By Outi Tervo
Research on baleen whales in Disko Bay has concentrated on the ecology and behaviour of bowhead whales. Topics of interest have included feeding and diving behaviour of bowhead whales [Laidre et al., 2007, Simon et al., 2009], area usage and migration patterns of bowhead whales [Heide-Jørgensen et al., 2003; Heide-Jørgensen et al., 2006] and vocal behaviour of bowhead whales [Tervo, 2006; Stafford et al., 2008; Tervo et al., 2009].
Bowhead whale females can reach 20 m in length but lengths of 16 – 18 m are more common [Nerini et al., 1984]. The males are smaller like in all the baleen whale species. The weight of an entire specimen has never been managed to measure but it has been estimated that bowhead whales can weigh 70-100 t [Reeves & Leatherwood, 1985]. Bowhead whales are dark in colour ranging from dark grey to black except for a white patch under the chin. The head is proportionally larger than in other baleen whales and the upper jaw is typically arched. The width of the fluke may reach two-fifths of the body length and it is smooth on the margins and has a deep notch in the middle. Unlike the other right whales the skin of the bowhead whale is clear from barnacles and other external parasites. Bowhead whales do not posses ventral grooves.
1. Bowhead whale movements in the Davis Strait
Bowhead whales are found from the north western part of Hudson Bay from mid-May to mid-September (Fig. 1) [Moore & Reeves, 1993]. There are some observations that suggest that bowhead whales would be present in the area throughout the year [McLaren & Davis, 1982]. Bowhead whales in the Davis Strait are generally divided into Davis Strait and Hudson Bay stock but individuals from the both stocks are likely to use same wintering grounds in Hudson Strait [Heide-Jørgensen et al., 2006].
Figure 1. Seasonal distribution of bowheads in Davis Strait and Hudson Bay. Cross hatched areas illustrate the summer distribution and the hatched area winter distribution [Moore & Reeves, 1993].
Davis Strait and the northern parts of the Labrador Sea are important wintering areas for bowhead whales [Richardson & Finley, 1989]. The northernmost point for winter distribution is thought to be Disko Bay [Moore & Reeves, 1993]. Bowhead whales arrive to the vicinity of Qeqertarsuaq in mid January to early February and departure in early May to early June [Eschricht & Reinhardt, 1861; Heide-Jørgensen et al., 2003; Heide-Jørgensen et al., 2006] after which they head across Baffin Bay to Lancaster Sound and to the surrounding areas (Fig. 2) [Heide-Jørgensen et al., 2006]. Approximately 250 bowhead whales occupy West Greenland between the months of March and May [Heide-Jørgensen & Acquarone, 2002]. The other possible route takes them up north through Smith Sound into Kane Basin (Moore & Reeves, 1993). The summer distribution extends from the Canadian High Arctic archipelago southwards off the north eastern coast of Baffin Island. An autumn migration southwards along the coast of Baffin Island takes place in late September extending to October (Moore & Reeves, 1993; Heide-Jørgensen et al, 2006). The population inhabiting Davis Strait and Hudson Bay is thought to be separated from the Bering Sea stock by heavy ice in Viscount Melville Sound and in adjacent areas (Reeves & Mitchell, 1985) but due to decreasing ice coverage in the Arctic this conjecture should be reassessed.
Figure 2. Tracks of three bowhead whales tagged in Disko Bay in May 2002 and 2003 [Heide-Jørgensen et al., 2006].
3. Acoustic behaviour of bowhead whales
Songs of bowhead whales have been described during the spring migration off Alaska (Ljungblad et al. 1982; Clark & Johnson 1984; Cummings & Holliday 1987), during the winter and spring in Western Greenland (Tervo 2006; Stafford et al. 2008; Tervo et al. 2009) and during fall and winter from the Chuckhi Sea (Delarue et al. 2009).
Simple calls have not been assigned to any particular behaviour [Ljungblad et al., 1984] but they, particularly simple calls with ascending and descending frequency referred often as up and down calls [Richardson & Finley, 1989] have been recorded in the presence of socially and sometimes sexually active whales [Ljungblad et al., 1984; Richardson & Finley, 1989]. Low complex AM calls with minimum frequencies starting from 25 Hz have been recorded from socially active bowhead whales in Isabella Bay in August and September [Richardson & Finley, 1989]. Complex calls have often recorded in the presence of mildly socialising (within a body length) or actively socialising (body contact) bowhead whales [Ljungblad et al., 1984; Würsig et al., 1984] and in the presence of sexually active whale groups [Richardson & Finley, 1989].
Humpback whale song has been suggested to function as an advertisement display [Tyack & Clark, 2000] and the same could be the case for bowhead whale song even if the sex of the singing individuals is unknown. Songs of bowhead whale have been recorded during April and May off Point Barrow during the spring migration but this particular singing is thought to be only a remnant from the winter breeding season and therefore fail to represent the entire richness of the singing repertoire [Ljungbald et al., 1982; Ljungblad et al., 1984; Clark & Johnson, 1984].
The first recordings from bowhead whale winter repertoire were made in Disko Bay in February and March 2005 where a vast amount of song notes, simple FM calls and complex AM calls were present [Tervo, 2006; Tervo et al. 2009]. Compared to April and May, February and March had a profoundly higher signalling activity of song notes, simple FM calls and complex AM calls (signals per minute), the repertoire of song notes was substantially broader and there were typically more than one animal vocalizing at the same time. These findings indicate that bowhead whales in Disko Bay engage more in sexual and social behaviour in February and March than later in the season in April and May making Disko Bay a potential mating area for the Davis Strait bowhead whales.
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