Sowerby's beaked whale (Mesoplodon bidens) COSEWIC assessment and status report: chapter 4

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Distribution

Global range

The distribution of Sowerby’s beaked whales is limited to the North Atlantic Ocean (Figure 2), where it is considered the most northerly of Mesoplodon species (MacLeod 2000). Details of the distribution of Sowerby’s beaked whale are not well understood. Definitive sightings in the field are rare due to the difficulty of distinguishing the species from other beaked whales (especially sympatric Mesoplodon species), its apparent preference for deeper, offshore waters, and its evasive behaviour (Mead 2002). What is known of the distribution of this species is based on relatively few strandings and opportunistic sightings (MacLeod et al. 2006). The use of strandings data to determine the range of offshore species has limitations; for example, a carcass may be transported over a great distance by wind and currents before it reaches a shoreline (Mead 1989). Only species-specific reports will be considered in this section, although there are reports of “unidentified beaked whale” and “Mesoplodon sp.” from several areas, including waters off Iceland (Sigurjónsson et al. 1991) and the northeastern United States (Kenney and Winn 1986), the Scotian Slope (Wimmer 2003), and Davis Strait (Whitehead pers. comm. 2004).

Sowerby’s beaked whales, like other beaked whales, are thought to prefer the deeper waters of the continental shelf breaks and open ocean, only occasionally being found in coastal waters (Kenney and Winn 1986; Kenney and Winn 1987; Lien and Barry 1990). In the eastern North Atlantic, Sowerby’s beaked whales range from as far north as the Norwegian Sea (Carlström et al. 1997), the waters off Iceland and the British Isles (Lien and Barry 1990; Sigurjónsson et al. 1989; Weir et al. 2001), and south to Madeira and the Azores (MacLeod 2000). A higher number of strandings in the eastern North Atlantic may indicate a greater abundance of Sowerby’s beaked whales than in the western North Atlantic (Lien and Barry 1990; Moore 1966). Strandings and sightings in the western North Atlantic (Table 1) suggest that the species is found off Newfoundland and Labrador (Lien and Barry 1990), Nova Scotia (Hooker and Baird 1999) and the northeastern United States (Lien and Barry 1990; MacLeod 2000). A recent stranding occurred on St. Catherine’s Island, Georgia (Tech Times 2004), and a stranding also occurred in the Gulf of Mexico, although the latter has been considered extralimital (Bonde and Oshea 1989).

There are no data regarding annual movements or site fidelity in Sowerby’s beaked whales.

Canadian range

The northern limit of confirmed sightings or strandings of Sowerby’s beaked whales in Canadian waters is Notre Dame Bay, Newfoundland (Lien and Barry 1990; Table 1; Figure 2), although it is likely that this species occurs further north. mesoplodonts sighted in Davis Strait (60º07´N, 60º34´W) during the summer of 2003 (Whitehead pers. comm. 2004) were probably Sowerby’s beaked whales. As Sowerby’s beaked whales have been seen in U.S. waters (Lien and Barry 1990), the southern Canadian limit should be considered the Hague Line. Given the apparent preference of this species for deeper, offshore waters, Sowerby’s beaked whale likely ranges to the seaward limit of the 200 mile Exclusive Economic Zone and beyond. All sightings and strandings have occurred in the summer months. This likely does not mean that these whales are absent from Canadian waters during other times of the year, but rather may reflect the poor sighting conditions and relative lack of search effort in non-summer months. However, two aerial surveys for cetaceans conducted in the fall of 2002 and 2003 (of limited extent and mostly near shore; G. Stenson pers. comm. 2006) did not observe Sowerby’s or any other beaked whales (Lawson pers. comm. 2004). Sowerby’s beaked whales may be quite widespread within Atlantic Canadian waters, but details of the range are unknown (e.g., the proportion of the total range that falls within Canadian waters, discontinuities in range, or temporal changes in range).

Reports of Sowerby’s beaked whales are less common in waters off Nova Scotia (Table 1). Sowerby’s beaked whales have been sighted in the Gully, a large submarine canyon located 150 km offshore on the edge of the Scotian Shelf (Hooker and Baird 1999; Whitehead pers. comm. 2004). The only recorded stranding in Nova Scotia was on Sable Island near the Gully (Lucas and Hooker 2000). There has been only one tentative sighting of a Sowerby’s beaked whale in the Bay of Fundy (McAlpine and Rae 1999; Table 1), which suggests that, given the relatively large amount of whale-watching and research effort in that area, the species is probably rare there.

There is a stranding record in the Gulf of St. Lawrence (New Brunswick coast), although the species identification is considered tentative (McAlpine and Rae 1999; McAlpine pers. comm. 2004). There are no records of sightings or strandings in Quebec (Measures pers. comm. 2004), Prince Edward Island (Daoust pers. comm. 2004) or along the Gulf of St. Lawrence coastlines of Newfoundland or Nova Scotia (Lien and Barry 1990).

In the absence of any data to suggest population structure within Canadian waters, a single Designatable Unit is recognized.