Assumptions about humpback breeding blown out of the water

An Australian study has discovered humpback whale calves are being born much further south than previously thought, before they migrate northwards alongside their mothers and sometimes on their backs.

Their post-birth route takes them through busy shipping zones.

Newborn humpbacks were seen as far south as Tasmania, about 1,500km outside of the existing calving zone, says the report by lead author Jane McPhee-Frew, PhD candidate at Australia’s University of New South Wales and a whale watching skipper.

The new study challenges the long-held assumption that humpback whales only give birth in warm tropical waters, raising important questions about the ability of existing management practices to protect calves from human interference.

“Historically, we believed that humpback whales migrating north from the nutrient-rich Southern Ocean were travelling to warmer, tropical waters such as the Great Barrier Reef to calve,” says McPhee-Frew.

Humpback whales (Megaptera novaeangliae) are found in all oceans around the globe. The highly migratory species has been known to journey hundreds  of km from high latitude summertime “feeding grounds” to warmer low latitudes “breeding grounds” in the winter.

In Australia, distinct populations migrate along Western Australia and the east coast, with calving habitats accepted to occur no further south than23-28° south of the equator. The New Zealand population is thought to winter in waters around New Caledonia, Tonga, and Fiji.

It’s thought that warmer waters could allow calves to devote more energy to growing, or that breeding grounds help them survive by providing calm waters and refuge from predators such as orca.

McPhee-Frew’s new research, which brought together more than 200 sightings of humpback calves from whale watching operators, citizen scientists and government wildlife agencies in Australasia, shows that breeding and calving is not restricted to the endpoints of humpbacks’ migration routes.

“We had reports [of calves] right to the bottom of Tasmania, the southernmost points of Western Australia and to the South Island of New Zealand,” says McPhee-Frew.

“Eventually, we just ran out of land to see them from,” she says, “so we don’t actually know where the limit is.”

Mothers and calves were also migrating north. McPhee-Frew says the study highlights the critical need for increased awareness to protect the newborn whales throughout their winter journey north which, in some cases, could span more than 2300km.

“The pattern we’re seeing is mother whales with calves travelling through some of the busiest shipping lanes and urbanised regions,” says McPhee-Frew.

“This means these vulnerable animals are exposed to risks like boat strikes, entanglements, pollution – and just general public unawareness.”

Co-author Professor Tracey Rogers, also from UNSW, says newborn humpbacks are not as strong as adult whales.

“Mums with newborns swim much more slowly,” Rogers says.

“Newborns are like Great Dane puppies. They have those long, enormous fins that they need to grow into, and they’re not very strong swimmers. So they rest a lot of the time on their mum’s back.

“It’s heartbreaking to think of these young whales travelling through busy ports and dangerous shipping lanes with those long, clumsy fins.

“And it’s not just happening here in NSW – this is off WA, Victoria, Tasmania, New Zealand – it’s something we just didn’t know before.”

The researchers say that understanding the true distribution of calving is important for proper management of risks from both a conservation and animal welfare perspective.

The question remains of why humpback mothers continue to use the “humpback highway” to travel north after giving birth because, according to McPhee-Frew, “…in the tropics, there’s really no food for them.”

She says rather than holding a strict view of migratory patterns with fixed endpoints, the focus is shifting to how humpbacks use different marine environments on their journey.

The study has been published in the journal Frontiers in Marine Science.