Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 5
Habitat
Sei whales are second only to blue whales in their use of offshore, pelagic habitats. They are usually found in deep waters, and appear to be associated with the continental shelf edge in the northwest Atlantic (Hain et al. 1985). Off British Columbia, less than 0.5% of the catch by coastal stations for which positions were recorded was on the continental shelf (Gregr 2002).
The wintering grounds used by sei whales remain largely unknown (Perry et al. 1999). Rice (1974) hypothesized that the majority of animals from the eastern North Pacific stock spend winters far offshore. The Nova Scotia stock likely winters somewhere off the continental United States.
Habitat requirements
The main characteristic of sei whale feeding habitat is likely a high concentration of prey organisms, particularly copepods. However, Nemoto and Kawamura (1977) suggested that the sei whale’s preference for open, pelagic waters may be more important than a preference for any particular prey, as the species was rarely observed in inland seas or gulfs, despite high concentrations of copepods found there.
Studies of the distribution of baleen whales in relation to oceanographic conditions indicate a strong association with oceanographic fronts (Uda 1954, Nasu 1966). Sei whales are reportedly found along major mixing zones and in eddies that had broken away from the fronts. Nasu (1966) also suggested that sei whales followed the fronts throughout the season. These fronts can be relatively permanent as in those surrounding the Alaskan gyre and major upwelling areas. Alternatively, they can be more dynamic, associated with eddies forming near topographic features, or shearing off the major currents (Horwood 1987).
Trends
Historic shifts in habitat may have an effect on how the distribution of the species is viewed today. Nemoto and Kawamura (1977) suggested that sei whales expanded their range to higher latitudes after the depletion of fin and blue whales. If so, the data on which the “pre-exploitation” distribution and population estimates are based would be biased by the previous exploitation of other species.
Describing the change in habitat over time for a migratory, pelagic species is a difficult task. Sei whales are capable of searching large areas for suitable habitat. Thus, while localized, periodic changes in habitat quality may alter the spatial distribution of the species, it is unlikely that this variability reduces the overall habitat available. The effects of long-term oceanographic trends (such ocean warming) are less clear, and depend on the trophic interactions between sei whales, their prey, and their competitors.
Habitat protection/ownership
The habitat for this species is currently not provided any degree of protection. However, a portion of the range on both the Pacific and Atlantic coasts falls within the Exclusive Economic Zones of both the United States and Canada (see Existing Protection or Other Status).
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