Sowerby's beaked whale (Mesoplodon bidens) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Like most cetaceans, Sowerby’s beaked whales are thought to have a low reproductive rate (Mead 1984), which will limit a population’s ability to adapt to, or recover from, a disturbance.

There is increasing evidence that mass strandings of beaked whales can be caused by military sonar (Balcomb and Claridge 2001; Frantzis 1998; Jepson et al. 2003; Fernández et al. 2005; Cox et al. 2006).  Although sonar has not been directly associated with deaths of Sowerby’s beaked whales, the susceptibility of beaked whales in general suggests that whales from all species in the family Ziphiidae can be harmed by exposure to high-energy, mid-frequency sonar.  When mass strandings of beaked whales have occurred in association with military sonar deployments, they seem to have involved most or all of the ziphiid genera that inhabit the area (cf. Brownell et al. 2005). In other words, there is no reason to believe that the effects are specific to a single species or species group within the Ziphiidae, and therefore it is reasonable (and prudent) to infer that mid-frequency sonars such as those deployed by many modern naval vessels can have lethal effects on all species of Mesoplodon, including Sowerby’s beaked whales. A number of Canadian military exercise areas are designated on the Scotian Shelf directly offshore from the Maritime Forces Atlantic Headquarters near Halifax (DFO 2006b).

Sowerby’s beaked whales also may be vulnerable to other sources of acoustic pollution.  Seismic surveys have influenced the behaviour and distribution of other cetaceans (e.g., Miller et al. 2006).  Although there have been no direct studies of the effects of seismic activities on any species of beaked whale, fatal strandings of Cuvier’s beaked whales (Ziphius cavirostris) in the Gulf of California have been linked to seismic activities (Hildebrand 2005; Cox et al. 2006).

Seismic surveys are common in the offshore waters of Atlantic Canada where licensing and prospecting for oil and gas is being pursued extensively (e.g., see CNSOPB 2006, DFO 2006a for the Scotian Shelf; CNLOPB 2006a, 2006b, 2006c for Newfoundland and Labrador). Such surveys have been increasing over the last decade in the deeper shelf edge and slope waters that comprise the habitat of Sowerby’s and other beaked whales.

Although not conclusive, the above observations suggest that Sowerby’s beaked whales are vulnerable when exposed to intense underwater sounds. As is true of other beaked whales, exposure to loud sounds may, in some circumstances, lead to serious injury or death. Although Cuvier’s beaked whales are the commonest of the beaked whales to mass strand, this may reflect, in part, that their carcasses usually float after death, whereas the carcasses of at least one of the Mesoplodon species (M. densirostris) more often sink (IWC in press).

Beaked whales, but not specifically Sowerby’s beaked whales (identifications were often made only to family or genus), were taken incidentally (bycaught) in the U.S. swordfish/tuna/shark drift gillnet fishery before this fishery closed in 1999 (Blaylock et al. 1995; G. Waring pers. comm. 2004).  Since this closure, no by-catch of beaked whales has been reported off the U.S east coast (G. Waring pers. comm. 2004).  In 1984, a single Sowerby’s beaked whale was found entangled in fishing gear in Manuels Cove, Newfoundland.  The live whale was successfully disentangled; however, it stranded two days later in the same area (Dix et al. 1986).  It is unclear what role the entanglement played in the death of this animal, as Sowerby’s beaked whales are only rarely found in coastal waters.  Although Sowerby’s beaked whales have not been reported as by-catch in the U.S. swordfish/tuna/shark longline fishery, there have been several reports of another beaked whale species (the northern bottlenose whale, Hyperoodon ampullatus) entangled in similar gear in Atlantic Canadian waters (e.g., Gowans et al. 2000). Sowerby’s beaked whales also may be susceptible to at least occasional entanglement in longline gear.

The Sowerby’s beaked whale that stranded on Sable Island in 1997 (Table 1) had several long fresh gashes on its sides and eight broken ribs below these gashes.  This type of injury is consistent with a ship strike and suggests that, like other cetaceans, Sowerby’s beaked whales are sometimes hit by vessels (Lucas and Hooker 2000). 

Many chemical pollutants bioaccumulate in the blubber and other tissues of cetaceans.  A mature male Sowerby’s beaked whale that stranded in eastern England contained higher levels of chromium, nickel, copper, zinc, arsenic, selenium, cadmium and mercury than many other stranded cetaceans in the UK, although most of these levels were within previously established ranges for marine mammals (Law et al. 2001). Mercury concentrations were particularly high in this individual (Law et al. 2001).

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