An international science team has been eavesdropping on the second largest animal on earth, the fin whale, using deep-sea sound recorders designed and built by the Australian Antarctic Division (AAD) to collect data in the remote waters off Antarctica.

Dr Brian Miller, marine mammal acoustician with the AAD, said a total of 285,000 hours of recording was collected from 2002 to 2019 across 15 ocean sites around east Antarctica and Australia.

“We tracked the largest animal you’ve never heard of by detecting nearly a million extremely powerful sounds that we can’t actually hear,” Dr Miller said.

Until now, seasonal migrations of fin whales from polar to temperate waters have been reported in the Northern Hemisphere, but not in the Southern Hemisphere between Antarctica and Australia.

A study by scientists from Curtin University, AAD, Ensta Bretagne (France) and NIWA (New Zealand), published in ‘Frontiers in Marine Science’, analyses the seasonal presence of fin whales from a total of 812,144 recorded calls.

Deep listening

Fin whales communicate with distinctively repetitive acoustic pulses at specific frequencies, commonly at 20 Hz, which are below the range of human hearing.

Passive acoustic monitoring is a cost-effective technique to map the distribution of whale species over time, where microphones are dropped off ships to record sound thousands of metres deep on the seabed over a year or more, before being retrieved.

Lead author Meghan Aulich of Curtin University is undertaking her PhD research to identify the soundscape ecology of the fin whale in Antarctic and Australian waters, and verified the more than 800,000 calls used in this study.

“By ‘listening’ to the marine world over the past two decades we gained new knowledge of the contemporary distribution and migrations of fin whales through the Indian, Pacific, and Southern Oceans,” she said.

“We identified two migratory pathways, from the Indian Ocean sector of Antarctica to the west coast of Australia, and from the Pacific sector of Antarctica to the east coast of Australia.”

“Fin whales are present in Australian waters from around May to October on east and west coasts, with the longest seasonal presence detected at Cape Leeuwin in WA, at the earliest in April and the latest in November.”

Greyhounds of the sea

Described as the ‘greyhound of the sea’ for its fast swimming speed, the fin whale is the second-longest species of cetacean after the blue whale, up to 27 metres long with an estimated maximum weight of 120 tonnes.

The Southern Hemisphere sub-species of fin whale (Balaenoptera physalus quoyi) is listed as vulnerable on the IUCN Red List.

Industrial whaling in the 20th century reportedly killed over 725,000 fin whales in the Southern Hemisphere, with the southern population now thought to number less than 40,000.

The scientists said understanding where and when the fin whales move in the Southern Hemisphere is vital to inform the conservation management of this vulnerable sub-species, especially with ongoing threats of population decline from climate change and habitat disturbance.

“From the acoustic information we recovered in this study, we believe there is limited mixing between the whales in east and west migration routes, which is preliminary evidence for separate sub-populations between the Indian and Pacific sectors,” Ms Aulich said.