From looking at the skin colour patterning and dentition of the smaller whale which was accessible on the beach, it became apparent that onlookers had witnessed an important event, the stranding of rarely seen Shepherd’s beaked whale Tasmacetus shepherdi. Identification was confirmed from photographs the next day by cetacean expert Peter Best in South Africa who was quick to affirm this as a ‘very important and exciting event’.
Shepherd’s beaked whale is only known from the southern hemisphere, with all records to date having occurred between the latitudes of 30 ° S and 55 ° S. The majority of stranding records are from New Zealand but there are six previous records of strandings on Tristan, but in each case the whale or whales found were already dead. This is the first time that the stranding event of a Shepherd’s beaked whale has been recorded here. In 1983 two beaked whales were found on the beach south of Anchorstock Point; in 1984 one was found at Deadman’s Beach, Stony Hill Point; in 1987 two were found dead at Runaway Beach and in 1995 a large beaked whale was found dead at Noisy Beach.
The colour pattern of Shepherd’s beaked whale is quite distinctive: the beak is dark but the ‘melon’ or forehead, as far back as the blowhole, is a pale bluish-grey. The rest of the body is a darker bluish-grey brown above and paler grey below, with extensions of grey up onto the flanks behind the dorsal fin and from just behind the flipper onto the shoulder. The dentition of Shepherd’s beaked whale is unique among ziphiids (beaked and bottlenose whales) in that there are multiple teeth in both upper and lower jaws, with the pair at the tip of the lower jaw much larger than the others. This larger pair of teeth only erupts in males.
Over the next few days the bodies of the whales shifted position along the beach, but eventually it was possible to reach both of them to determine the sex of each. The larger adult female measured approximately 6.75m as the beak had been heavily damaged during the stranding and a portion of both upper and lower jaws was missing. Two mammary slits were visible confirming its sex, these lie on either side of the genital opening. On the male which measured 5.05m in length from the tip of the upper jaw to the middle of the trailing edge of the tail flukes, the single genital slit was visible.
For the first 24 hours after stranding the distinctive colour pattern of the skin on the male was visible. After this time, the skin darkened and had the whale been first noticed at this time, the pattern would have been difficult to make out. With advice from Peter Best on what data and samples to collect, it was important not to lose the opportunity to contribute to furthering the knowledge of this species. Stomach contents were collected from the two large stomachs of the female and skin and blubber samples were taken for analysis from both whales. Stomach contents have been taken from only two other Shepherd’s beaked whales, these are needed to determine what the whales feed on. From an initial examination of the contents, lenses from the eyes of squid and some fish bones were visible. The contents will be examined more closely by scientists in South Africa along with the skull and teeth of the young male that are being preserved.
It may never be known what caused these two whales to beach themselves on Tristan. There are theories that whales may strand themselves due to illness or injury, and other possible causes put forward include weather conditions, underwater seismic activity, magnetic field anomalies and unfamiliar underwater topography. Loud sounds associated with seismic exploration or military activity have been implicated elsewhere in mass strandings of other beaked whales (Best, 2007).
With thanks to Trevor Glass, Sean and Marina Burns and Stanley Swain for their accounts of the stranding event, to the Fisheries Department for collecting samples and to Brad Robson and Peter Best for their advice.
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