Atlantic white-sided dolphins are robust and deep-bodied, with a short beak and tail stock that is “keeled” above and below. The color pattern is complex. The back and upper sides, upper jaw, dorsal fin, flippers, and flukes are black or dark gray, and a dark line runs backwards from the beak and surrounds the eye. The lower jaw and belly, as far as the urogenital area, are white. In between, the sides from just ahead of the eye to the base of the flukes are light gray. Along the upper margin of the gray flank is a white patch from below the dorsal fin to midway along the tail stock. There is another narrow band, this one ochre in color, at the lower margin of the dark upper flank, from mid-tail stock to just in front of the flukes.
Adult Atlantic white-sided dolphins reach 2.8 m (males) or 2.5 m (females) in length and about 235 kg (males) or 182 kg (females) in weight. Newborns are 1.1-1.2 m. Each tooth row contains 30-40 pointed teeth.
Can be Confused With
Confusion is most likely with the white-beaked dolphin, which shares a nearly identical range. The two can be distinguished most readily by color pattern differences. The Atlantic white-sided dolphin’s color pattern is much more bold and striking.
Atlantic white-sided dolphins are widely distributed throughout the cold temperate to subpolar waters of the North Atlantic. Their preferred habitat appears to be the deep waters of the outer continental shelf and slope. It is possible that three stock units exist Gulf of Maine, Gulf of St. Lawrence, and Labrador Sea.
In the western North Atlantic, recent evidence indicates that the southerly limit of their distribution is at 38°N, on the coast of the Delmarva Peninsula. They are common in the Gulf of Maine and Bay of Fundy, with probable southerly migration in winter and spring; in the waters of Cape Cod; on the Scotian Shelf, Newfoundland, and the “Gully” south of Sable Island; on the Labrador Coast; in the Gulf of St. Lawrence including waters surrounding the Mingan Islands; and on the Flemish Cap (probably the easternmost sighting). The northernmost limit is not well known in this part of the range, but may be the southern waters of Greenland.
In the eastern North Atlantic, the range includes the waters south of Iceland in the Irminger Sea from 50°N to 60°N in summer; the Faroe and Shetland Islands; the Barents Sea; the waters off southwest Norway; the North Sea including the coasts of Denmark, Sweden, and the Netherlands; the waters of the United Kingdom, peaking in summer and possibly exploiting the mackerel migration; and in winter and spring, the Bay of Biscay, the Azores, and possibly the Straits of Gibraltar.
Ecology and Behavior
Atlantic white-sided dolphin habitat includes shelf-slope waters
Herds of several hundred are seen, with some age and sex segregation of herds. Older immature individuals are not generally found in reproductive herds of mature females and young. Atlantic white-sided dolphins are lively and acrobatic. Much of what we know of this species’ biology comes from examination of several mass strandings.
Sexual maturity is reached by females at around 2 m and 6-12 years, and 2.3-2.5 m and 7-11 years for males. Calves are born in summer with a peak in June and July after a gestational period of 10-12 months, and lactation may last 18 months. Stranded females show evidence that lactation and pregnancy overlap.
Feeding and Prey
Atlantic white-sided dolphins feed by seizing prey, which composes a broad diet dominated by: fish > squid. Atlantic white-sided dolphins feed on small schooling fish, shrimp, and squid. They often feed in association with large whales, and groups apparently cooperate when feeding on schooling fish.
Prey species include:
Fish: Clupea harengus, Scomber scombrus, Trisopterus sp., Trachurus trachurus, Argentina sphyraena, Gadiculus argenteus, Micromesistius poutassou, Osmerus mordax, Merluccius bilinearis, Ammodytes americanus, Ammodytes dubius, Merlangius merlangius, Gadus morhua, Trisopterus minutus
The Atlantic white-sided dolphin is not listed as threatened or endangered by the IUCN or the United States government. Atlantic white-sided dolphins have been hunted in drive fisheries in Norway, Newfoundland, Greenland, and the Faroe Islands. Incidental catches are known from many different areas of the species’ range, and captures in midwater trawls and sink gill nets appear to be the most significant. Recent catches in trawls off Ireland have been quite large. The population of the Gulf of Maine stock is estimated to be 51,640 (CV = 0.38), although there is insufficient data to analyze population trends. In the Gulf of St. Lawrence, population is estimated at 11,740 (CV = 0.47).
Cipriano, F. 1997. Antitropical distributions and speciation in dolphins of the genus Lagenorhynchus a preliminary analysis. pp. 305-316 in A.E. Dizon, S.J. Chivers and W.F. Perrin, eds. Molecular genetics of marine mammals. The Society of Marine Mammalogy.
Cipriano, F. 2002. Atlantic white-sided dolphin Lagenorhynchus acutus. pp. 49-51 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Gaskin, D.E. 1992. Status of the Atlantic white-sided dolphin, Lagenorhynchus acutus, in Canada. Canadian Field-Naturalist 106:64-72.
Palka, D., A. Read and C. Potter. 1997. Summary of knowledge of white-sided dolphins (Lagenorhynchus acutus) from US and Canadian Atlantic waters. Reports of the International Whaling Commission 47:729-734.
Reeves, R.R., C. Smeenk, R.L. Brownell, Jr. and C.C. Kinze. 1999. Atlantic white-sided dolphin Lagenorhynchus acutus (Gray, 1828). pp. 31-56 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 6: The second book of dolphins and the porpoises. Academic Press.
Waring, G.T., Quintal, J.M. and C.P. Fairfield. 2002. U.S. Atlantic and Gulf of Mexico marine mammal stock assessments – 2002. U.S. Department of Commerce, NOAA Technical Memorandum, NMFS-NE-169.