Report #4: The Fine Scale Krill Survey (Map 4)

Due to the low abundance of krill in the original research site located off Mawson, the decision was made to attempt conducting the fine-scale krill survey component of the research in an area east of Mawson where previous voyages had located large krill aggregations. An initial excursion eastward eventually located krill aggregations off Cape Darnley. These were marked with an Argos buoy before steaming back to Mawson for a fuel transfer from the m.v. Polar Bird. After a successful fuel transfer, followed by strong katabatic winds keeping the ship in Mawson’s Horseshoe Harbour for 3.5 days, the krill research was continued at the ‘new’ site (Map 4). En route to the east 4 groups of fin whales were sighted, the largest of which included 9 animals. Unfortunately, prior commitment to a lifeboat muster precluded the launching of the FRC for biopsy and faeces collection from these whales.

Map 4. Whale sightings from fine-scale krill survey

An initial large box configuration allowed delineation of the size and position of the krill swarms, followed by ever decreasing survey designs eventually culminating in a box-type survey grid measuring only 2 nm × 2 nm. This research was aimed at determining the diel changes in krill swarm dynamics and distribution within the water column, plus their movements relative to current direction and strength. Such a fine-scale survey crisscrossing the same patch of water for 24 hours inevitably increases difficulty in ensuring whales are not repeatedly counted. Observer awareness of the positions of previously seen whales is therefore critical.

During the fine-scale survey of the denser krill swarms, 3 days were ‘lost’ due to inclement weather and only one minke whale and a group of 3 killer whales were sighted during the remaining 2.5 days allocated to this research. The sightings were within 1.5 nm of each other and unfortunately the killer whales were lost during our continual changes in direction. A sonobuoy was deployed and only an extremely short burst of killer whale acoustics recorded. As the ship departed for the Kerguelen Plateau, we crossed an oil slick that smelled of dead whale and included several birds swooping overhead the slick. The position of this slick was in the region where the killer whales were last seen. This suggests that the group of killer whales may have completed a successful predation event, possibly explaining the silence of the whales while hunting.

Julie deploying sonobuoy

Later that day (15:00 LT), 5 groups of fin whales (3; 4; 4; 8; 6 individuals per group) and three pairs of humpback whales were seen within a few miles of steaming. Due to the sea state of Beaufort Scale 2-3, the voyage leadership allowed launching of the FRC to collect biopsy samples and any whale faeces encountered. No faeces were detected, even though the fin whales appeared to be feeding. One pair of humpback whales was successfully biopsied and two fin whale biopsies were collected from each of two of the fin whale groups (i.e., a total of 4 fin whale samples). For more details on the biopsy program, see the relevant section of this web site.

A total of 33 whale sightings was made east of Mawson during the 10 days spent surveying the waters around the fine-scale krill survey site offshore of Cape Darnley. 13 categories of whales were recorded, comprising a minimum of 75 individuals of which the majority (32) comprised fin whales (see Table 4). Fin whales were seen in all water depths; however, the survey track was biased to searching primarily over the continental shelf break (>1000 m) (Map 4). Analysis of odontocete distributions indicated that killer whales were seen both over the continental shelf and in deeper waters off the shelf edge, while the two sperm whales appeared closely associated with the 1000 m contour.

Fin whales

Table 4. Total whale sightings for fine scale krill box

Cumulative species	Cumulative sightings/animals
like fin whale	1:2
unidentified small whale	1:1
unidentified baleen whale	-
undetermined minke whale	3:3
long-finned pilot whale	-
like minke whale	1:1
killer whale	4:14
humpback whale	5:10
like humpback whale	-
sperm whale	2:2
Antarctic minke whale	3:3
unidentified cetacean	-
unidentified whale	3:3
unidentified large baleen whale	-
fin whale	7:32
unidentified large whale	1:2
sei whale	-
unidentified small baleen whale	-
like sperm	-
like southern bottlenose whale	-
like killer whale	2:2
like sei whale	-
small baleen whale	1:1