The so-call ‘bio-duck’ sound first came to the attention of sub-mariners at the height of the Cold War in the 1960s. Did it come from a Soviet underwater detection system or was it produced by a sea creature? Listening devices, placed on the Antarctic seabed, recorded the mysterious sound but the source remained elusive. Now a team, led by Denise Risch at the Northeast Fisheries Science Centre, Woods Hole, Massachusetts, claim to have solved the mystery. The low-pitched repetitive pulses, with three- to four-second intervals between them, are said to resemble the low-pitched quacking of a duck. However, listening to them on the internet, I couldn’t detect a resemblance. A slight drop in frequency as the sound progresses was a clue to its possible source — whale calls usually fall in pitch.
Timing provided another piece of the jigsaw: the noises are heard in winter and spring. If their source is an animal, it’s likely to be one present only at these times of year. Minke whales became the prime suspects. They frequent Antarctic waters but their newborn calves can’t cope with extreme cold, so they migrate northwards in winter and spring to give birth. Being the name of a man, ‘minke’ should perhaps begin with a capital letter. Meincke was a Norwegian whaler who mistook this species for the blue one and was lampooned for his incompetence.
There are two kinds, those with teeth and those with a sieve-like structure known as a baleen. The minke, up to ten metres long, is the second smallest of the baleen tribe. It is fairly common off Ireland, although ours is now considered to be of a different species to the southern one.
You sometimes see minkes from ferries between Ireland and France. They can come close to shore. I once saw them off the western tip of Dursey. When sailing among a school of minkes off Iceland, some came close as if to investigate our boat. Very lively animals, they rise from the water when surfacing to breathe, making them easy to identify. The nose is pointed and there are conspicuous white bands on the black pectoral fins.
Being small and fast helped save the minke from whalers. It is difficult to harpoon an animal swimming at up to 30km per hour and its size meant a poor return for the effort of doing so. Whalers, therefore, targeted larger species. The minke’s respite didn’t last however. During the 1920’s, 30,000 blue whales were killed. By the 1950’s their numbers were so reduced attention turned to the fin whale. Eight species of whale were exploited, in turn, until their populations collapsed. Then, in the 1960s, it was the turn of the minke. Last week, the Japanese whaling fleet began hunting them; minke flesh is an expensive delicacy in Japan. Icelanders and Norwegians also kill them.
In proving that minkes are responsible for the mysterious sound, scientists faced a similar problem to the whalers; attaching listening devices to such fast-moving creatures is not easy. In February 2013, Risch’s team managed to do so. Two whales were approached using an inflatable boat off the Antarctic Peninsula and monitoring tags were placed on them with a long pole. The animals were tracked and their behaviour observed. Bio-duck sounds were heard and matched to recordings in the researchers’ archive. There being no other whale species in the vicinity, it was concluded that the sounds came either from the tagged individuals or minkes close to them.
Just what the calls mean to the whales is a mystery. However, this doesn’t imply that these findings are trivial. Knowing that the bio-duck sound comes from minkes enables the distribution of the species to be more easily mapped and estimates can be made of their abundance.
* Denise Risch et al. 2014. Mysterious bio-duck sound attributed to the Antarctic minke whale. Biology Letters 10.