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Life history parameters of the Pacific white-sided dolphin (Lagenorhynchus obliquidens) and its diet and occurrence in the coastal waters of British Columbia Heise, Kathy Ann


Throughout most of their range, Pacific white-side dolphins (Lagenorhynchus obliquidens) inhabit pelagic waters, or the continental shelf and slope. In recent years, however, their abundance has increased markedly in the nearshore waters of British Columbia, and the species is likely the most numerous cetacean in the region. In this study I examined the timing of this increase, and ecological and population factors that may have contributed to it. No systematic population censuses of Pacific white-sided dolphins have been conducted in British Columbia. However, because the species is gregarious, active at the surface, and interacts with vessels, many mariners take note of it. I surveyed mariners on the timing and locations of their sightings, and received 165 written responses. They reported relatively few sightings between 1978 and 1984, and frequent sightings after 1985. I estimated life-history parameters for Pacific white-sided dolphins using data taken from animals that were killed accidentally or deliberately during fisheries, or for scientific reasons, or recovered as stranded carcasses. The average age at first reproduction was 8.5 years for females, gestation was 12.0-12.2 months, and the calving interval was 4.67 years (S.E.=1.81). The rate of population increase was estimated to be between 0.94-1.02/yr, reflecting a stationary population. This suggests that the increase in abundance in British Columbia cannot be explained by population growth alone, and must have resulted, at least partly, from a movement of dolphins from other areas. To examine the potential role of prey in the movement of dolphins, I examined their diet in inshore waters, using prey fragment sampling (n=64) and stomach content analyses (n=11). Prey consisted mostly of herring (Clupea harengus), salmon (Oncorhynchus spp.), cod (Family Gadidae), and shrimp (Order Decapoda). Previous studies found that in offshore areas, dolphins feed on mesopelagic fish and squid, thus dolphins moving inshore may be exploiting new food resources. The inshore movement of dolphins appears to coincide with a 'regime shift', characterised by a change in water temperatures. It is suggested that this may have caused changes in the relative abundance of the prey of Pacific white-sided dolphins.

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