Essay Preview: Marine Mammals
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Genus Mesoplodon – Beaked whales: Introduction and Sources
The distribution of many Mesoplodon species is known almost entirely from records of stranded individuals. This situation is due to the difficulty in making specific identifications of these animals at sea and the relative rarity of sighting them at all (Mead, 1989). However, the distributional conclusions that are drawn from stranded animals are tentative due to the likelihood that these animals were diseased and strayed from their normal range. It is only when there is a large sample of strandings that have come from the same area that any relatively firm distributional conclusions can be drawn. Care must also be taken in the weight which one gives to negative distributional data. In some cases there may be animals frequenting the waters and stranding upon the shores but there has not been enough cetological activity in the area to bring the strandings to the attention of scientists (Mead, 1989).
Unfortunately, correct identification of Mesoplodon specimens also seems to be fraught with difficulties. Dalebout et al. (1998) report that to assist in the species-level identification of stranded and hunted beaked whales, they compiled a database of reference sequences from the mitochondrial DNA control region, for 15 of the 20 described ziphiid species. Reference samples for eight species were obtained from stranded animals in New Zealand and South Australia. Sequences for a further seven species were obtained from a previously published report. This database was used to identify 20 test samples obtained from incompletely documented strandings around New Zealand. Analyses showed that four of these test specimens (20% !) had initially been misidentified. These included two animals of particular interest: a Blainvilles beaked whale (Mesoplodon densirostris), the first record of this species in New Zealand waters, and a juvenile Andrews beaked whale (Mesoplodon bowdoini).
Populations size: According to Pitman (2002) so few mesoplodonts have been reliably identified at sea that it is impossible to accurately determine the population status of any species, although, based on stranding data, at least some species may not be as rare as the sightings records indicate. M. grayi, M. layardii and M. densirostris seem to widespread and fairly common, whereas e.g. M. bowdoini and M. hectori are rather rare.
The best available abundance estimate of beaked whales for the western North Atlantic stock is 3196, where the estimate for the northern USA Atlantic is 2600 and for the southern USA Atlantic 596 (data from 1998, in Waring et al. 2001).
Habitat: According to Pitman (2002) mesoplodont whales normally inhabit deep ocean waters (>2000 m deep) or continental slopes (200 – 2000 m) and only rarely stray over the continental shelf. Whereas M. densirostris is found in all tropical and warm temperate oceans, most species are restricted to one or two broad ocean areas. The distribution of M. perrini could be considered localized (MacLeod, pers. comm.).
Migration: M. layardii may undergo some limited migration to lower latitudes during local winter (Pitman 2002) and M. bidens may undergo migration in the eastern Atlantic (MacLeod et al. Unpublished).
Food: Mead (1989) reports that all beaked whales feed primarily on deep-water mesopelagic squid, although some fish may also be taken (Pitman 2002; MacLeod et al. 2003). Most prey are probably caught at depths exceeding 200 m via