Sowerby's beaked whale (Mesoplodon bidens) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

Survey effort for cetaceans in deep waters off Atlantic Canada has been limited. In addition, Mesoplodon beaked whales are difficult to sight and identify, and they may avoid boats. Stranding networks in Atlantic Canada have become more active since the late 1970s and the number of strandings reported has increased (Table 1). However, it is difficult to determine if this increase is due to increased effort or an increased number of carcasses ashore (Lien and Barry 1990).

Abundance

No estimate of population size or number of mature individuals within Canadian waters exists for this species. The rarity of sightings suggests that Sowerby’s beaked whales are rare, but this may reflect limited effort in the deepwater areas and the difficulties of detecting and identifying the species at sea.

Several surveys have been conducted off the coasts of Nova Scotia and the United States by the US National Marine Fisheries Service and US Bureau of Land Management to assess cetacean populations. Due to difficulties in distinguishing one species of Mesoplodon from another at sea, population estimates have been produced only for undifferentiated Mesoplodon species. These estimates were based on ship or aerial line transect surveys and were not corrected for g(0), the probability of failure to detect animals on the track line. Thus estimates are negatively biased, especially considering the deep-diving behaviour of mesoplodonts. Each survey covered a different portion of the habitat considered suitable for mesoplodonts and occurred at a different time of the year, therefore the estimates are not directly comparable. However, the surveys from 1978 to 1994 indicated that there were several hundred mesoplodonts in the area from Cape Hatteras to Nova Scotia during the summer months, and that the Georges Bank region may be an area of relatively high density (Blaylock et al. 1995).

Fluctuations and trends

No information is available on fluctuations or changes in population size. As the numbers of sightings and strandings are very low (Table 1), apparent increases or decreases in these cannot be attributed to changes in population size.

Rescue effect

Sowerby’s beaked whales are found near the continental slope off the Atlantic coast of the United States and likely are part of the same population as those found in Canadian waters. However, there are no data indicating the frequency of movement between Canadian and American waters.

No sightings or strandings of Sowerby’s beaked whales have been reported from western Greenland, although search effort in that area is low so the lack of records does not mean these whales do not occur there. Sowerby’s beaked whales have been observed off Iceland (Sigurjónsson et al. 1989) but it is unknown if individuals move between the eastern and western Atlantic.

A rescue effect is at least plausible.

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