Stenella frontalis (G.
English: Atlantic spotted dolphin
Spanish: Delfín pintado
French: Dauphin tacheté de l'Atlantique
Stenella frontalis © Würtz-Artescienza (see "links")
Atlantic spotted dolphins have a moderately long, stocky beak,
with a distinctive crease between the base of the beak and the melon.
The dorsal fin is tall and falcate and the flippers are curved backwards.
Juveniles are unspotted and look similar to bottlenose dolphins,
with their dark cape, spinal blaze, light gray sides and white belly.
As the animals age, spots on both ventral and dorsal surfaces develop
and some individuals become so heavily spotted that the underlying
colour pattern becomes obscured. However, adults in some offshore
and temperate populations may remain unspotted (Jefferson et al.
2008). S. frontalis can be distinguished from S.
attenuata, which also occurs in the tropical Atlantic, by
its spinal blaze which sweeps up into the dorsal cape. In addition,
the peduncle does not exhibit the dorsoventral division into darker
upper and lighter lower halves observed in S. attenuata.
Adult size ranges from 166 cm to 229 cm, and mass reaches 143 kg
There is a marked regional variation in the size and shape of the
skull and in adult body size (Perrin et al. 1987, in Rice, 1998).
The largest individuals inhabit the coastal waters of the southeastern
United States; these are the animals that long went under the name
S. plagiodon (Cope, 1866) (Rice, 1998). Genetic differentiation
of Atlantic spotted dolphins in the western North Atlantic, including
the Gulf of Mexico was recently tested by Adams and Rosel (2006)
who presented evidence for 3 populations coupled to known biogeographic
transition zones at Cape Hatteras, North Carolina and Cape Canaveral,
Florida, USA. This also supports previously documented morphotypes
of Atlantic spotted dolphins in coastal and offshore waters.
Stenella frontalis ranges in the tropical and warm-temperate
Atlantic, north to the Gulf of Mexico, Cape Cod, the Azores, and
the Canary Islands, and south to Rio Grande do Sul in Brazil, Saint
Helena, and Gabon. A synonym is Stenella froenata (F. Cuvier,
1829) (Rice, 1998). The species is well documented from Equatorial
Guinea and Côte d'Ivoire, with recent sightings at sea off
Senegal (Van Waerebeek et al. 2000 and refs. therein) and Angola
(Weir, 2008) . The range extends to about 50°N to 25°S (Hammond
et al. 2008; Jefferson et al. 2008).
Distribution of Stenella frontalis (Hammond
et al. 2008; © IUCN; enlarge
Warm, temperate, subtropical and tropical waters in the North and
3. Population size
In the US waters of the North Atlantic Ocean (EEZ) south of Maryland,
the most abundant species in 1998 was the Atlantic spotted dolphin
(14,438; CV = 0.63) (Mullin and Fulling, 2003). Subsequent abundance
estimates are significantly higher: the best recent abundance estimate
for Atlantic spotted dolphins stems from two 2004 western U.S. Atlantic
surveys: Maryland to the Bay of Fundy (3,578; CV = 0.48) and Florida
to Maryland (47,400; CV = 0.45) (Waring et al. 2008).The best available
abundance estimate for the northern Gulf of Mexico is the combined
estimate of abundance for both the outer continental shelf (fall
surveys, 2000-2001) and oceanic waters (spring and summer surveys,
2003-2004), which is 37,611 (CV=0.28) (Waring et al. 2008).
There are no data on abundance available from West Africa (van
Waerebeek et al. 2000) or for other populations in the Atlantic
ocean (Hammond et al. 2008; Perrin, 2009).
4. Biology and Behaviour
Habitat: Atlantic spotted dolphins are encountered primarily
in continental shelf (<200 m) and continental slope waters (200-2000
m)(Mullin and Fulling, 2003). The large, heavily spotted form of
the Atlantic spotted dolphin along the south-eastern and Gulf coasts
of the United States inhabits the continental shelf, usually being
found inside or near the 100-fathom curve (within 250-350 km of
the coast) but sometimes coming into very shallow water adjacent
to the beach seasonally, perhaps in pursuit of migratory forage
fish (Perrin et al. 1994 and refs. therein; Jefferson and Schiro,
1997). In the eastern Gulf of Mexico between Tampa Bay and Charlotte
Harbor, Florida, S. frontalis was the most common shelf species
at depths of 20-180 m. Although its habitat has elsewhere been described
as broadly extending over the shelf, these data suggest that in
the eastern Gulf of Mexico the species prefers midshelf habitat
(Davis et al. 1996; Griffin and Griffin, 2003).
The smaller and less-spotted forms that inhabit more pelagic offshore
waters and waters around oceanic islands are less well known in
their habitat requirements. In the Bahamas, Atlantic spotted dolphins
spend much time in shallow water (6-12 m) over sand flats (Perrin
et al. 1994 and refs. therein; Jefferson and Schiro, 1997). Off
the Azores Archipelago they were found around each group of islands,
where they were also more abundant in offshore (9 to 28 km) as opposed
to coastal areas (to 9 km from shore) (Silva et al. 2003).
Schooling: Small to moderate groups, generally of fewer
than 50 individuals, are characteristic of the Atlantic spotted
dolphin. Coastal groups usually consist of 5 to 15 animals (Jefferson
et al. 1993). However, on both coasts of northern Florida, moving
groups may consist of up to 100 individuals and may attract other,
smaller groups that join the large group briefly. Segregated schools
of subadults and adults, or of adults with calves have also been
observed (Perrin et al. 1994 and refs. therein). In a report from
the Canary Islands, maximum group size of S. frontalis is
given as 650 animals (mean 40) in 321 sightings between 1994-2001
(Ritter, 2003). In the Gulf of Mexico Atlantic spotted dolphins
feed in a co-ordinated manner and herd schools of clupeid fish into
dense balls against the sea surface. While such feeding activity
for other delphinid species has been well-described nearshore, co-ordinated
feeding offshore is rarely reported (Fertl and Würsig 1995).
Herzing and Johnson (1997) observed interactions between free-ranging
Atlantic spotted dolphins and bottlenose dolphins (Tursiops
truncatus) in Bahamian waters. Mixed-sex, mixed-species
adult groups (including pregnant females) were seen foraging together
and travelling together. In the central Azores, the presence of
large concentrations of bait fish in the area each summer gives
rise to mixed-species feeding aggregations, usually at dawn and
dusk (Clua and Grosvalet, 2001). The encircling of prey initiated
by common dolphins (Delphinus
delphis), often mixed with spotted dolphins, results in
the formation of a compact 'ball' of several thousand prey fish
close to the surface. Other dolphins, in particular bottlenose dolphins,
also eat the prey fish, whose high concentration makes them easy
to capture. Large tunas (Thunnus thynnus, Thunnus albacares)
some-times participate in the phenomenon. Seabirds (mainly Cory's
shearwaters, Calonectris diomedea borealis) are always present
throughout the few minutes during which the entire collective food
hunt takes place. Clua and Grosvalet (2001) showed that it is the
tunas that generate and benefit from the aggregation with dolphins,
rather than the contrary. Finally, in the Bahamas, interspecific
aggression with bottlenose dolphins was also observed (Herzing et
Food: A wide variety of fish and squid are taken by this
species (Jefferson et al. 1993): The stomach of a specimen captured
off northern Florida contained a large number of small cephalopod
beaks, and S. frontalis have been observed to feed on small
clupeoid and carangid fishes and large squid and to follow trawlers
to eat discarded fish. Observers in the north-eastern Gulf of Mexico
have reported that small squid have been regurgitated during captures
of spotted dolphins (Perrin et al. 1994). Dives to 40-60m and lasting
up to 7min have been recorded, but most time is spent at less than
10m (Davis et al. 1996).
Reproduction: Age at sexual maturity is estimated at 8-15
years in females (Herzing, 1997). The average calving interval is
about 3 years, nursing lasting for up to 5 years (Perrin, 2009).
In the Bahamas, genotypes of females and offspring revealed that
more than two males were required to explain the progeny arrays,
indicating promiscuous mating among females. Males mate within their
social cluster or with females from the next-closest cluster (Green,
2008). In southeastern Brazil the oldest specimen was 23 y old,
and the asymptotic length of 224.4 cm predicted by the growth curve
occurred at about 20 y (Sicilliano et al. 2007).
Davis et al. (1996) reported on the diving behaviour and daily
movements of a rehabilitated Atlantic spotted dolphin that was tracked
in the northwestern Gulf of Mexico for 24 d using satellite telemetry.
During that time, the animal travelled a total of 1,711 km at a
mean travelling speed of 0.8 m/s. The mean minimum distance travelled
daily was 72 km. Although this single animal can hardly be considered
representative of the species, it illustrates the habitat use and
movements within the marine habitat. International borders (e.g.
Between Texas and Mexico) are not limiting for wild populations.
Over the west-Florida continental shelf, monthly surveys conducted
between 1998 and 2001 between the coast and the 180 m depth contour
showed significant seasonal variation in Atlantic spotted dolphin
densities. Lowest abundances were recorded during the warm season
(June-October) and highest densities during the cool season (November-May).
Densities significantly decreased during 2000 and 2001, suggesting
a species response to short-term environmental variation (Griffin
and Griffin, 2004).
Mignucci et al. (1999) assessed cetacean strandings (including Atlantic
spotted dolphins) in Puerto Rico and the United States and British
Virgin Islands. Between 1990 and 1995, the average number of cases
per year increased from 2.1 to 8.2. There was a seasonal pattern
of strandings,, with a high number occurring in the winter and spring.
The monthly temporal distribution showed an overall bimodal pattern,
with the highest number of cases reported for February, May and
Direct catches: Atlantic spotted dolphins were taken in
a direct fishery for small cetaceans in the Caribbean. Direct takes
may also occur off the Azores and off West Africa (Jefferson et
al. 1993; Perrin et al. 1994).
Incidental catches: The total annual fishery-related mortality
or serious injury to the US west Atlantic stock during 2001-2005
was estimated at 6 (CV=1) spotted dolphins (S. frontalis
and S. attenuata combined). There was no reported fishing-related
mortality of spotted dolphins in US Gulf of Mexico waters during
1998-2006 (Waring et al. 2008).
In Campeche Sound, Mexico, Atlantic spotted dolphins stay behind
shrimping vessels and eat the discarded bycatch (mainly at night).
Because dolphins respect trawl net position, the probability of
incidental catch appears to be low (Delgado, 1997).
For Puerto Rico and the Virgin Islands, the most common human-related
causes observed in strandings are entanglement and accidental captures,
followed by animals being shot or speared (Mignucci et al. 1999).
Atlantic spotted dolphins are also captured incidentally in gill
nets in Brazil and Venezuela (e.g. Zerbini and Kotas, 2001). In
Venezuela, the dolphin carcasses are utilised for shark bait and
for human consumption (Perrin et al. 1994).
Some are probably taken incidentally in tuna purse seines off the
West African coast. While there are no reliable estimates of the
number of animals taken in these fisheries (Jefferson et al. 1993;
Carwardine, 1995), but it may be considerable. Nieri et al. (1999)
reported that in 1995, a large number of Atlantic spotted dolphins
washed ashore on the sandy beaches north of Nouakchott, the capital
of Mauritania, presumably due to fishery interaction.
Pollution: In specimens found stranded along the coastal
waters of Florida, USA, during 1989 to 1994, PCBs were the most
predominant contaminants followed by DDTs, chlordanes, tris(4-chlorophenyl)methane
(TCPMe), tris(4-chlorophenyl) methanol (TCPMOH), hexachlorobenzene,
and hexachlorocyclohexane isomers. Concentrations of TCPMe and TCPMOH
were greater than those in the blubber of marine mammals of various
other regions, suggesting the presence of sources for these chemicals
along the Atlantic coast of Florida (Watanabe et al. 2000).In specimens
from Brazilian coastal waters, concentrations of DDTs and PCBs were
the highest, followed by CHLs, TCPMOH, dieldrin, TCPMe, heptachlor
epoxide, HCB, and HCHs. Unexpectedly, the significant pollution
of PCBs, DDTs, TCPMe, and TCPMOH implies the occurrence of local
sources in the Southern Hemisphere comparable to those in the Northern
Hemisphere, probably by high industrialization in Brazil (Kajiwara
et al. 2004).
Red tide blooms: Between August 1999 and May 2000, 152 bottlenose
dolphins died coincident with red tide (K. brevis) blooms and fish
kills in the Florida Panhandle (Waring et al. 2008).
Noise pollution: Off Angola a dual-source airgun array used
in oil field prospection resulted in overt response by Atlantic
spotted dolphins; they were observed at a significantly greater
distance from the airgun array during full-array operations than
during guns-off. However, there was no evidence for prolonged or
large-scale displacement from the region during the 10-mo survey
duration (Weir, 2008).
Range states (Hammond et al. 2008) :
Antigua and Barbuda; Bahamas; Barbados; Brazil; Cameroon; Cape Verde;
Cayman Islands; Colombia; Côte d'Ivoire; Cuba; Dominica; Gabon;
Ghana; Guinea; Guinea-Bissau; Haiti; Honduras; Jamaica; Mauritania;
Mexico; Morocco; Netherlands Antilles; Nicaragua; Panama; Portugal
(Azores); Puerto Rico; Saint Helena; Saint Vincent and the Grenadines;
Senegal; Spain (Canary Is.); United States; Venezuela; Western Sahara.
The species is listed in Appendix II of CITES. Classified as "Data
Deficient" by the IUCN. Not listed by CMS.
Atlantic spotted dolphins seem to prefer inshore waters on both
sides of the tropical Atlantic and may venture even further. Satellite
telemetry showed that the species is capable of moving considerable
distances, and stranding data show seasonal peaks. These data show
that movements and home range size are likely to stretch across
international boundaries. Inclusion in Appendix II of CMS is therefore
Atlantic spotted dolphins also occur in South America, so please
see Hucke-Gaete (2000) in
Appendix 1 for further recommendations. Range states in the
Caribbean should be encouraged to investigate into and reduce accidental
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· Delgado EA (1997) Interaction of bottlenose dolphins, Tursiops
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© Boris Culik (2010) Odontocetes.
The toothed whales: "Stenella frontalis". UNEP/CMS
Secretariat, Bonn, Germany.http://www.cms.int/small-cetaceans
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.