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Bluefin tuna farming moves to the deep sea

Moving tuna farms further offshore can produce bigger, healthier fish, says Dr Nicole Kirchhoff.

December marks the beginning of the five-month tuna fishing season in the Great Australian Bight.

Juvenile southern bluefin tuna (Thunnus maccoyii) are caught in nets and towed to cages in waters just off the coast of Port Lincoln in South Australia. Here they are fattened before they are ready for export to the high-end Japanese sashimi market.

But recent research by Dr Nicole Kirchhoff, published last year in PLoS One, could see the Australian southern bluefin tuna aquaculture industry move much further offshore.

"My speciality is making aquaculture more sustainable and economical," says Kirchhoff, a US marine scientist from Florida, who now researches and lectures at the University of Tasmania.

The Australian tuna industry moved to aquaculture (fish farming) in the 1990s following a rapid decline in stocks of southern bluefin tuna, which are listed as critically endangered by the International Union for Conservation of Nature.

Now more than 98 per cent of the Australian southern bluefin tuna catch, which is managed by quotas set by the Commission for the Conservation of Southern Bluefin Tuna, is farmed.

"[Farmed tuna] are pretty healthy and they have very high survival rates compared to other aquaculture and fishery industries around the world, but there are always ways you can try to improve your product a little bit more," says Kirchhoff, who studied the industry for her PhD.

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Commercial scale success

While there has been worldwide interest in moving aquaculture further out to sea where the water quality is better, it's more expensive than near-shore farming.

Kirchhoff's study provides the first evidence on a commercial scale that moving fish kilometres offshore may benefit them — and provide an economic boon not only for tuna companies but potentially other aquaculture industries.

Kirchhoff worked closely with one South Australian southern bluefin tuna aquaculture company to compare the health effects of cages located 46 kilometres offshore compared to those located 29 kilometres offshore in the traditional near-shore zone.

During the farming season she travelled out to sea with the pre-dawn fish feeding vessels at least once a week to monitor the performance and health of farmed fish.

She took samples of the fish's blood to test their glucose, pH and lactate levels, which indicate stress; and haemoglobin levels, which indicate oxygen levels. At harvest the fish were also examined for parasite infestations and weight.

"The offshore fish as well as the near-shore fish had no indications of stress with glucose, lactate and pH. They were perfect, the only difference was that the offshore fish had higher haemoglobin," she says.

"The results were better than any of us had ever expected. Especially since these fish are very healthy and have high survival near-shore to begin with."

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Healthy fish

While their blood tests may have been similar, Kirchhoff found that fish ranched offshore were completely free of sea lice and blood fluke — two parasites that also occur in wild fish — while all those near-shore were infected.

Significantly, around 10 per cent of the near-shore fish died, while less than 2 per cent of the offshore fish died.

"Eight per cent is a pretty big economic benefit when these fish are worth so much money," she says.

The offshore fish also reached market weight much faster than fish raised closer to shore.

"They usually take about 13 weeks or so to get to this really great condition where they can be marketed, and these offshore fish got there in six weeks."

In research yet to be published, Kirchhoff also investigated whether the fish maintain their condition if they are brought near-shore after they reach market weight at six weeks.

"We actually found that after being offshore for several weeks then bringing them inshore the fish maintained their condition, they maintained their superior survival, they maintained their health status, they maintained everything."

"So even temporarily leaving fish offshore is having amazing results."

Kirchhoff has also been collecting samples from wild tuna to compare with the farmed tuna, and the results are favourable.

"They [the wild tuna] come from the wild and they're pretty darn healthy. This work we did was to show that the work we did in the ranching cages was directly related to the fish that are swimming around before we catch them."

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International interest

Four of the 12 Australian tuna companies have now moved offshore based on her study, and it has drawn attention from aquaculture industries around the world, says Kirchhoff.

She says the research may be particularly beneficial for industries in urbanised areas where there are environmental concerns about near-shore farming.

"Offshore aquaculture in areas [that use cages in the same location all year round], specifically might work even better than the tuna industry here where we have temporary cages due to the ablation effect [of fast moving ocean currents on the cages]," she says.

Moving offshore would also allow some industries to expand their operations. But the move offshore may raise other issues that need to be carefully assessed by each industry, Kirchhoff says.

"You need to [consider] things such as interactions with wild populations, interaction with currents and wave action."

"You can't just put a cage where you want."

Dr Nicole Kirchhoff is a lecturer and researcher at the University of Tasmania. She spoke to Genelle Weule.

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