Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 11

Summary of Status Report

Sei whales were the last commercially exploited species in Canadian waters, and the majority of exploitation by factory whaling occurred in the second half of the 20th century (1950s to 1980s). Considerably more information is available from the North Pacific and Antarctic whaling grounds than from the North Atlantic.

Very little is known about current population sizes or trends. Contemporary efforts to establish population sizes for this species using line-transect studies are likely complicated by the species’ extensive range, its unpredictable use of high latitude feeding areas, and the difficulty in distinguishing it from the fin whale. The expense associated with extensive offshore surveys is also a significant limiting factor.

The diet of sei whales is one of the more diverse for baleen whales. Along with the fin whale, this species is able to forage on zooplankton as well as schooling fish, and possibly on other aggregations of small prey (i.e. squid). The diet differences observed between sei whales in the three major basins (North Pacific, Antarctic and North Atlantic) could well be the result of dietary adaptations to the trophic distribution of prey species within those basins.

While faced with similar threats as the other baleen species, the sei whale may be more adaptable to changing ecological conditions due to its intermediate morphology. Habitat degradation resulting from acoustic pollution, chemical contamination and ecosystem-level competition for prey is the most likely threat to this species.

We will likely never completely understand the ecosystem effects of removing so much biomass from the oceans through commercial whaling. However, we should not be surprised if the species that do recover exhibit significantly different distributions and habitat use patterns than they did prior to exploitation.

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