The Port of Townsville in north Queensland is planning to dump silt and sediment at sea within days of coral spawning.

The timing has residents of nearby Magnetic Island concerned that poor water quality will compromise local reef reproduction.

James Cook University coral ecologist Andrew Baird said Queensland inshore reefs were expected to begin spawning within days.

"Coral spawning is a very important part of reef recovery," Professor Baird said.

"If reefs get disturbed, the main source of recovery will be sexual recruits from the coral spawning."

The Port of Townsville said it was planning not to dredge, due to coral spawning, until November 4.

It said it would then dump naturally occurring sediment build-up from shipping channels at sea 6 kilometres offshore of Magnetic Island's Florence Bay.

Community concern

Diving instructor Chad Bertram, who lives and runs a scuba and snorkel business on the island, said dredge spoil muddied the local waters.

"Easterly winds and the currents that we do have generally brings [the spoil] to the shores of Magnetic Island and on our fringing reefs," he said.

"There are times we have to cancel for safety when there is visibility of next to nothing."

He said he feared that dredge spoil in the water, particularly during coral spawning, could have long-term implications for the reefs.

"I think anything we can do to minimise our human impact can only be a good thing, so concerned is an understatement."

The Magnetic Island Nature Care Association, led by Gethin Morgan, has spent years raising concern about the port dumping spoil at sea.

The area in Cleveland Bay, between Townsville and Magnetic Island, is excised from the Great Barrier Reef Marine Park and has been an approved disposal site for decades.

"It is a real shock to hear that they are going to do dredging and dumping in the middle of the spawning season," Mr Morgan said.

"We all know that October and November are months in which corals spawn, and they can also be very windy months."

Silt and sediment affect spawning

Australian Institute of Marine Science coral ecologist Katharina Fabricius said coral larvae were sensitive to poor water quality well after the initial spawning.

"They are looking for clean reef surfaces and that could be up to two weeks after the spawning event, sometimes longer," she said.

"During those times the larvae are searching for places that are free of sediment, then they settle and become baby corals.

"Any type of silt and sediment is affecting coral spawning, no matter where it comes from."

Port says dredging is essential

The Port of Townsville said it had voluntarily planned for no dredging during the main predicted spawning window in October as a precaution.

"Maintenance dredging of the port's main shipping channels and harbour is scheduled to recommence from November 4 for 10 days to remove naturally occurring sediment build-up," a spokesperson said.

"Maintenance dredging is an essential and routine activity that supports the safe movement of vessels, which is a statutory responsibility for the port.

"The dredging program is being undertaken some kilometres from sensitive coral reef habitats, and environmental modelling does not predict impacts on those reefs."

The spokesperson said the port had undertaken maintenance dredging and sea placement at this time of year previously, most recently in 2024.