Feeding Area Re-Discovered in the Sub-Tropical Convergence
CWR has just completed a 12 day voyage from Fremantle to Jervis Bay to pre-position for its next job for the Department of Defence. The transit provided an opportunity to explore an old whaling ground south of the Australian continent, interestingly in the same area that would have been targeted by the super trawler recently stopped by Tony Burke. This is the subtropical convergence (STC), an area renowned as the “roaring forties” where warm sub tropical waters mix with the cold southern ocean and create a nutrient mixing area which stimulates phytoplankton growth, which in turn feeds krill, which in turn feeds whales – the biggest whales.
No one has been down there since the Russian whalers in the 60’s and 70’s. The Centre wants to find out how this area, a possible global conveyor belt of whale food, relates to Australia’s blue whale population which doesn’t seem to be able to stage a comeback at the moment – unlike their prolific cousins, the humpbacks.
Loaded with high tech underwater listening gear, the ultra-quiet RV Whale Song is normally tasked to assist the Australian Defence Department across a wide range of system testing. Seven days into the transit, and deliberately timed to coincide with the team’s previous experience in tracking blue whales to the STC using satellite tags, the sonobuoy speakers in the wheelhouse began to shudder with low frequency calls. At 20Hz, the sound these whales make is below an adults hearing threshold but incredibly, as loud as a jumbo jet taking off! The team closely examines tiny differences in times of arrival at our sonar sensors on each call. This is called beam forming and allows us to create a bearing to run down to find the “singer” – a huge blue whale.
It takes over a day to track the whale and as we travel the song unfolds into what initially appears to be a “Z” call. A 2 tone call made only by Antarctic blue whales, one of the rarest whales on the planet. From original numbers of over 350,000, it is thought that there may be as few as 5,000 left. Is one of them actually in the STC just south of Australia? And if so, why isn’t it down against the ice edge in the middle of what should be the peak of the Antarctic feeding season? Certainly the smaller pygmy blue whales are expected in the STC but not their huge cousins.
January 27 dawns and the research team are eager to actually see the source of all the calls they’ve been recording. It doesn’t take long and a wild shout from one of our observers indicates it is going to be a great morning. Closing with the large whale in the 4m swell takes some time but eventually we recognise this first sighting to be a pygmy blue whale. The whale is photographed and more tracking sonobuoys are deployed and it’s off to the next singer, only 10km away again identified as a pygmy blue whale. It’s third time lucky and as we set off towards the east another tall straight blow is sighted. Closing quickly we realise that this whale isn’t “just like the others”. It’s a blue alright, but its huge, over 24m long, and importantly, its head is tapered instead of rounded like that of the pygmy blue whales. Scaling the mast to gain a height advantage Micheline Jenner sets her camera on hyper drive as the whale slowly rises to the surface through the rich blue-green water. She takes what may be one of the first series of photos of an Antarctic blue whale in Australian waters!
The whale looks healthy and well fed, as did the pygmy blues sighted early that day and we remark how important this far flung feeding area must be to this species. As if to reinforce this thought, later that day the sonobuoy pick up the unique calls of southern right whales – they’re not far away but therse is no time to close with them. It’s enough to know that they too use this hidden feeding ground that for some reason has attracted the attention of an Antarctic blue whale.
The Centre is mounting an expedition to the Australian Antarctic territory later this year to continue their investigations into the health of our blue whale populations.