At the same time every year around 150 orcas meet 50 kilometres off the south coast of Australia, and now we may know why.
The extraordinary congregation happens between January and April and the orcas spend their time feeding and playing in a surprisingly deep patch of water around 40 kilometres across and around 1 kilometre down. Initially, researchers were puzzled that the orcas found that particular region so appealing as the surrounding ocean is low in nutrients.
Jochen Kaempf at Flinders University, Australia, suspected the geometry of the ocean floor might hold the answers.
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He and his colleagues mapped deep and narrow canyons slicing into the continental shelf under the surface. And using computer modelling they discovered that a current running along the southern coastline would concentrate waters through the contours of the canyons and out as a narrow stream toward the orcas鈥� feeding ground.
鈥淭his region hence serves as a 鈥榖ottleneck鈥� for all marine species,鈥� says Kaempf.
Some of these are squid, which are a typical food source for orcas, but giant squid that live in the ocean depths may also get swept up. Kaempf says this may explain some of the spectacular feeding frenzies that are observed when these fearsome predators are joined by sharks and flocks of birds as they feast on a much larger marine animal.
According to modelling and observational data, the influence of a separate current emerges in April and suppresses the upwelling. This means squid are no longer funnelled into that region, and explains why orca numbers start to dwindle in April.
This phenomenon may also explain why many other orca hotspots exist near certain submarine canyons in the world, Kaempf says.
Orca hotspots
However, there are other potential explanations. A hydrocarbon leak on the seafloor could produce fertiliser in the form of methane into the ocean depths and would explain the vibrant marine life in this 20 kilometre radius patch of water. But there is little evidence this exists, and it wouldn鈥檛 explain why the orcas prefer to visit at the same time every year, says Kaempf.
He initially thought that the submarine canyons could push enough nutrients into shallower waters to create blooms of plankton that might support a web of marine life. But they found no evidence for that, and that would not have explained why the orcas were further into the open ocean.
Bec Wellard at Curtin University, Australia, has been studying this population of killer whales for the past six years and says that it鈥檚 unique to see orcas regularly meeting in such great numbers, so far offshore.
But this isn鈥檛 the only unusual orca gathering in the world.
Several years ago researchers realised that type B orcas, known for spending most of their lives in the cold waters of the Antarctic, were migrating briefly to the subtropical waters off the coast of Brazil to shed their skin. A thick layer of algae builds on these orcas over the winter before they head north for their 鈥� spa bath鈥�, says Wellard.
Orcas have also mysteriously been visiting the strait near the Shiretoko Peninsula in Japan. The Orcas have specific saddle-patch markings and visit during spring and early summer. But this is unexplained, because the orcas are not believed to eat mammals and it is not the season for any known fish migration to the area, says Yoko Mitani as Hokkaido University in Japan.
But a recent tagging mission by Mitani and her colleagues suggests that they are eating squid or non-migratory fish that live in deeper waters at around 700 metres.
She says it is possible that they visit to breed, but nobody has seen them mating there yet.
