Atlantic White-sided Dolphins (Lagenorhynchus acutus)

Atlantic White-sided Dolphins on the Ledge: White-sided dolphins appear to be found on the ledge year-round, with a peak in sightings during late summer and early fall. These are also high-abundance times for the other species of whales and dolphins which eat herring. Groups of up to several thousand have been seen travelling together. This also appears to be an important nursery area as well; in 1994 we even spotted a dolphin calf which we felt was no more than a week old! Calves become commonly sighted in groups during August and September, leading us to suspect that June and July are peak calving times. Dolphin groups are rarely residential on the Ledge, but rather often appear to be transiting through the area, often moving to the south when sighted. Size: 5-8 feet as adults Weight: approximately 300-600 pounds Distinctive Characteristics: A white and yellow stripe on both flanks which is highly visible at sea; a single blowhole (characteristic of all the toothed whales); a tall erect dorsal fin mid-way on the back Diet: A combination of squid and fish, often herring and its relatives. Atlantic white-sided dolphins, like all dolphins, feed on single prey items, so they are less likely to feed on the large numbers of small fish that the whales usually feed on. Distribution: Atlantic white-sided dolphins are found only in the North Atlantic, generally from waters just south of New England north to Norway. A close relative, the Pacific white-sided dolphin, is found in similar latitudes in the North Pacific ocean. Atlantic white-sided dolphins are members of the genus Lagenorhynchus, which are typically adapted to colder water; one species (the hourglass dolphin) is only found in the Antarctic! Recent genetic work indicates that this genus is only distantly related to the other genuses within the dolphin family, and may be split into its own taxonomic family in the near future. Atlantic white-sided dolphins appear to be very nomadic and rarely stay in one spot for long, but do not show any set seasonal migrations. Life History: Atlantic white-sided dolphins are born in the summer, usually anywhere from late June to late July. It is believed that they stay with their mother for up to a year. Interval between calves is not known, but we know of one case where a distinctively marked dolphin was seen with what appeared to be young-of-the year calves in two straight years. Dispersal may be sex-related (see below). Unlike the baleen whales, males are slightly larger than females. Dolphins are thought to mature at 6-10 years of age, and can live for at least 25 years and possibly more. Social Organization: Atlantic white-sided dolphins are sighted in groups as small as 10-15 animals, and as large as several thousand together. Groups of 100 or more are generally called "superpods", and are believed to be temporary associations which may form for foraging, cooperative feeding, or just travel. Smaller groups may be more stable; in many dolphin species, these groups are comprised of related females and their young, while males move from group to group. Although little is known about the social organization of this species, it appears groups are either primarily female-young groups or "bachelor" male groups without any females present. We have typically seen group size increase through our field season, and most superpod sightings occur in August through October. Population Status: Although there is no good estimate of the number of Atlantic white-sided dolphins, there is no question they are an abundant species. While they are not commercially hunted, several animals may be killed each year by native groups in northern climates. Today the primary threats to Atlantic white-sided dolphins are from pollutants (all dolphin species seem to build up unusually high pesticide loads), and entanglement in fixed fishing gear, which is often fatal to the dolphin. Other Sites: Link #1    Link #2    Link #3