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Research published

Underwater Pacific white-sided dolphins (c) Alexandra Morton
Underwater Pacific white-sided dolphins (© Alexandra Morton)

Pacific white-sided dolphins (Lagenorhynchus obliquidens) are sporadic residents on the B.C. coast. While their teeth are abundant in some First Nation middens signifying several thousand years of presence, they were absent from approximately 1915-1984. Seven showed in the Broughton Archipelago in December of 1984. For the next few years they appeared occasionally, always in winter. Then in 1992 hundreds of dolphins appeared in the fall and stayed till spring in groups of tens to hundreds. At the same time a small smelt called capelin (Mallotus villosus) appeared, an Arctic population moving south. The dolphins fed predominately on capelin and then when the Pacific sardine or pilchard (Sardinops sagax) suddenly reappeared in 1997 after a forty year absence in the Broughton area, the dolphins fed on this species as well.

Many marine species pulse in and out of coastal areas in response to oceanographic events, such as El Niņo, which increase or decrease food resources. By examining seafloor core samples Baumgartner et al. (1992) discovered sardines have crashed and flourished in Monteray Bay on 50 - 70 cycles over the past 1,700 years. This pattern would appear widespread and the reason the dolphins come and go from British Columbia.

Raincoast Research documented the most recent chapter in dolphin history and has identified over 900 individual dolphins. Photo-identification is a powerful tool to examine social structure and site fidelity. Most groups seem flexible, but there are pairs and perhaps trios of dolphins that have been photographed side by side years apart. Most groups have tight coalitions of males (thick-based fins with a bit of a forward hunch). Any photos from other areas are extremely useful in mapping travel patterns and range.

The dolphins are very aggressive towards other species chasing porpoises, killing juvenile harbour seals, even harassing the fish-eating resident orca (though they flee from the mammal-eaters). How do they tell the difference? A silent orca is a dangerous orca, but by the time they see the whale it can be too late! On occasion both Steller and California sea lions have been sighted among the dolphins. Calves are extremely rare in the Broughton.

Pacific white-sided dolphins can be distinguished from all other B.C. cetaceans by their often large groups upto 1,000, their ability to jump several body-lengths above the water and their grey and white streaking, not black and white like Dall's porpoise.

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