Lagenorhynchus acutus (Gray,
English: Atlantic white-sided dolphin
Spanish: Delfín de costados blancos
French: Dauphin à flancs blancs
Lagenorhynchus acutus © Würtz-Artescienza
Atlantic white-sided dolphins are robust and , with a maximum girth
of up to 60% of total length. The tail stock is laterally compressed
into vertical keels and the beak is short (Jefferson et al. 2008).
These dolphins are impressively patterned and more colourful than
most dolphins. Below the black or very dark grey back and dorsal
fin a narrow, bright white patch on the side extends back from below
the dorsal fin, overlaying a yellow blaze above a thin dark stripe
running towards the flukes. The belly and lower jaw are white, and
the sides of the body are light grey. A black eye ring extends in
a thin line to the upper jaw and a very thin stripe extends backward
from the eye ring to the external ear. A faint grey stripe may connect
the leading edge of the flipper with the rear margin of the lower
jaw. Male Atlantic white sided dolphins reach 270 cm and 230 kg,
whereas adult females are about 20 cm shorter and 50 kg lighter
L. acutus is a deepwater species which ranges across the
North Atlantic, from Cape Cod in the western North Atlantic to southern
Greenland, across the Barents Sea to Svalbard and from there south
to the North and Irish Seas as far south as Brittany (France) (Reeves
et al. 1999; Cipriano, 2009). The species rarely enters the Baltic
sea (Kinze et al. 1997 and pers. obs.). It has been seen as far
south as Strait of Gibraltar (Hammond et al., 2008).
Distribution of Lagenorhynchus
acutus (Hammond et al. 2008; © IUCN): cool, temperate
and subarctic waters of the northern North Atlantic (for
large map click here).
Mikkelsen and Lund (1994) found no evidence of separate
populations based on a study of metrical and non-metrical skull
characters of 123 Atlantic white-sided dolphins from much of the
3. Population size
Evans (1987, in Reeves et al. 1999) suggested a total population
throughout the North Atlantic of tens of thousands to low hundreds
of thousands, which is supported by Kaschner (2004). Compton et
al. (2007) found that L. acutus was the second most commonly
sighted species during a crossing of the North Atlantic covering
waters between the UK, Iceland, Greenland and Canada.
The best available current abundance estimate for white-sided dolphins
in the US western North Atlantic is 63,368 (CV=0.27), an average
of surveys conducted in August of 2002 and 2006, with no apparent
trends compared to earlier estimates. An abundance estimate of 109,141
(CV=0.30) white-sided dolphins was obtained from an aerial survey
conducted in July and August 2002 which covered 7,465 km of trackline
over waters from the 1000-m depth contour on the southern edge of
Georges Bank to Maine (Waring et al. 2009). There are at least 12,000
in the Gulf of St. Lawrence (Kingsley and Reeves, 1998).
In the Eastern North Atlantic, Weir et al. (2001) carried out surveys
to the north and west of Scotland (UK) and found that Atlantic white-sided
dolphins were the most abundant species in the region with a total
of 6,317 animals recorded. However, based on data obtained during
a shipboard survey conducted in 1998 within an area to the west
of Scotland commonly known as the Atlantic Frontier, MacLeod (2004)
estimated an uncorrected Atlantic white-sided dolphin abundance
of 27,194 (CV = 0.29). After correction for g(0), the abundance
was re-estimated as 21,371 (CV = 0.54) to the west of the Outer
Hebrides and 74,626 (CV = 0.72) in the Faroe Shetland Channel.
The first SCANS (Small Cetacean Abundance in the North Sea), survey,
conducted in summer 1994 yielded a Lagenorhynchus spp. abudance
of 11 ,760 (95% CI 5900-18 500) (Hammond et al. 2002), while SCANS
II yielded an abundance of only 1,860 (95% CI 611 - 5,661) for pooled
common, striped, white-sided and white-beaked dolphins (Burt et
4. Biology and Behaviour
Habitat: L acutus seems to prefer areas with high
seabed relief along the edge of the continental shelf (Carwardine,
1995). It is more pelagic than the white-beaked dolphin, occurring
mainly along edges or seaward of continental shelves, over depths
of 100-300 m. However, it sometimes does come onto the continental
shelf and may enter fjords and inlets with depths of less than 50
m (Evans, 2009).
Along the Mid-Atlantic Ridge from Iceland to the Azores, white-sided
dolphins tend to aggregate in areas of steep slopes, but actual
bottom depth appears to be less important. Based on spatial correlations
between dolphin occurrence and candidate prey organisms recorded
acoustically and by midwater trawling, mesopelagic fishes and squids
were assumed to be important prey items, with Benthosema glaciale
probably being their most important prey (Doksaeter et al. 2008).
Waring et al.(2008) sighted white-sided dolphins mainly in the cold
(5-16°C) and less saline (34.8-36.7 PSU) water masses along
the Reykjanes Ridge.
From the Sea Watch database, 75% of sightings in NW European seas
were recorded at SSTs of 7-13°C (total range including outliers
6-17.5°C) (Anderwald, 2002). In eastern United States, the species
occupies waters of 1-13 °C in spring and autumn, but mostly
occurs in waters of c. 5-11° C (Selzer and Payne, 1988).
Behaviour: L. acutus an acrobatic and fast swimmer
and frequently breaches (though not as often as white-beaked or
common dolphins) and lobtails. It surfaces to breathe every 10 to
15 seconds, either leaping clear of the water or barely breaking
the surface and creating a wave over its head. L. acutus
is wary of ships in some areas (Palka and Hammond, 2001) but will
swim alongside slower vessels and may bow-ride in front of faster
ones. Sometimes it can be observed riding the bow-waves of large
whales. Individual and mass strandings are relatively common (Carwardine,
1995; Jefferson et al. 1993). The species is presumably not a deep
diver, as maximum recorded dive times were 4 min and most dive times
were shorter than 1 min (Cipriano, 2009).
Photo of L. acutus © Hal Whitehead
Schooling: Herds of up to several hundred are seen, and
there is some age and sex segregation among these. Older immature
individuals are not generally found in reproductive herds of mature
females and young (Jefferson et al. 1993; Reeves et al. 1999). Gaskin
(1992) hypothesized that Atlantic white-sided dolphins split into
small groups for feeding and that such small groups merge into large
aggregations "while migrating". Groups often associate
and probably feed with fin whales (Balaenoptera physalus),
humpback whales (Megaptera novaeangliae) and long-finned
pilot whales (Globicephala melas). Mixed herds of Atlantic
white-sided dolphins and white-beaked dolphins have been observed
in the North Sea (Reeves et al. 1999, and refs. therein).
Reproduction: Females reach sexual maturity between 6 and12
years of age, males 7-11 years. Maximum recorded ages were 27 and
22 years, respectively (Cipriano, 2009). Parturition in the western
North Atlantic usually takes place between May and August, with
a peak in June and July, following an estimated 11-month gestation
period. The timing of parturition is apparently similar in the eastern
North Atlantic, where sightings have been interpreted to suggest
"breeding areas" offshore in the North Sea and in the
Atlantic to the north and west (Reeves et al. 1999 and refs. therein).
Food: Atlantic white-sided dolphins feed on small schooling
fish and squid. These include herring (Clupea harengus),
small mackerel (Scomber scombrus), silvery pout (Gadiculus
argenteus), blue whiting (Micromesistius poutassou),
American sand lance (Ammodytes americanus), smelt (Osmerus
mordax), silver hake (Merluccius bilinearis) and short-finned
squid (Illex iilecebrosus) (Jefferson et al. 1993; for details
see Reeves et al. 1999). In the North Sea, oceanic cephalopods seem
to be their main diet (Das et al. 2001). Different prey species
may predominate at different times of year, representing seasonal
movements of prey, or in different areas, indicating prey and habitat
variability in the environment (Cipriano, 2002). For instance off
the coast of New England, pelagic Atlantic herring
(Clupea harengus) was the most important prey in summer but was
rare in winter. (Craddock et al. 2009). Atlantic white-sided dolphins
apparently co-operate in their efforts to contain and attack schools
of fish, a behaviour which is similar to that described for dusky
dolphins off Argentina (Reeves et al. 1999 and refs. therein).
There may be inshore-offshore movements with the seasons in some
areas (Carwardine, 1995). Selzer and Payne (1988) suggested that
L. acutus moves south along the continental shelf edge in
winter and spring, in association with the relatively cold, less
saline Gulf of Maine water flowing southwards through Northeast
Channel during these seasons. Seasonal variation in sea-surface
temperature and salinity and local nutrient upwelling in areas of
high sea floor relief may affect preferred prey abundances, which
in turn may affect dolphin distribution. The occurrence of Atlantic
white-sided dolphins off Newfoundland seems also to be seasonal,
mainly from July to October (Reeves et al. 1999). Data from one
satellite-monitored dolphin indicated an ability to travel long
distances at a speed of at least 14 km/hr (Mate et al. 1994).
Weinrich et al (2001) reported that off New England they sighted
1,231 groups of Atlantic white-sided dolphins between April and
October from 1984 through 1997, primarily on Stellwagen Bank and
Jeffreys Ledge (two shallow glacial deposits along the coasts of
Massachusetts, New Hampshire, and Maine). Mean group size was 52
and was significantly larger from August through October (71.9)
than April through June (35.0).
Couperus (1997) investigated the occurrence of incidental cetacean
catches in the Dutch pelagic trawl fishery. These are largely restricted
to late-winter early-spring in an area along the continental slope
southwest of Ireland. Available evidence indicates that annual variations
are large. It seems that the Atlantic white-sided dolphin is normally
a more oceanic species but will actively search for mackerel (Scomber
scombrus) closer to shore in early spring. Fresh mackerel remains
were found in the stomachs of nearly all white-sided dolphins taken
as by-catch, whereas deep-water fish otoliths suggested that the
dolphins had a completely different diet before moving to the southwest
Direct catch: Some hunting for this species occurred in
the past, especially in Norway. Some are still taken in Greenland,
the Faeroe Islands, and eastern Canada (Jefferson et al. 1993; Reeves
et al. 1999 and refs. therein). There is still a substantial hunt
in the Faroe Islands today, with 310 animals landed in 2005 and
617 in 2006. Furthermore, the drive hunt is far from perfect, with
animals "struck and lost", e.g. in Fuglafjord (29 animals
in 2006) (NAMMCO, 2006).
Incidental catch: Incidental mortality in fishing gear has
been documented off Canada, the United States, the United Kingdom
and Ireland. Total annual estimated average fishery-related mortality
or serious injury to the western North Atlantic stock during 2002-2006
was 352 (CV=0.10). These occurred in northeast sink gillnet (34
mortalities p.a.), northeast bottom trawl (193), northeast midwater
trawl (19), mid-Atlantic midwater trawl (77), and mid-Atlantic bottom
trawl (29). Numbers for Canadian fisheries are unknown (Waring et
Starting in 1990, a deep water trawl fishery for Greenland halibut
(Reinhardtius hippoglossoides) in the NAFO Regulatory Area
was developed by Spain. 42 cetaceans were caught, which included
Atlantic white-sided dolphins. It seems that the Greenland halibut
fishery has a relatively low level of incidental marine mammal mortality
Morizur et al. (1999) investigated marine mammal by-catch in 11
pelagic trawl fisheries operated by four different countries in
the Northeast Atlantic. One of the main marine mammal species identified
in by-catches was L. acutus. Mean dolphin catch rate for
all fisheries combined was 0.048 per tow (one dolphin per 20.7 tows),
or 0.0185 per hour of towing (one dolphin per 98h of towing). All
dolphin by-catches occurred during the night. White-sided dolphins
were observed feeding around the net during towing, and this behavior
may make them more vulnerable to capture. Substantial numbers have
also been by-caught in pelagic trawl fisheries for horse mackerel
and mackerel southwest of Ireland (Reeves et al. 1999 and refs.
Pollution: A juvenile dolphin from the north-west coast
of Ireland was found to have a relatively high concentration of
mercury in its liver (44 ng per g wet weight). An adult male from
Nova Scotia had moderately high levels of organochlorines in its
blubber (Reeves et al. 1999 and refs. therein).
Hexabromocyclododecane (HBCD), a brominated flame retardant used
primarily in expanded polystyrene foams and other styrene resins,
was found in blubber and liver samples of Atlantic white-sided dolphins
stranded on the eastern coast of United States between 1993 and
2004. However, concentrations were lower than in cetaceans from
Western Europe (Kucklick et al. 2008), which is not really reassuring.
Range states (Hammond et al. 2008) :
Belgium; Canada; Denmark; Faroe Islands; France; Greenland; Iceland;
Ireland; Netherlands; Norway; Russian Federation; Sweden; United
Kingdom; United States of America.
The North and Baltic Sea populations are listed in Appendix II
of CMS, but inclusion of the NW Atlantic stock in CMS Appendix II
is recommended on the basis of observed migrational behaviour: Atlantic
white-sided dolphins seem to be migratory in North America, where
range states are the USA, Canada and France (St. Pierre et Miquelon).
IUCN Status: "Least Concern" (Hammond et al. 2008).
The species is listed in appendix II of CITES.
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© Maps by IUCN.