Researchers worried by decline in migrating humpback whales off WA

A drop-off in humpback whale numbers during their annual migration along Western Australia's coast has researchers concerned.

Experts say the trend reflects an overall drop in whale sightings and an increase in strandings across Southern Hemisphere oceans.

A Broome tour operator says there have been a noticeable drop in numbers, with his tours having to travel much further afield to find whales.

Researchers say a drop-off in humpback whales migrating along Western Australia's coast this year reflects a developing trend across the Southern Hemisphere.

Observable whale numbers making the annual journey from the Southern Ocean to WA's northern waters dropped by 60 per cent this year.

Whale researchers and watchers also saw fewer calves and more strandings and entanglements along the state's coastline.

Mothers with calves were also scarce on the southern migration after birthing off the north Kimberley.

"It certainly has been an odd year for the humpback whales," Geographe Marine Research team leader Capri Jolliffe said.

Marine Scientist Olaf Meynecke also described the season as "strange."

"Reports around the Southern Hemisphere, from different populations of humpbacks, are all telling the same story of low numbers," Dr Meynecke said.

The Griffith University researcher said the earlier sea ice melts meant the whales were spending less time in their feeding grounds.

At the same time, lower food supplies due to changing krill populations and phytoplankton blooms were also having an impact.

"We had mums looking quite thin," Dr Meynecke said.

"You can see their fat reserves have been depleted and that's concerning because they need enough milk for their calves.

"We've had strandings of unhealthy whales, seen unfit whales."

Veteran Broome whale watcher Cameron Birch said he travelled much further than normal from the coast to find whales for tourists.

"We went nearly twice as far out, 10 or 12 nautical miles offshore," he said.

"And definitely less on the water, instead of interacting with 10 or 15 whales a day, it might only be two or three."

But Mr Birch believes the drop-off is not yet cause for alarm.

"Over the last 25 years we've seen this cyclic seasonality — we'll have amazing numbers for a few years, then they'll thin out," he said.

"And we've had times, say three years in a row, of low numbers then bang; they just bounce back again.

Dr Meynecke said whale entanglements had risen to 60 this year across Australia, a jump of 20 per cent.

But he feared for their survival rate and that there could be many more unreported entanglements.

"There's a tragedy behind every number, but what are the grey numbers? They could be double," he said.

Dr Meynecke said ongoing research was vital, as humpbacks were often a bellwether for other whale populations.

"If humpback whales start to struggle then all the other large whales start to struggle," he said.

"They're all signals telling us they're experiencing change."