Industry partnership to improve tuna health and grow aquaculture

Researchers are developing a new test for two tiny parasites that threaten Australia’s ranched Southern Bluefin Tuna, and are working with industry to develop best practice for treatment.

The first findings from the collaboration, published in the Journal of Aquaculture, revealed the effectiveness of targeted treatments of ranched tuna and a reduction in parasite numbers.

These husbandry techniques are widely used in fish farming worldwide under veterinary supervision. It is one of many reasons that Australian tuna ranching has such high productivity.

The parasitic blood flukes, Cardicola forsteri and Cardicola orientalis, asexually reproduce in a polychaete worm found on the sea floor before emerging to infect tuna, which can be deadly for the species if left untreated.

Though naturally occurring in approximately five percent of wild tuna, the parasites are of particular concern for fishers who capture and keep fish alive for ranching in areas where the polychaete is more common.

RMIT lead researcher Dr Bott said as current diagnostic testing methodology is fatal and involves taking samples from the tuna's gills, blood and internal organs, new methods were needed to enable accurate samples to be taken without harming live fish.

“A lack of cost effective and timely diagnostic testing means ranched fish are being preventatively treated at significant expense to the industry,” he said.

“We’re hoping to develop a new test that would only require a mucus swab from the gill of a live fish to test for the presence of the parasite,” said Bott.

“The aim is for this to be a portable diagnostic test that can be used by aquaculture workers on site in Port Lincoln.”

The parasite found in tuna is restricted to the fish’s circulatory system and cannot transfer to humans.  It becomes a problem for tuna when the parasite lays eggs that enter the fish’s blood stream and lodge in the gills, causing respiratory failure that can be fatal.

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

The research will also make recommendations on best practice for managing the handling of fish once caught to reduce any effects the parasite may have.

Humans and livestock can get similar types of parasites; when this occurs they are treated with the medication praziquantel to get rid of them.

Praziquantel must be prescribed by a veterinarian under a permit and administered at a particular time in the life cycle, meaning a wait of up to 21 days before the tuna can be harvested and sold.

Bott said he hoped improved diagnostic testing would help to provide more accurate guidelines for when to treat affected fish, reducing the cost of treatment and lowering mortality.

The study is being undertaken by researchers at RMIT University in collaboration with the Australian Bluefin Tuna Industry Association (ASBTIA) and the Fisheries Research and Development Corporation.

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