Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 4
Distribution
Global range and stock structure
Sei whales are considered to be cosmopolitan in distribution (Figure 1), occurring in all the world’s oceans. However, they appear somewhat restricted to temperate waters, occurring within a more restricted range of latitudes than all other rorquals except Bryde’s whales. Gregr et al. (2000) suggested a northern limit in the northeastern Pacific of 55ºN based on comparing the catch records from British Columbian and Alaskan shore stations (Figure 2). While pelagic catches of sei whales appear to have been made at least occasionally at higher latitudes in the North Pacific (Masaki 1977), their appearance in Alaskan waters is irregular (Kate Wynne, personal communication). In the eastern North Atlantic, sei whales were regularly caught between 60 and 65ºN (Jonsgård and Darling 1977, Mitchell and Chapman 1977). This may be due to the intrusion of warmer Gulf Stream waters to higher latitudes.
Sei whales have historically been managed as “stocks” by the IWC. However, the stock boundaries are often based more on political or commercial convenience than on the biology of the species (Donovan 1991). The IWC managed sei whales in the North Pacific as a single stock until the species was protected from whaling in 1976. In the North Atlantic, three stocks (Nova Scotia, Iceland-Denmark Strait, and Northeast Atlantic) were defined in 1977, apparently based on the distribution of whaling at the time (Donovan 1991). The subsequent admission that identifying stock structure was extremely difficult effectively ended the discussion of sei whale stock identity by the IWC (Donovan 1991). However, a number of researchers considered the distribution and biology of sei whales in more detail and proposed biologically based stock structures for both the North Pacific and the North Atlantic (Fijino 1964, Rice 1974, Masaki 1977).
Figure 1. Global distribution of sei whales. From Perry et al. (1999). Reprinted with permission.
Figure 2. Sei whale distribution in and around Canadian waters. Stippled areas show possible areas of sporadic occurrence.
Masaki (1977) examined marking studies, catch distributions, sightings, and baleen morphology and concluded that the North Pacific contained three stocks, separated by 175ºW and 155ºW longitude. Examination of blood types (Fujino 1964) indicated differences between animals caught in the inner Gulf of Alaska and off Vancouver Island. Finally Rice (1974) described the different forms and species of parasites observed at opposite sides of the Pacific, implying the existence of at least an eastern and a western stock.
The US National Marine Fisheries Service (NMFS) has divided the North Pacific into an eastern and a western stock, separated by 180ºW (Carretta et al. 2001). This arbitrary stock boundary is likely due to the inconclusive nature of the above analyses. It is also in keeping with a management-based analytical approach commonly employed by the IWC (Donovan 1991).
Historically, sei whales ranged throughout the North Atlantic, from the shelf waters of eastern North America, through the Labrador Sea, to the Denmark Strait and the Norwegian Sea (Figure 1). The IWC continues to recognize 3 stocks in the North Atlantic (Perry et al. 1999). Mitchell and Chapman (1977) proposed that the Northwest Atlantic contained two stocks, one off Nova Scotia and one in the Labrador Sea. These stocks were considered different from those in the Northeast Atlantic. The IWC scientific committee (IWC 1976) identified eight concentrations of sei whales throughout the North Atlantic, which were subsequently interpreted by Horwood (1987) as presumptive stock units. The Nova Scotia stock, which is the basis for NMFS stock assessments, is considered to range from the shelf waters of the Northeastern United States to the south of Newfoundland (Waring et al. 2001).
Canadian range
There have been no recent sightings of sei whales off Canada’s Pacific coast. However, there is no reason to believe that sei whales from the eastern North Pacific stock do not continue to frequent Canadian and U.S. waters given the lack of survey effort, and the difficulty in distinguishing this species from fin whales. On the Atlantic coast, a major portion of the Nova Scotia stock is centered on the Scotian Shelf (Mitchell and Chapman 1977). However, this stock makes use of both Canadian and U.S. waters (Figure 2).
A recent study of the Scotian Shelf (Breeze et al. 2002) reviewed both the historic whaling data (Sutcliffe and Brodie 1977) and contemporary sighting data (Reeves 1999). While these data are biased towards particular areas, seasons and species (Breeze et al. 2002), they were sufficient to produce an occurrence map for the shelf (Figure 3).
Figure 3. Modeled distribution of sei whales on the Scotian Shelf based on whaling records and sighting data. Reprinted from Breeze et al. (2002), with permission of the author.
Sei whale distributions on the Atlantic coast may have shifted around 1992, moving from Roseway Basin to the mouth of the Bay of Fundy. This corresponded with a similar shift by right whales. Sei whales continued to appear off the Bay of Fundy for about a decade, usually with right whales but sometimes with fin whales. The distribution may now be shifting back to the Roseway Basin area although this is still to be determined (Laurie D. Murison, personal communication).
A small number of sightings have been reported off the west coast of Cape Breton. Six observations were recorded by Parks Canada in 1982 (Corbett 1984), and one by charter operators in each of 2001 and 2002 (Gilbert Dubé, personal communication). While these observations may be misidentified fin whales, the occasional occurrence of the species on the edge of the Gulf of St. Lawrence cannot be ruled out.
The putative Labrador Sea stock proposed by Mitchell and Chapman (1977), does not fall into any IWC stock boundary, and this area has remained relatively unexplored since the cessation of commercial whaling for this species in the late 1970s. Only two confirmed sightings of sei whales have been reported since 1978, both off the southeastern shore of Newfoundland. Aerial (1980) and vessel-based (1992-1994) surveys of the Newfoundland and Labrador coasts reported no sightings of sei whales (Jon Lien, personal communication). However, observer records from the Northwestern Atlantic pelagic fishery, which carried observers from 1987-1993, appear to contain a significant number of new sightings – at least 20 sei whale sightings were reported for 1988 alone. Many of the locations appear to be off the northeast coast of Newfoundland and along the Labrador shelf. Recovery of these data is in progress (Jack Lawson, personal communication). The degree to which this region continues to be used by sei whales, and whether it is occupied by a unique stock, or is used by animals from the Nova Scotia or Iceland-Denmark Strait stock, is unknown.
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