Lagenorhynchus obscurus (Gray,
English: Dusky dolphin
Spanish: Delfín oscuro
French: Dauphin sombre
Lagenorhynchus obscurus © Wurtz-Artescienza
Dusky dolphins are small and moderately robust. They have virtually
no beak, as the head slopes evenly down from the blowhole to the
tip of the beak. The tip of the dorsal fin is rather blunt and is
not markedly hooked. The tail and back are bluish-black, and a dark
band runs diagonally across the flanks from below the dorsal fin
towards the vent and along the tailstock. The underside of the body
is white, and whitish-grey colour extends over the flanks. The tips
of the snout and lower jaw are dark. A grey area extends from the
eye down to the flipper. Two diagonal whitish streaks run forward
from the tail up past the base of the dorsal fin (Baker, 1990, Jefferson
et al. 2008). The largest dusky dolphin males and females reach
211 and 205 cm, respectively, attaining a body mass of rarely higher
than 100 kg (van Waerebeek and Würsig, 2009).
Yazdi (2002) reported a possible hybrid between a dusky dolphin
and a southern right-whale dolphin (Lissodelphis
peronii), south of Peninsula Valdés in Golfo Nuevo,
Lagenorhynchus obscurus is widespread in the southern hemisphere,
but its distribution is probably not continuous. Populations in
the South American, African, and New Zealand sectors of the range
are sufficiently distinct to be regarded as subspecies, according
to Van Waerebeek et al. (1993), although he did not apply scientific
names to them. However, Cassens et al. (2005) found only low levels
of genetic differentiation among most dusky dolphin populations.
Only the Peruvian dusky dolphin stock is highly differentiated.
Distribution of the various subspecies of Lagenorhynchus
obscurus: coastal temperate waters
off Australia, New Zealand, South America and Southern Africa and
(Hammond et al. 2008; © IUCN; enlarge
Rice (1998) listed three subspecies which are still
recognized (Hammond et al. 2008; Jefferson et al. 2009):
L. o. fitzroyi (Waterhouse, 1838), ranges in coastal waters
of South America from Isla Mazorca to Mar del Plata. However, Cassens
et al. (2003) using genetic analysis found no evidence for recent
female gene flow between Atlantic and Pacific waters, indicating
that the eastern South Pacific dusky dolphins stock (e.g. the Peruvian
stock) should be considered a separate management unit.
L. o. obscurus ranges in coastal waters of southern Africa
from Lobito in Angola south to Cape Agulhas in Cape Province. It
has been reported from Prince Edward Islands (subspecies?) and Ile
Amsterdam (subspecies?). Purported sightings and specimens from
Iles Crozet and Iles Kerguelen are erroneous or unverified (Rice,
L. o. subsp.: Ranges on the east coast of New Zealand from
Whitianga on North Island south to Stewart Island and is also found
on Campbell, Auckland and Chatham Islands (Rice, 1998).
In addition, there may be an unrecognised subspecies at the Falkland
Islands/Islas Malvinas, and another around Gough Island in the South
Atlantic Ocean (Rice, 1998), a melanistic form of L. o. obscurus
(van Waerebeek, 2002).
Gill et al. (2000) reported the sighting of a school of 15 dusky
dolphins off eastern Tasmania, suggesting that the species does,
in fact, also occur in Australian waters. However, The low rates
of observation or stranding, compared to those of other inshore
dolphins such as Delphinus
delphis, which is well-known along the southern Australian
coast, strongly suggest that dusky dolphins occur rarely in coastal
waters of southern Australia and are unlikely to be resident. Dusky
dolphins may occur far offshore, visiting coastal waters in response
to unusual oceanographic conditions. Another possibility is that
members of the population around St Paul and Amsterdam Islands may
visit Australian waters (Gill et al. 2000).
Based on new mitochondrial and nuclear DNA sequence data, Harlin-Cognato
et al. (2007) proposed a Pacific/Indian Ocean origin of the species,
with a relatively early and continued isolation of Peru from other
regions. Dispersal of the dusky dolphin into the Atlantic would
be correlated with the history of anchovy populations, including
multiple migrations from New Zealand to South Africa. They suggest
that changes in primary productivity and related abundance of prey
played a key role in shaping the phylogeography of the dusky dolphin,
coincident with periods of ocean change.
3. Population size
The population off Kaikoura, New Zealand was estimated at 12,000.
Markowitz, 2004). In the southern Ocean, during the Southern Hemisphere
minke whale assessment cruises between 1978/79 and 1987/88, a total
of 2,665 dusky dolphins in 27 schools were observed. These observations
were made while in transit between home ports and the Antarctic,
but no abundance estimates were calculated (Brownell and Cipriano,
1999 and refs. therein). The total number of dusky dolphins in the
fishing area off the Patagonian cost was estimated to be close to
7,252 individuals (Dans et al. 1997), and the number given by Schiavini
et al. (1999) for the area between Punta Ninfas and Cabo Blanco,
Argentina is 6,628. Off the Peruvian coast, dusky dolphins were
the third most abundant cetacean species sighted (Sanchez et al.
4. Biology and Behaviour
Habitat: This coastal species is usually found over the
continental shelf and slope (Jefferson et al. 1993; Aguayo et al.
1998), in waters up to 2,000 m deep (Würsig et al. 2007) The
distribution of dusky dolphins along the west coast of South Africa
and both coasts of South America is associated with the continental
shelves and cool waters of the Benguela, Humboldt and Falkland Currents.
Around New Zealand these dolphins are associated mainly with various
cold water currents (Brownell and Cipriano, 1999; Würsig et
al. 2007). Off Argentina, dusky dolphins have been sighted from
the coast to just before the 200 nautical miles Exclusive Economic
Zone border (Crespo et al. 1997). They seem to prefer waters with
sea surface temperatures between 10°C and 18°C (Brownell
and Cipriano, 1999).
Behaviour: Dusky dolphins are highly inquisitive and usually
easy to approach. They seem to enjoy the contact with boats and
people and readily bow-ride (Carwardine, 1995). They are one of
the most acrobatic of dolphins, frequently leaping high out of the
water, at times tumbling in the air (Jefferson et al. 1993). Mean
dive time for 10 radio tagged dolphins off Argentina was only 21.0
sec, and the number of long dives (>90 sec), probably associated
with feeding, peaked in mid-day to afternoon in summer (Würsig,
Subgroups of dusky dolphins within larger schools off New Zealand
sometimes were observed to dive synchronously, and occasionally
almost the entire group would be underwater for several minutes
(Brownell and Cipriano, 1999 and refs. therein). Group sizes, behaviours,
social affiliations and general habitat-use patterns differ between
Kaikoura and Admiralty Bay, Outh Island, even though some of the
same animals utilise both habitats in different seasons. Off Kaikoura,
there are daily and seasonal differences in behaviours and movement
patterns, with daytime rests around midday, social activities in
the mornings and afternoons, and feeding in deeper oceanic waters
at night (Würsig et al. 2007).
Schooling: The species is highly gregarious and seems to
welcome the company of other species as well as its own: it is often
seen with seabirds and frequently associates with other cetaceans.
Its own group sizes vary according to the time of year, with larger
numbers living together during the summer (Carwardine, 1995). School
size is fairly variable, with a range of 2-500 and a mean of 98.7
individuals. During the winter months, groups of less than 20 are
more common than at other times of the year.
Stable subgroups were observed within a more fluid society of changing
group size (Würsig and Würsig, 1980) and probably displayed
a high degree of individual-to-individual fidelity (Würsig
and Bastida, 1986).
In Argentina large groups are more efficient at herding schools
of anchovy than small ones, and it appears that methods for calling
in distant groups evolved because the food benefit for each dolphin
is increased when groups join forces. Cooperative herding appears
essential in their effort to feed on small schooling fish. An original
group of eight to 10 dolphins often increases to more than 200 by
the time feeding is completed. After they have fed, high levels
of social and sexual activity take place in the large group (Würsig
et al. 1989).
In New Zealand, the feeding and social behaviour of dusky dolphins
are very different. Instead of traveling, as their Argentine kin
do, in a widespread school with small groups some distance apart,
New Zealand dusky dolphins move in closely knit schools, made up
of subgroups of about ten individuals. There is usually an unbroken
and tight perimeter surrounding an entire school so that two- or
three hundred animals cover an area generally no larger than one
square kilometer. The entire school travels in search of food as
a directed unit rather than meandering in groups. Like the dusky
dolphins of Argentina, they split into small groups to rest near
shore during the day (Würsig et al. 1989).
Off Kaikoura, thousands of dusky dolphins gather, feeding nocturnally
on deep scattering layer prey, resting and socializing diurnally.
Group size, distance from shore, ranging along shore, traveling,
inter-individual distance, and noisy leaping peaked in winter; during
the spring- summer-autumn reproductive seasons dolphins maintaining
closer proximity to each other in smaller, more quiescent groups,
closer to shore (Markowitz, 2004). In the same area, mother-calf
pairs are often found in small groups with other mother-calf pairs,
with calves of roughly the same age. Nursery groups were encountered
in shallow waters (=20 m) significantly more often than in deeper
waters, thus effectively avoiding marine predators such as sharks
and killer whales (Weir et al. 2008).
Dusky dolphins have been observed in mixed cetacean schools with
southern right-whale dolphins (Lissodelphis
peronii) off Namibia. In summer, L. obscurus groups
off Kaikoura, New Zealand were occasionally accompanied by small
groups of common dolphins (Delphinus delphis), which travelled
as a cohesive subgroup within the larger dusky dolphin group. Dusky
dolphins were also observed with pilot whales (Globicephala
sp.) off Southwest Africa and the Prince Edward Islands (Brownell
and Cipriano, 1999, and refs. therein). Off Argentina, dusky dophins
were also observed in association with 2 Delphinus
capensis females and one Tursiops
truncatus male (Yazdi, 2000).
Reproduction: In New Zealand and Argentina, calving is believed
to peak in summer (November to February; Jefferson et al. 1993).
In Peruvian waters most births ocurred in late winter (August, September,
and October; Van Waerebeek & Read, 1994). In Peru, sexual maturity
in females is estimated at 4.3 - 5 years and in males at 3.8-4.7
years (Van Waerebeek and Würsig, 2009).
Food: L. obscurus take a wide variety of prey, including
southern anchovy and mid-water and benthic prey such as squid and
lanternfishes. They may also engage in nocturnal feeding. Co-operative
feeding is practised commonly in some areas (Jefferson et al. 1993).
Their most important prey in Peruvian coastal waters is anchoveta
(Engraulis ringens). It constituted almost 90% of the dusky
dolphin´s diet by percent gross energy (Mc Kinnon 1994). Other
prey species commonly found in dolphin stomachs were horse mackerel
(Trachurus symmetricus), hake (Merluccius gayi), sardine
(Sardinops sagax), Patagonian squid (Loligo gahi)
and jumbo flying squid (Dosidicas gigas) (Mc Kinnon, 1994).
The most important prey of Patagonian dusky dolphins off Argentina
is the southern anchovy (Engraulis anchoita), representing
39% of prey by number and 46% by weight (Alonso et al. 1998). The
most frequent prey was the patagonian squid (Loligo gahi), which
was present in 84% of stomachs. Other prey species found were hake
(Merluccius hubbsi), the "pampanito" (Stromateus
brasiliensis), the southern cod (Nothotenia sp.), shortfin
squid (Illex argentinus), a sepiolid (Semirossia tenera)
and an octupus (Octupus tehuelches).
Stomachs from 24 dusky dolphins incidentally killed in fishing operations
in New Zealand waters contained remains of mesopelagic fishes, mainly
myctophids and hoki (Macruronus novaezelandiae), and squids
(Nototodarus spp., Moroteuthopsis spp. and Teuthowenia
spp.; McKinnon, 1994; Brownell and Cipriano, 1999). In Admiralty
Bay and Current Basin, New Zealand, dolphin feeding tactics were
different from May through July than from August to November. From
May through July, dolphins fed on mobile prey at depth; from August
to November, they herded small schools of fish (including pilchard
Sardinops neopilchardus) to the surface (Vaughn et al. 2007).
Photo of L. obscurus © P.
Dusky dolphin may cover large distances: Würsig and Bastida
(1986) equipped two animals with spaghetti tags in Jan. 1975 off
Golfo San José Argentina. The two dolphins were sighted approximately
20 km (1 day after tagging) and 35 km (5 days after tagging) from
the tagging site, respectively. In December 1982, both dolphins
were observed swimming side by side in a school of approximately
150 dusky dolphins about 10 km off Mar del Plata, approximately
780 km north-east of the original tagging location.
In New Zealand, photographic identification data indicate a seasonal
shift in residency of dolphins between Kaikoura and the Marlborough
Sounds (approx. 200 km apart) (Harlin et al. 2003; Markowitz, 2004).
On a smaller scale, the Argentinian and New Zealand populations
exhibit inshore-offshore movements both on a diurnal and on a seasonal
rhythm (van Waerebeek, 2002). They were found during most of the
year in Golfo San José, Argentina, with a seasonal low in
abundance during winter and a peak in summer (Würsig and Würsig,
1980). In summer, these dolphins were also found more often in deeper
water near the mouth of the bay, at a time when southern anchovy
(Engraulis anchoita) was probably moving into deeper water.
In the Kaikoura area off South Island, New Zealand, dolphins moved
from nearshore to off-shore waters during the course of the day,
apparently feeding on mesopelagic fishes in deep water during evening
and night but consistently remained closer to shore than during
winter months (Brownell and Cipriano, 1999 and refs. therein). Residence
in Kaikoura is seasonal, with 1,969 from a population of 12,626
dolphins spending 103 consecutive days in the area per year (Markowitz,
2004). Mark-recapture data indicate that more than 1000 dusky dolphins
used Admiralty Bay, Marlborough Sound, over the course of a 5-year
study, with an average of 220 individuals inhabiting the bay on
any given week during the winters of 1998- 2002. As many as 55%
of individuals returned to Admiralty Bay in consecutive winters
(Markowitz et al. 2004). Off the west coast of South Island, dusky
dolphins occurred almost exclusively in summer in groups of 2-150
individuals, often with calves, especially at Cape Foulwind and
Jackson Head (Braeger and Schneider, 1998).
The intermittent nature of 12 records in Tasmania, Australia, over
175 years is puzzling. Setting aside concerns about identification,
the dates of records are quite seasonal, occurring from October/November
(8) through January (1) and March/April (3). Such seasonality suggests
a causal link with changes in one or more oceanographic features
in this region, perhaps, for example, in the position of the Subtropical
Convergence, a feature which appears to coincide with the northern
limit of distribution for this species off eastern New Zealand,
and/or ENSO events (Gill et al. 2000).
Direct catch: Large catches (approximately 10,000) of small
cetaceans were reported from the coastal waters of central Peru
in 1985 (Read et al., 1988). In the 1991-1993 period, an estimated
7,000 dusky dolphins were captured per year, an exploitation thought
to be unsustainable. It is believed, but not confirmed, that this
level of exploitation has diminished since dolphin hunting was banned
by law in 1996 and due to depletion of the population (van Waerebeek,
2002). Captured dusky dolphins and other small whales are still
used in Perus as shark bait in long-line and gillnet fisheries and
landed illegally to be sold in public markets, largely due to a
lack of law enforcement (Van Waerebeek and Würsig, 2009).
Incidental catch: Of 722 cetaceans captured mostly in multi-filament
gillnets and landed at Cerro Azul, central Peru, in 87 days during
January-August 1994, 82.7% were dusky dolphins. The total kill estimate
for a seven-month period, stratified by month, was 1,567 cetaceans.
Data collected at 16 other ports showed that high levels of dolphin
and porpoise mortality persisted in coastal Peru at least until
August 1994 when an unimplemented 1990 ban on small cetacean exploitation
was renewed. Circumstantial evidence suggests that, thereafter,
increasing enforcement reduced direct takes and illegal trade in
meat but also hampered monitoring. The absence of abundance data
precludes any assessment of impact on populations (van Waerebeek
et al. 1997).
Dusky dolphins continue to be affected by the trawl fishery off
Patagonia (Crespo et al. 2007) and mortality estimates for 1994
reached a minimum value of 36 dolphins per year, mostly females
and young matures (Dans et al. 1997). Dans et al. (2003) concluded
that this incidental mortality could be, or may become, a threat
for the regional dusky dolphin population.
In New Zealand, some dusky dolphins are entangled in gill nets.
Incidental mortality at one fishing port was estimated to be 100
to 200 animals per year (Jefferson et al. 1993) and within the range
of 50-150 during the mid-1980s (Würsig et al. 1997). However,
this is not repeated in the recent literature (Würsig et al.,
2007), whereas in Australian waters the pelagic drift-net fishery
in Tasman Sea as well as entanglement in drift-nets set outside
Australian Economic Exclusion Zone and in lost or discarded netting
are a cause for concern (Ross, 2006).
Aquaculture: Aquaculture may threaten dusky dolphin foraging
habitat in Admiralty Bay, Marlborough Sound, New Zealand, where
an estimated 220 dolphins gather to feed each winter. Overlap of
dusky dolphin habitat use with proposed marine farms is high, and
dolphins rarely used areas within the existing farms (Markowitz
et al. 2004).
Pollution: The maximum concentration (ppm wet weight) of
DDT reported in the blubber of this species in New Zealand waters
was 175 (Brownell and Cipriano, 1999 and refs. therein).
Tourism: Commercial dolphin watching and swim-with-dolphin
operations started in the late 1980s and are a major industry in
Kaikoura, New Zealand. During summer, boats approach the same dolphin
groups throughout the day. While there are behavioural reactions
by the dolphins, no large-scale or long-term adverse reactions to
human tourism have been documented to date. It is presently unknown
whether more subtle chronic effects could be detrimental to the
population (Würsig et al. 1997). The discovery of midday resting
has led to a voluntary 'down-time' of no tourism boats by a local
dolphin tourism enterprise, Dolphin Encounter (Würsig et al.
2007). Since 1997, dusky dolphin watching activities have also increased
in Patagonia, from 1,393 tourists in 1997 to 1,840 in 2000, with
unknown effects on the animals (Coscarella et al. 2003).
Range states (Hammond et al. 2008):
Argentina; Australia; Chile; Falkland Islands (Malvinas); French
Southern Territories (the) (Amsterdam-St. Paul Is.); Namibia; New
Zealand; Peru; South Africa (Marion-Prince Edward Is., Northern
Cape Province, Western Cape Province).
L. obscurus is included in Appendix II of CMS. IUCN Status: "Data
Deficient" (Hammond et al. 2008) . The species is listed in
Appendix II of CITES.
Bycatch in gillnets occurs at an unknown and ins some regions like
Peru and Patagonia, presumably intolerable level. This needs to
be investigated. For South American stocks, see recommendations
in the Hucke-Gaete (2000) report in Appendix
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© Boris Culik (2010) Odontocetes.
The toothed whales: "Lagenorhynchus obscurus".
UNEP/CMS Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.