PORT Douglas adventurer Ben Cropp believes he may have solved the puzzle of the Queensland groper deaths.

Twenty-five giant gropers, weighing up to 120kg each, have washed up in varying states of decomposition between Cairns and the Daintree since March.

Scientists remain baffled by their deaths.

The cause, the renowned film-maker and diver told The Cairns Post, could be heavy-metal poisoning.

"The groper is the top of the food chain," Mr Cropp said.

"It is eating other fish and crabs which have a smaller amount of metals. But they're cumulative - they stay in the body for a long, long time.

"Eventually, it's like the straw that broke the camel's back.

"It's too much and they become sick."

Mr Cropp said natural heavy metals leaching from mangroves, run-off from acid-sulfate soil and chemicals from boats boosted heavy-metal levels in estuary waters.

"There's mercury, arsenic, even the copper leaching from the holds of boats from anti-foul," he said.

Department of Primary Industries and Fisheries scientist Dr Richard Knuckey said scientists had established the fish have died from blood poisoning caused by bacteria.

But the cause of infection and type of bacteria remain a mystery.

Heavy-metal poisoning was one of several possibilities under the microscope, he said.

But he said: "We're a bit sceptical because we’re also seeing it in the Daintree river as well, where you’re not subject to the same level of human
interaction."

An absence of deaths among other large predators, such as estuary cod and crocs, cast further doubt on heavy metals as the cause of death, he added.

Scientists were testing samples from a relatively fresh groper carcass found at Trinity Beach this week for heavy metals, pesticides, algal toxins, bacteria and viruses.

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