Millions of crayfish are set to descend into deep waters off WA's coast this week, in a "unique" migratory march.
The annual "whites run" is a natural phenomenon in which juvenile western rock lobsters from Bunbury to Kalbarri moult their red shells, becoming pale and soft.
They then turn to exactly 283 degrees north-west, and move from shallow coastal reefs to deep water.
Department of Primary Industries and Regional Development (DPIRD) scientist Simon de Lestang said the annual run was a bizarre event.
"It's a really unique thing to this species," Dr de Lestang said.
Choreographed crays like 'homing pigeons'
Each year between November and February, more than 10 million crays journey to waters between Geraldton and Shark Bay, where they settle on reefs.
Dr de Lestang said the crayfish use the Earth's magnetism to spin in the same direction for the migration.
"They all walk out at pretty much the exact same angle, 283 degrees, using magnetic reckoning, until they hit the deeper waters around about 100 to 200 metres," he said.
"And once they get into that loop and current, they turn and walk into the current and they walk northwards."
Dr de Lestang said lobsters tapped into magnetic forces in much the same way as homing pigeons.
"They can sense the magnetic fields underwater, and they can use that to know which direction they are going," he said.
Predicting the march
Dr de Lestang estimated this year's run would start around November 26 near Kalbarri, and lobsters in other areas would follow soon after.
But the prediction is not an exact science.
"That's been a question that a lot of people have sort-of answers to, like, it's got something to do with the Christmas trees flowering … or the seed coating the seagrass …or the moonphase," Dr de Lestang said.
The most scientific theory is water temperature, with warm waters prompting moulting and migration.
And if you were hoping to snap a photo of the march, you're out of luck — the crays prefer murky water.
"If it's dead calm, then they sort of wait for it to get a bit murkier," Dr de Lestang said.
Peak season for fishers
According to DPRID, cray stocks are high this year and record numbers of fishers are out to catch them, with 60,000 current licence holders.
Geraldton mayor and keen cray fisher Shane Van Styn said there were already plenty of mature red lobsters around ahead of the whites run, in what he called "the red run".
"The big ones move around to hide back in those holes and push out the smaller ones," he said
"But the whites run is truly the magic part of the year for catching crays.
"That's when the boat ramps fill up and cause all sorts of merry havoc down on the ramp."
