Report #2: The Mawson Krill Box – survey 1. (Map 2)

The research is being conducted in a 60x50 nautical mile 'box' approximately 30 nm off the coast of Mawson (see Map 2).  The position of this box is determined by the positions of penguins feeding.  Adelie penguins from the Bechervaise rookery off Mawson have been studied for many years, including researching their diets and related weight changes (using an electronic automated weigh-bridge that the penguins walk over when leaving & returning to the rookery).  Some of these penguins have been fitted with satellite tags and their movements can therefore be tracked while feeding out at sea. This provides an indication of where possible krill concentrations may occur (see the penguin tracks ‘live’ at the Australian Antarctic Division web site – http://www.aad.gov.au). 

MAP 2.

It transpires that the satellite-tagged penguins are feeding in about the same region as in yr 2000/1, so the 'box' has been positioned in the same place as we did during the KACTAS voyage.  The ship steams up & down in a series of parallel transect lines 5nm apart recording krill abundance below us using scientific echosounders and sampling where krill is abundant (or swarm dynamics look 'interesting', i.e. considerable structure within the swarm) using trawl nets. Additional programs, like ours, record predator (bird & whale) abundance. Several other projects are investigating plankton and nutrient ‘turn-over’ in the water, including one which deploys sediment traps to ‘catch’ sinking plankton and krill faeces.  The oceanographic data will mainly be collected along the outer edges of the box, but several drogue buoys fitted with Argos satellite telemetry equipment will be deployed to calculate current speed and direction.

In a 'nutshell', that's what we'll be doing for the next few weeks.

The first survey of the Krill Box took 9 days. Each alternate transect included a ‘night’ repeat of the transect to compare day & night distribution and abundance patterns of krill (‘night’ is considered appropriate from 10 AM to 4 PM with midnight equivalent to about 2am). Unfortunately almost half of the transects were ‘lost’ for the cetacean surveys due to inclement weather, i.e. either high sea states due to storm conditions, or snow-fall reducing visibility (see the red lines on Map 1). In some cases the severity of the seas led to the ship ‘heaving to’ and abandoning the transect lines to steam into the weather (see top of transects 10 and 11, Map 1).

MAP 1.

A total of 33 sightings of twelve identification categories were made during this survey (see Table 2). Generally the odontocete sightings (consisting primarily of sperm whale sightings, with the exception of one group of killer whales) were made beyond the continental shelf break. It is thought that the sperm whales will probably all be large males as the females and younger animals do not travel this far south during the austral summer. The sperm whales ‘log’ on the surface for a few minutes puffing away as they hyperventilate before their next deep dive (possibly in excess of 1500m). The single sighting of killer whales included some interesting behaviour: Although it was, unfortunately, approximately 2 nautical miles away, observations through the big-eye binoculars suggested they may recently have conducted a kill. The group was first sighted through observing a lot of splashing, followed by one animal leaping clear of the water in a rapid lunge. The group then spent some time underwater, coming up in 1s and 2s for a single breath before diving again. There was what looked like an oil-slick on the water surface and 2 southern giant petrels, a white chin petrel, South Polar skua and several storm petrels flying overhead and ‘dipping’ down to the surface (to pick up pieces of food?).

Mysticete sightings were spread throughout the ‘box’, although a series of Antarctic minke whale sightings were made at the ice edge when the ship headed south to allow investigation of the characteristics of the pack-ice (see the bottom of transect 9, Map 1). The sei whale sightings included one of 4 animals apparently feeding as they were surfacing synchronously and facing in the same direction at times (herding?), while at other times they surfaced in a tight group at different angles to each other (feeding?).

No obvious feeding behaviour was recorded for humpback and minke whale sightings.

Penguins foraging trips changed dramatically during the period that we were conducting the survey:

For the first few days (11-16 January) the penguins were primarily foraging in the continental shelf break region within the perimeters of the krill box. From the 17^th through to approx. the 25^th January they started spreading through a wider area and many appeared to be spending more time in deeper waters (approx. 2500m).

Both ‘target’ trawls (aimed at specific features detected on the acoustics screen) and ‘oblique’ trawls (conducted at the middle and ends of each alternate transect) yielded few Antarctic krill. Several trawls contained fishes, while most comprised a ‘mixed bag’ of invertebrates including ctenophores and salps, plus Antarctic and crystal krill, the latter particularly found over the shelf. However, some trawls did yield sufficient live krill for growth studies conducted on board.

Table 2: Categories of whale sightings during the first survey of the Mawson Krill Box.

Whale species sighted	Common name	Total number of sightings: Total number of animals seen
Balaenoptera physalus	Fin whale	1:1
Physeter macrocephalus	Sperm whale	9:9
Like sperm whale	Like sperm whale	1:1
Orcinus orca	Killer whale	1:5
Balaenoptera borealis	Sei whale	2:6
Undetermined species of minke	Undetermined species of minke	1:1
Like minke whale	Like minke whale	3:3
Balaenoptera bonaerensis	Antarctic minke whale	5:5
Megaptera novaeangliae	Humpback whale	3:5
Unidentified whale	Unidentified whale	2:2
Unidentified baleen whale	Unidentified baleen whale	4:4
Unidentified small baleen whale	Unidentified small baleen whale	1:1