For the past fortnight a team of researchers have swapped their lab coats for waders and wellies in South Australia's Spencer Gulf in a quest to battle a tiny parasite that threatens the $150 million dollar tuna industry.

Key points: DNA from blood fluke will be used to create a portable diagnostic test for the tuna industry

DNA from blood fluke will be used to create a portable diagnostic test for the tuna industry Until now, whole pens of ranched fish have had to be treated with an expensive medicine to guard against the parasite

Until now, whole pens of ranched fish have had to be treated with an expensive medicine to guard against the parasite Scientists believe blood fluke will always be a threat to the industry, but will be better managed with diagnostic testing

The back deck of a harvest boat is not for the faint-hearted. Hundreds of the warm-blooded tuna are swiftly killed and gutted on board the boat.

The researchers collect vials of blood, hearts, gills and samples of the tuna's kidneys, spleen, and liver.

Dr Dan Austin and and Dr Claire Webber examine a heart during a tuna harvest at sea. ( ABC Eyre Peninsula: Jodie Hamilton )

What they are looking for are two tiny blood flukes, Cardicola forsteri and Cardicola orientalis, that killed vast quantities of the fish a decade ago and cost the industry millions.

A monitoring program has reduced the risk considerably and now researchers are looking at a DNA test for the blood fluke.

The project, involving Australian Bluefin Tuna Industry Association (ASBTIA), RMIT University Melbourne, and the Fisheries Research and Development Corporation, aims to create a portable diagnostic test and best practice to manage the blood flukes and monitor the health of tuna fish.

Once the diagnostic test has been created, it will mean producers will no longer have to treat whole pens of farmed fish — an expensive exercise.

Tuna ranchers were facing a crisis 10 years ago when the mortality rates of farmed product spiked to an average of 13 per cent, with some farms losing almost a quarter of their stock.

Researchers have spent the past eight years learning about the blood flukes' life cycle and how to beat it, ensuring the industry's 900 jobs and the health of individual fish that export for more than $500 each.

Nasty parasite

Dr Claire Webber prepares some tuna blood samples for testing. ( ABC Eyre Peninsula: Jodie Hamilton )

Researchers are only able to test the fish after the fish are killed because it is the heart and gills that need to be examined.

ASBTIA researcher Claire Webber hoped the new research would be used to develop a mucus swab from the gill of a live fish to test for the presence of the parasite.

The adults live in the tuna's heart and their eggs enter the blood stream, becoming lodged in the gills.

"[The eggs] block the blood flow and oxygen exchange for the fish and when they hatch out as well they can cause haemorrhaging," Dr Webber said.

Nathan Bott from RMIT University Melbourne said the respiratory system was affected.

"If left unchecked blood fluke will cause mortalities, which will obviously reduce the value of the industry," Dr Bott said.

"The larvae want to basically burst out of the gills so the fish can suffer from quite severe respiratory distress."

Blood flukes occur in 5 per cent of wild tuna but Dr Webber said ranching stimulated a greater infection rate because of the parasite's complicated life cycle.

"Over years of bringing those fish back we end up with quite high infection rates in farms not reflective of what happens naturally in the wild," Dr Webber said.

"It has a two-stage life cycle where the tuna is the definitive host, and hosts the adults, and that's where they have their eggs.

The adult blood fluke lives in the heart of a tuna. ( Supplied: Australian Southern Bluefin Tuna Industry Association )

"However it then goes into the water column once it hatches and down to the sediment where it has an intermediate host in a polychaete worm.

"It sort of incubates in there and then comes back and reinfects the tuna through the water column and that cycle is around five weeks."

Dr Webber said deeper water reduced infection rates.

"We raise our tuna in fairly deep water, 25 metres," she said.

"It's not deep enough to escape this worm, however with ranching further offshore and in deeper water we've learned over time that they've had a much harder time reinfesting the tuna say in 30 or 40 metres of water."

But there were extra economic and logistical concerns prohibiting farming in deeper water.

Blaslov Fishing managing director Justin Nelligan said his company's losses to blood fluke peaked at close to a quarter of their fish.

"We had one year at 22 per cent, which was massive in terms of tuna losses," Mr Nelligan said.

Blaslov Fishing's Justin Nelligan says his company suffered 22 per cent mortalities one season due to blood flukes. ( ABC Eyre Peninsula: Jodie Hamilton )

He said the cost to treat his tuna each season was $60,000 to $70,000 and the whole industry bill would be close to $1.4 million.

"We're at the stage now where the blood fluke levels have dropped significantly and guys haven't treated cages so we're trying to track down how safe it is not to treat now and save that money," he said.

"But I'm pretty wary about not treating."

He said the diagnostic test would be hugely beneficial and provide savings to tuna operators.

Dr Bott said developing a portable diagnostic test that the industry could use itself in the field on live fish could reduce costs to about $4 a test.

The tuna farmers could test for the parasites quickly and have an answer within a couple of hours, eliminating unnecessary treatments and increasing farm efficiency.

The parasite is restricted to the circulatory system and does not transfer to humans.

The sardines emerging from the marinating machine where they were injected with medicine for the tuna to consume. ( Supplied: Australian Southern Bluefin Tuna Industry Association )

Until now, producers have been using a converted chicken marinating machine to treat farmed tuna.

The only way to test for blood fluke is when the tuna is killed at harvest time. ( ABC Eyre Peninsula: Jodie Hamilton )

Sardines are placed in the marinating machine where they are injected with medicine for the tuna to consume. The bait is then frozen until required to treat a whole cage of fish.

Praziquantel is a medicine used in livestock and even humans, but it is expensive, needs to be administered at particular time in the life cycle, and means a wait of up to 21 days before the tuna can be harvested and sold.

"When the vet writes the prescription then the medicated feed will be taken out and fed to the farm over two days to make sure we get good coverage of every fish in the pontoon," Dr Webber said.

"There are a lot of efficiencies to be gained there by being able to treat specifically at the right time for the problem instead of just going out … and treating because you think that's the right time to do it.

"This is bringing a lot more science into what we're doing."

Blood fluke will always be around

Dr Webber said ranch mortalities had reduced to just 2 per cent because of the treatment, and those were due to causes other than blood fluke.

She said there might come a time when there were so few infected tuna that treatment might be administered in a fallow basis, but it would be impossible to wipe out the blood fluke completely.

She said it was a matter of managing their levels scientifically.

"I don't think there'll ever be a time where we don't have blood fluke," she said.