Lagenorhynchus acutus (Gray, 1828)

Russell A. Mittermeier & Don E. Wilson, 2014, Delphinidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 410-526 : 489-490

2.

Adantic White-sided Dolphin

Lagenorhynchus acutus

French: Dauphin a flancs blancs / German: Atlantik-Weif 3seitendelfin / Spanish: Delfin de flancos blancos del Atlantico

Other common names: Atlantic White-sided Porpoise, Jumper, Skunk Porpoise

Taxonomy. Delphinus acutus Gray, 1828 ,

type locality unknown. Later designated by J.E. Gray in 1846 as “North Sea, Faroe Isls (Denmark).”

Taxonomy of Lagenorhynchus is currently in dispute. Recent molecular analyses have revealed that the genus is not monophyletic. L. acutus may actually belong to an isolated lineage within Delphinidae .

The genus Leucopleurus has been suggested as an alternative placement for L. acutus . Monotypic.

Distribution. Cold temperate to subarctic waters of the N Atlantic Ocean (including mouth of the Saint Lawrence River, Canada, but excluding the Baltic Sea), S to ¢.38° N in W Atlantic Ocean, and from S Svalbard to the Brittany coast, France, in the E Atlantic Ocean.

Descriptive notes. Total length ¢.280 cm (males) and ¢.250 cm (females); weight c.235 kg (males) and c.182 kg (females). Neonates are 110-120 cm long and weigh c.25 kg. Like the White-beaked Dolphin ( L. albirostris ), the Atlantic White-sided Dolphin has robust body shape and short beak. Dorsal fin, positioned just posterior to halfway on back,is tall (more than 12% of total body length), falcate, and pointed. Flippers are broad, pointed at tips, and covered in tubercles along leading edges. Tailstock is also characteristically deep. The Atlantic White-sided Dolphin has a complicated color pattern. Dorsal surface, including upper jaw, top of head, dorsal fin, and flukes are a dark gray to black, but lower jaw, throat, lower anterior flanks, and belly up to urogenital area are white. Lateral body in between these dorsal and ventral areas and ventral tail posterior to urogenital area are typically medium-todark gray. Thin, dark bands stretch between dark edges of upper jaws to small, dark patches around eyes and from eyes down to anterior bases of flippers. There is a white streak at lateral dark-gray/medium-gray borderstretching from leading edge of dorsal fin to midway along tailstock, and yellow-orange streak at lower margin of lateral dark-gray section stretching from midway along tailstock to base of flukes. Male Atlantic White-sided Dolphins tend to have deeper tailstocks than females, and immature individuals have more muted coloration. There are 30-40 small, conical teeth in each jaw.

Habitat. Cold temperate and subarctic waters, 100-300 m deep, over outer continental shelves. Nevertheless, Atlantic White-sided Dolphins may enter fjords and inlets less than 50 m deep. They prefer sea-surface temperatures of 5-16°C.

Food and Feeding. The Atlantic White-sided Dolphin primarily feeds on small fish and squid. Preferred prey species include Atlantic herring ( Clupea harengus), small Atlantic mackerel ( Scomber scombrus), silvery pout ( Gadiculus argenteus), blue whiting ( Micromesistius poutassou), American sand lance (Ammodytes americanus), rainbow smelt (Osmerus mordax), silver hake ( Merluccius bilinearis), and northern shortfin squid (/llex illecebrosus). Although little is known about foraging strategies, Atlantic White-sided Dolphins have been observed hunting cooperatively by herding fish into tight balls against the water’s surface.

Breeding. Breeding of Atlantic White-sided Dolphins peaks in June-July in the West Atlantic Ocean and extends into autumn in the East Atlantic Ocean. Gestation is c.11 months, and young are weaned by 18 months. Females are sexually mature at 6-12 years, and males reach sexual maturity at 7-11 years. Longevity is ¢.27 years for females and c¢.22 years for males. Breeding females will typically have an offspring every two years. Studies of genetic variability of stranded groups suggest that mating is random.

Activity patterns. The Atlantic White-sided Dolphin is known to frequently tail-slap, breach acrobatically (especially in larger groups), bow ride, and surf in stern wakes of vessels. Frequent breaching is the source of the common name “Jumper.” They have even been observed riding “bow waves” of mysticetes. Dives usually last less than a minute, and maximum dive time is about four minutes.

Movements, Home range and Social organization. Group sizes of Atlantic White-sided Dolphins tend to vary geographically. Inshore groups near Newfoundland, Canada, typically contain 50-60 individuals. Off New England, USA, groups vary from a few to several hundred individuals, but average c.40 individuals. Off Iceland and around the British Isles, group size is usually less than ten individuals. Most groups consist of less than 100 individuals, but groups of several thousand have been observed. Within these large groups, there may be a few stable subgroups. Reproductive groups contain breeding females, adult males, and young offspring and tend to remain segregated from groups that contain older juveniles. Studies of genetic variability of stranded groups suggest that both sexes disperse from their natal groups. Mixed-species aggregations with the Fin Whale (Balaenoptera physalus), the Humpback Whale (Megaptera novaeangliae), and the Long-finned Pilot Whale ( Globicephala melas ) have been observed. In some areas, such as off Newfoundland where Atlantic White-sided Dolphins are more abundant in July-October, they appear to have seasonal inshore—offshore movements. These may be correlated with seasonal variation in prey abundance related to upwelling and sea-surface temperature.

Status and Conservation. CITES Appendix II. Classified as Least Concern on The IUCN Red List. Abundance of the Atlantic White-sided Dolphin is estimated to be more than 100,000 individuals, and there are no reports of population declines or major threats. There are ¢.51,640 individuals off eastern North America, c.11,740 individuals in the Gulf of Saint Lawrence (Canada), and ¢.96,000 individuals off western Scotland. The Atlantic White-sided Dolphin has historically been taken in drive fisheries in Greenland, Norway, the Faroe Islands, and Newfoundland. Currently, they are hunted opportunistically in Greenland and eastern Canada. Drives in the Faroe Islands also continue: 300-600 Atlantic White-sided Dolphins were killed per year in 2004-2006. These drives do not appear to have as long a history as drives for pilot whales. Incidental catches of the Atlantic White-sided Dolphin have been reported in fisheries from Canada, the USA, the UK, and Ireland. They may be especially vulnerable to capture in pelagic trawl fisheries that target mackerel ( Scomber spp. ). Incidental catch rates of the western North Atlantic stock in gillnet and trawl fisheries were ¢.352 ind/year in 2002-2006. Atlantic Whitesided Dolphins will feed behind trawl nets, which may make them more vulnerable to incidental catch. Contamination from heavy metals and organochlorine pollutants have been reported in a few specimens, but effects of pollutants are not well understood.

Bibliography. Cipriano (2009), Couperus (1997), Craddock et al. (2009), Gaskin (1992b), Gray (1846), Hammond et al. (2008b), Jefferson et al. (2008), LeDuc et al. (1999), May-Collado & Agnarsson (2006), Mirimin et al. (2011), Reeves, Smeenk, Brownell & Kinze (1999), Weinrich et al. (2001).

Figures

On following pages: 3. Peale’s Dolphin (Lagenorhynchus australis); 4. Hourglass Dolphin (Lagenorhynchus crucigen; 5. Pacific White-sided Dolphin (Lagenorhynchus obliquidens); 6. Dusky Dolphin (Lagenorhynchus obscurus); 7. Irrawaddy Dolphin (Orcaella brevirostris); 8. Australian Snubfin Dolphin (Orcaella heinsohni).

Distribution. Cold temperate to subarctic waters of the N Atlantic Ocean (including mouth of the Saint Lawrence River, Canada, but excluding the Baltic Sea), S to ¢.38° N in W Atlantic Ocean, and from S Svalbard to the Brittany coast, France, in the E Atlantic Ocean.