Humpback whales calve further south than previously thought

A study that looked at where humpback whales give birth along the coast of Western Australia has shown that the calving grounds extend more than 1000 kilometres further south than currently recognised.

The research by IMAS PhD student Lyn Irvine, published in the journal Marine Mammal Science, included aerial surveys that recorded large numbers of young humpback calves along North West Cape, 1000km from the identified calving range between Broome and Camden Sound.

Ms Irvine said that this is a real paradigm shift in our understanding of humpback whales.

“Until now, our understanding was that humpback whales calved in localised areas of the Kimberley, such as Camden Sound,” Ms Irvine said.

“This research indicates that in fact humpback whales are born along a lengthy area of their migratory corridor extending from Camden Sound in the Kimberley to at least the Ningaloo coast in the Pilbara.”

Ms Irvine said the humpback population that migrates along the WA coast is the world’s largest.

“Up to 30 000 humpbacks are part of the population which migrates along the WA Coast each year as they move from their Antarctic feeding grounds to breeding and calving sites in tropical coastal waters.

“Pregnant females favour shallow, calm and protected waters to give birth, where it’s believed they shelter from predators and harassment by male humpbacks.

“In Western Australia, the principal humpback calving ground is currently recognised as being around Camden Sound and extending south to Broome

“However, our aerial surveys along North West Cape suggest that more than 20 per cent of the estimated 3000 calves born each year along the WA coast did so in this region.

“The light colour, small size and northward movement of the majority of calves recorded off the Cape indicate that they were born near, or south of, the area during the whales’ migration.

“This result is consistent with observations made during the commercial whaling period, where a considerable number of young calves were recorded along the southern end of the Cape.”

Ms Irvine said data about where the whales are born is important because it can help to inform strategies to manage and conserve the species.

“The recognised calving grounds further north were identified in 1997 when the population which uses WA waters was still recovering from commercial whaling.

“Over the last 20 years this population has increased at a rate of more than 10 per cent per annum, and is now estimated to number between 20 000 and 30 000 individuals.

“However, there has been no recent comprehensive survey along the WA coast to identify or describe the calving areas.

“As a result, it is unclear whether the observations off North West Cape means the calving grounds are expanding to their pre-whaling locations, or whether increasing abundance simply means the whales are now easier to observe in areas where they’ve always calved.

“Identifying critical habitat such as calving grounds will become increasingly important in coming years as the humpback whale population continues to grow and potentially overlap with human activities,” Ms Irvine said.