Italian and Greek coasts in the mid-1990's contained the anti-fouling
component tributyltin (TBT) and its degradation products, monobutyltin
(MBT) and dibutyltin (DBT), in the liver and kidney. BTs were found
to be transferred from mother to fetus. (Focardi et al. 2000). In
waters around Japan butyltin concentrations in coastal T. truncatus
were higher than in offshore populations, indicating land-based
sources (Le et al. 1999), e.g. shipyards where contaminated paint
is stripped during repair.
Specimens stranded along the Corsican coast, Mediterranean, France,
in the mid-1990's showed high levels of mercury accumulation in
the liver, with concentrations as high as 4,250µg Hg/g dw
suggesting a life-long uptake of this heavy metal (Frodello et al.
Finally, there are also biological sources of toxic substances.
In Sarasota Bay, Florida, USA, brevetoxins produced by blooms of
the harmful alga Karenia brevis were measured at high levels in
bottlenose dolphin carcasses after large-scale mortality, and levels
in animals stranded during non-bloom conditions were also detectable
(Fire et al. 2007). Since masss-occurrence of these algae is linked
to high concentrations of fertilizers in terrestrial runoff, this
toxic substance can also be indirectly attributed to anthropogenic
activities (NOAA, 2009).
Noise pollution: Anthropogenic sounds in the ocean are increasing
from such influences as shipping, drilling, sonars, and scientific
exploration, and several marine mammal strandings have been linked
to anthropogenic noise-induced events. Odontocetes rely on utilizing
sound in the ocean and are particularly affected by man-made noise.
In April 2006, an exceptional mass stranding event occurred in the
northern part of Zanzibar involving more than 600 dolphins. The
dolphins were alive at the stranding spot and the locals ate the
meat without any problems, which excludes red tides as a mortality
cause. Amir and Jiddawi (2007) speculated that a seismic event which
at the time was taking place in the southern part of Tanzania for
gas exploration, a sea quake or sonar activities could have caused
Mooney et al. (2006) showed that temporary threshold shifts in
the bottlenose dolphin can be induced by long exposure times or
high sound pressure levels. Whereas bottlenose dolphins may have
a protective mechanism that reduces harmful physiological noise
damage at shorter duration exposures, the inverse might be true
for long duration exposures at lower levels. In Teignmouth Bay,
UK, stationary boats elicited no response, but speedboats and jet
skis were associated with aversive behaviours, even when boats were
not directly approaching the dolphins (Goodwin and Cotton, 2004).
In Aberdeen harbour, Scotland, dolphins were usually concentrated
around the harbour entrance. Their responses to boats varied considerably
according to boat size, activity and speed, but there was evidence
of habituation to boat traffic (Sini et al. 2005). A more subtle
effect of noise is the acoustic detection range of female dolphins
and their dependent calves. Quintana-Rizo et al. (2006) found it
to be noise limited as opposed to being limited by hearing sensitivity.
In shallow-water seagrass areas, low-frequency (7-13 kHz) whistles
with a 165 dB source level can be normally heard by dolphins at
a distance of 487 m, which is larger than usual mother-calf separation
Tourism: Excessive and unregulated visiting of wild dolphins
habituated to humans has raised concern in several areas, in particular
in Europe (Reyes, 1991 and refs. therein). Off Sarasota, Florida
animal behavioral observations conducted during boat approaches
detected longer interbreath intervals compared to control periods
(no boats within 100m). Dolphins decreased interanimal distance,
changed heading, and increased swimming speed significantly more
often in response to an approaching vessel than during control periods
(Nowacek et al. 2001). These findings provide additional support
for the need to consider disturbance in management plans for cetacean
conservation (P. Yazdi, pers. comm. 2003).
In South Carolina, USA, and New Zealand multiple boats were found
to have a greater influence on dolphin behaviour and movement than
the presence of a single boat (Constantine et al. 2004; Mattson
et al. 2005). Dolphin-watching boats, motorboats, shrimp boats,
and jet skis affected group size and behaviour of dolphin groups,
with jet-skis having the most pronounced effects. Boat-related effects
on bottlenose dolphin behaviour are considered "harassment"
under the USA Marine Mammal Protection Act (1972) and should be
scrutinized (Mattson et al. 2005).
However, in Zanzibar waters, Western Indian Ocean, local fishermen
realised that the touristic value of dolphins far exceeds that of
using them as bait for shark. As many as 2,000 tourists visit the
dolphin site at Kizimkazi per month and dolphin-tourism has become
a popular economic activity. It is hoped that successful management
of the dolphin-tourist trade will ensure continued visitors to coastal
villages and thus add to local income while contributing to management
and conservation (Ali and Jiddawi, 1999) This is also a problem
in Australia, where excessive interaction with tourists has led
to reduced survivorship of juveniles at Shark Bay (W. Perrin, La
Jolla, CA, USA, 2010, pers. comm.).
Range states (Hammond et al. 2008) :
Albania; American Samoa; Anguilla; Antigua and Barbuda; Argentina;
Aruba; Australia; Bahamas; Bangladesh; Barbados; Belgium; Belize;
Benin; Bermuda; Brazil; Brunei Darussalam; Bulgaria; Cambodia; Cameroon;
Canada; Cape Verde; Cayman Islands; Chile; China; Cocos (Keeling)
Islands; Colombia; Comoros; Cook Islands; Costa Rica; Côte
d'Ivoire; Croatia; Cuba; Cyprus; Denmark; Djibouti; Dominica; Dominican
Republic; Ecuador; El Salvador; Falkland Islands (Malvinas); Fiji;
France; French Guiana; French Polynesia; Gabon; Gambia; Georgia;
Germany; Ghana; Gibraltar; Greece; Grenada; Guadeloupe; Guam; Guatemala;
Guinea; Guinea-Bissau; Guyana; Haiti; Honduras; Hong Kong; India;
Indonesia; Iran, Islamic Republic of; Ireland; Italy; Jamaica; Japan;
Kenya; Kiribati; Liberia; Madagascar; Malaysia; Maldives; Malta;
Marshall Islands; Mauritania; Mexico; Micronesia, Federated States
of; Montenegro; Morocco; Mozambique; Myanmar; Namibia; Nauru; Netherlands;
Netherlands Antilles; New Caledonia; New Zealand; Nicaragua; Niue;
Northern Mariana Islands; Oman; Pakistan; Palau; Panama; Papua New
Guinea; Peru; Philippines; Pitcairn; Portugal; Puerto Rico; Romania;
Russian Federation; Saint Helena; Saint Kitts and Nevis; Saint Lucia;
Saint Pierre and Miquelon; Saint Vincent and the Grenadines; Samoa;
Senegal; Singapore; Slovenia; Solomon Islands; Somalia; South Africa;
Spain; Sri Lanka; Suriname; Taiwan, Province of China; Tanzania,
United Republic of; Thailand; Timor-Leste; Togo; Tonga; Trinidad
and Tobago; Turkey; Ukraine; United Arab Emirates; United Kingdom;
USA; Uruguay; Vanuatu; Venezuela; Viet Nam; Virgin Islands, British;
Virgin Islands, U.S.; Wallis and Futuna; Western Sahara; Yemen
The common bottlenose dolphin is listed in Appendix II of CITES.
The North Sea, Baltic Sea, Mediterranean and Black sea populations
are listed in Appendix II of CMS.
The IUCN lists the species as "Least Concern" with the
exception of the Black sea bottlenose dolphin T. t. ponticus,
which is listed as "Endangered". Its current population
size is around several thousand animals, but the population was
significantly reduced by large directed takes, incidental mortality
in fisheries, live-catches and a mass mortality of unknown cause
in 1990, and it is currently suffering from a degradation of the
Black Sea environment (Birkun, 2008).
In the Mediterranean Sea, important ongoing threats include incidental
mortality in fishing gear and the reduced availability of key prey
caused by overfishing and environmental degradation throughout the
region. Additional potential or likely threats include the toxic
effects of xenobiotic chemicals, epizootic outbreaks, direct disturbance
from boating and shipping, noise, and the consequences of climate
change. Dolphin abundance is thought to have declined considerably
in the region and management measures are needed to prevent further
decline. Compliance with existing legislation and treaties, which
outline appropriate measures, is urgently required (Bearzi et al.
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© Boris Culik (2010) Odontocetes.
The toothed whales: "Tursiops truncatus". UNEP/CMS
Secretariat, Bonn, Germany.
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.