Report #5: Kerguelen Plateau oceanographic survey (Map 5).

 

The final portion of the Marine Science voyage of the R.S.V. Aurora Australis constituted an oceanographic survey of the Kerguelen Plateau, including a planned deployment of 8 oceanographic moorings plus 48 CTDs. As a result, the ship would be stopping at regular intervals during all deployment of oceanographic equipment. During these stations, the whale survey would be reduced to incidental observations from the bridge. As the CTD is lowered to the seabed, which in many cases exceeded 4600 m in depth, these stationary periods regularly continued for over 4 hours. The short distances between CTD and mooring sites led to only short whale surveys on full effort (Map 5). Confounding this complex survey layout was the inclement weather that reduced search effort for 11 of the total 17.5 days.

 

Map 5. Whale sightings in the Kerguelen Plateau.

 

 

The total of 94 whale sightings, of a minimum 333 animals (Table 5), is therefore surprisingly high considering the reduced search effort in the region. Due to the inclusion of a large group of 120 long-finned pilot whales in this total, a breakdown of the whales seen into three categories allows greater understanding of the data. Odontocete sightings were only of the family Delphinidae, including 2 groups of killer whales, the large group of pilot whales and the first sightings of hourglass dolphins for this voyage. The gregarious nature of this family led to a total count of 154 individuals in only 5 sightings. Conversely, almost the same number of individuals (159) was seen in 74 sightings of mysticetes. Fin whales were again the most commonly seen species, and a large proportion of the unidentified whales probably included this species. The large number of unidentified animals (15 sightings of 20 animals Table 5) can be attributed to high swells which reduced the effectiveness of the big-eye binoculars for species identification. Unfortunately, this unstable sea was encountered in the region of apparently highest cetacean abundance off the north-eastern drop-off of the BANZARE Bank (Map 5).

 

 

Table 5: Kerguelen Plateau whale sightings.

 

Species

Sightings/animals on the Kerguelen Plateau survey

like fin whale

7:10

unidentified small whale

-

unidentified baleen whale

1:1

undetermined minke whale

5:7

long-finned pilot whale

1:120

like minke whale

-

killer whale

2:14

humpback whale

23:53

like humpback whale

3:4

sperm whale

-

Antarctic minke whale

12:17

unidentified cetacean

-

unidentified whale

11:14

unidentified large baleen whale

6:9

fin whale

17:58

unidentified large whale

4:6

sei whale

-

unidentified small baleen whale

-

like sperm

-

like southern bottlenose whale

-

like killer whale

-

like sei whale

-

small baleen whale

-

hourglass dolphin

2:20

 

 

Fin whale blowing

 

Of the odontocetes seen, the two sightings of hourglass dolphins occurred within half an hour of each other on the BANZARE Bank. The long-finned pilot whales occurred in water deeper than 3000 m (Map 5), a similar depth within which the previous three sightings of this species occurred (Map 1). Both groups of killer whales seen were in the shallower waters of the continental shelf.

 

Hourglass dolphin

 

Identified baleen species included fin, humpback and minke whales, all three of which were seen throughout the region surveyed (Map 5). Both fin and humpback whales were seen feeding on the Kerguelen Plateau. Most humpback whales seen consisted of pairs swimming together, although a single animal swam up to the ship past the CTD door while at a CTD site. Similarly, the single sighting of a minke whale occurred when it swam around the CTD wire several times during an oceanographic site.

 

The route from Mawson to Hobart, Tasmania, will again transect the Kerguelen Plateau as it is hoped to complete the final three deep CTDs en route to Australia. This may allow an additional data set should the weather allow full search effort.