The number of whales migrating along Western Australia's coastline is at its lowest in decades, with researchers linking the decline to warming seas, shrinking krill numbers and a rise in whale carcasses washing up on Perth beaches over the past month, prompting shark warnings from authorities.
Capri Jolliffe, research chairwoman at Geographe Marine Research, said while there had been no quantifiable study, "it certainly does seem like we've had a lot more carcasses wash up on Perth beaches than we typically would."
Sorrento Beach was closed on Sunday morning following reports of a carcass floating just offshore. Days earlier, a juvenile whale washed ashore at South Cottesloe's Dutch Inn Beach, and at the end of September the carcass of a 13-metre humpback whale was discovered along South City Beach.
Jolliffe said a super El Nino event was underway, with ocean temperatures and currents different from normal, which could be favouring carcasses washing up on the beaches. She said it had been an unusual season, with a pattern emerging in the humpback population and resource limitations. Calf numbers were about 50 to 60 per cent below normal: in Exmouth, where scientists would typically see up to 12 calves a day, they averaged only two, and southbound observations through Geographe Bay recorded low numbers.
Jolliffe said whales, like all mammals, would not reproduce without enough energy, and krill numbers had fallen as global warming melted Antarctic ice. This year's southbound migration showed more foraging than ever seen before, along with pods moving south and north repeatedly off the Perth coast and a calf with a heavy whale lice infestation, which she described as an indicator that an animal is unhealthy.
John Totterdell, lead researcher at the Cetacean Research Centre WA, said this year's winter migration recorded "fewer animals than we've seen in decades," with the daily relative abundance of new-season calves off the Ningaloo Coast plummeting. He said colleagues in the Humpback Whale Sentinel Program reported similar trends across Southern Hemisphere populations, pointing to reduced sea-ice habitat for krill driven by warming seas off Antarctica.
Robert McCauley, adjunct professor at Curtin University's School of Earth and Planetary Sciences, said newborn calves had the highest mortality rates, followed by subadults up to the age of five. But he said the decline did not necessarily mean "anything dramatic happening." The population could have reached the carrying capacity of available resources after bouncing back from the end of whaling in Australia, overshot, and entered a first decline. "That would match up with more carcasses washing up," he said.
In 2016, McCauley calculated 5000 tonnes of carcasses were available along the WA coast for scavengers, "not an insignificant amount, and that's just with the normal mortality rate."
Analysis
Why This Matters
- The humpback population along WA's coast was recovering after whaling ended; a low migration count raises questions about whether that recovery has stalled or reversed.
- More carcasses on Perth beaches mean beach closures and shark warnings for swimmers, a direct impact on local communities.
- The findings add to a picture across Southern Hemisphere populations of reduced Antarctic sea ice and krill, with implications for whales well beyond Western Australia.
Background
Each year humpback whales migrate along the Western Australian coastline, feeding in Antarctic waters in summer before travelling north to breed. The population rebounded after the end of whaling in Australia. Researchers now point to two possible explanations for the decline: reduced krill linked to warming seas and melting Antarctic ice, and the possibility that the population has grown to the carrying capacity of its food supply and is now in a natural decline phase. Calf production is a key indicator, and this year it fell sharply at monitoring sites including Exmouth and the Ningaloo Coast.
Key Perspectives
Capri Jolliffe (Geographe Marine Research): says carcass numbers on Perth beaches appear higher than usual, and describes a pattern in the humpback population tied to resource limitations, with a super El Nino event altering currents that may favour carcasses washing ashore.
John Totterdell (Cetacean Research Centre WA): says the winter migration recorded fewer animals than in decades, with daily calf abundance off the Ningaloo Coast plummeting, and similar trends across Southern Hemisphere populations linked to reduced sea-ice habitat for krill.
Robert McCauley (Curtin University): says the decline may not mean "anything dramatic happening"; the population could have overshot the carrying capacity of its resources after recovering from whaling, and the resulting first decline would match the increase in carcasses washing up.
What to Watch
- Whale counts through the rest of the southbound migration, with strong numbers normally recorded through October.
- Further carcass strandings along Perth beaches and any new beach closures or shark warnings.
- Calf abundance at Exmouth and along the Ningaloo Coast in coming seasons, which will show whether reproduction is recovering.
Checked
12 claims in this story checked against 1 source: 11 supported.
Show each claim
- SupportedWhale migration along WA’s coastline is at its lowest in decades.Brisbane Times - Latest News: “The number of whales migrating along Western Australia’s coastline is at its lowest in decades”
- SupportedCarcasses have washed up on Perth beaches over the past month.Brisbane Times - Latest News: “the number of whale carcasses that have washed up along Perth beaches over the past month”
- SupportedScientists point to warming Antarctic seas and krill loss as explanations for the decline.Brisbane Times - Latest News: “reduced sea-ice habitat for krill, driven by warming seas off Antarctica.”
- SupportedA possible population overshoot is an explanation for the decline.Brisbane Times - Latest News: “it is quite possible we’ve reached that overshoot mark, and we’re now in the first decline.”
- Partly supportedResearchers link the whale decline to the rise in carcasses washing up, which prompted shark warnings.Brisbane Times - Latest News: “scientists believe there is a likely correlation between poorer conditions and the number of whale carcasses that have washed up along Perth beaches over the past month and triggered shark warnings from authorities.”
- SupportedJolliffe said, “it certainly does seem like we've had a lot more carcasses wash up on Perth beaches than we typically would.”Brisbane Times - Latest News: ““it certainly does seem like we’ve had a lot more carcasses wash up on Perth beaches than we typically would””
- SupportedSorrento Beach was closed on Sunday morning after reports of a carcass floating just offshore.Brisbane Times - Latest News: “Sorrento Beach was closed on Sunday morning following reports of a carcass floating just offshore”
- SupportedCalf numbers were about 50 to 60 per cent below normal.Brisbane Times - Latest News: “There’s been way fewer calves than what we would normally see, probably in the order of 50 to 60 per cent less”
- SupportedIn Exmouth, where up to 12 calves a day would typically be seen, the average was two.Brisbane Times - Latest News: “Up in Exmouth, where they would typically have seen up to 12 calves a day, they only had two on average”
- SupportedThis year’s southbound migration showed more foraging than ever seen before.Brisbane Times - Latest News: “this year we’ve seen more foraging along their southbound migration than we’ve ever seen before”
- SupportedTotterdell said this year’s winter migration recorded “fewer animals than we’ve seen in decades”.Brisbane Times - Latest News: “Cetacean Research Centre WA lead researcher John Totterdell said this year’s winter migration had recorded “fewer animals than we’ve seen in decades”.”
- SupportedMcCauley said the population could have reached carrying capacity, overshot and entered its first decline.Brisbane Times - Latest News: “it is quite possible we’ve reached that overshoot mark, and we’re now in the first decline.”
