Lagenorhynchus acutus (Gray, 1828)

English: Atlantic white-sided dolphin
German: Weißseitendelphin
Spanish: Delfín de costados blancos
French: Dauphin à flancs blancs

Family Delphinidae

Lagenorhynchus acutus © Würtz-Artescienza (see "links")

1. Description

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 (Cipriano, 2002).

2. Distribution

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 species' range.

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 al. 2006).

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).

5. Migration

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 of Ireland.

6. Threats

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 al. 2009).

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 (Lens, 2001).

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. therein).

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.

7. Remarks

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.

8. Sources

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© Boris Culik (2010) Odontocetes. The toothed whales: "Lagenorhynchus acutus". UNEP/CMS Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.