Spanish research centre achieves first tank-bred Atlantic bluefin as NGOs warn of poor welfare, more antibiotic use and water pollution

The first successful breeding of Atlantic bluefin tuna at a Spanish research centre has spurred at least two companies to ramp up plans for the industrial farming of land-bred tuna.

The companies would be the first to use only tank-bred Atlantic bluefin stocks of fertilised eggs or young tuna. Up to now, farming of Atlantic bluefin has relied on catching young wild fish and fattening them in open-sea cages.

After the breakthrough in July at the government-run Mazarrón aquaculture plant in Murcia, the company Next Tuna said it plans to begin building a tuna farm north of Valencia. Nortuna, a Norwegian company, has also signed a deal with Mazarrón for the firm’s pilot site in Cape Verde, off the west coast of Africa.

Commercial aquaculture companies argue that as more fish are farmed from stocks in closed tanks, fewer wild Atlantic bluefins will be caught for fattening or immediate consumption, reducing pressure on sea stocks. However, many NGOs note that an increase in farmed tuna would mean more fish taken from oceans to feed them. They have also raised concerns about animal welfare, antibiotic use and water pollution.

Two other species, the Pacific and southern bluefin, have been successfully bred on land before, but until July no one had successfully reproduced Atlantic bluefin (Thunnus thynnus) from tank-based adults. Atlantic bluefin are highly prized for sushi, but their commercial importance has attracted overfishing and populations have plummeted by as much as 80% in some areas of the Atlantic Ocean.

Aurelio Ortega, who leads the research team at the Mazarrón centre, said: “We have about 2,000 or 3,000 tuna fish now. They weigh about 5g to 10g each, and they will take two to three years to reach a size of about 30kg to 40kg.”

Under the plans, the plant will supply fertilised eggs and juvenile tuna to the newly created commercial firms, which will either continue the breeding cycle on land, or use a combination of land-based tanks and sea cages.

NGOs said this will pitch companies into unknown territory, given how little is known about the bluefin species. Catalina López, a veterinarian and director of the Aquatic Animal Alliance at the Aquatic Life Institute, said: “Very little is known about [tuna] requirements for adequate welfare, as it is a highly migratory species with complex hunting behaviours and migration patterns.

“Without extensive scientific research on the welfare of tuna, it is irresponsible to farm them intensively, and could lead to many welfare issues [including] stress, frustration and, ultimately, poor immunity.”

Environmentalists have also noted that more tuna in captivity will mean diverting fish supplies that could be eaten by humans. Tuna, said López, are “a very unsustainable species to farm, since 90% of the species used for fish meal and fish oil are food grade, meaning it could feed humans directly”.

More tuna farming would also mean increased use of antibiotics, which intensive farmers typically employ to avoid diseases spreading, given the relatively poor immunity of farmed animals.

Nortuna and Next Tuna have signed contracts with the Mazarrón research centre, Ortega said. Both companies claim to have plentiful space, which would allow low densities of tuna in the tanks. However, according to Claudia Millán, a fish welfare specialist with the NGO Equalia, even spacious captivity may be incompatible with the needs of a migratory species that crosses oceans to find food and reproduce.

Inefficient feeding practices can also produce toxic wastewater, said Salazar. “Left untreated, [this wastewater] can deplete surrounding waters of oxygen, causing algal blooms [or] dead zones, and public health issues,” she said.

The new breeding successes mean the supply of fertilised eggs could expand outside the species’ natural reproductive season, normally limited to about 45 days in June and July. By controlling the natural spawning triggers of water temperature and lighting, the researchers said they hope the reproduction period could last for 60 or more days.

Other tools to induce breeding include implanting female fish with a synthetic version of a hormone that causes them to release eggs for fertilisation.

Paul Sindilariu, a co-founder of Next Tuna, said the company’s farming plan involved “a closed system that will bring in seawater, but there will be no outflow, so no environmental impacts”. He said the firm’s model, known as a recirculating aquaculture system, would use floating cages that are on, but not in, the sea, and this would allow the company to control the water quality and temperature conditions in a similar way to a laboratory.

“The system has to be tuna-friendly,” said Andrew Eckhardt, who manages site selection and financing for Next Tuna. “They have to be comfortable. Our stocking density will be very low, less than 10kg [of fish weight] per cubic metre.”

The company said it plans to establish its own breeding programme and sell young fish to “grow-out” farms for fattening and sale. It aims to bring its first stocks from the research centre tanks to the new site at the port of Castellón de la Plana next year, with the goal of selling about 45 tonnes of juvenile Atlantic bluefin tuna by 2025, and 1,200 tonnes by 2028.

Nortuna also said it would keep stocking densities low. The company’s chairman, Anders Attramadal, said one of the attractions of the pilot site in Cape Verde was space. He said the company aims to get fertilised eggs from the Mazarrón research centre this year in order to produce saleable fish weighing between 12kg and 30kg by late 2024.

“We would buy eggs every week if we could. Right now, we can only get a six-to-eight-week supply,” he said.

Attramadal dismissed concerns about food supply, saying farmed tuna ate less than wild ones. “With formulated food we expect to be getting down to 3kg to 4kg of feed per kilo of [farmed fish] muscle mass,” compared with about 30kg per kilo of muscle for wild Atlantic bluefin, he said.

Eckhardt said that although the main food for the tuna would be fishmeal and fish oil, Next Tuna would be “working with our feed partners to add other ingredients, maybe plant proteins, algae, insects or krill”.

Nortuna said antibiotic usage would be minimal, only for brief periods if fish are sick, while Next Tuna said it would not use any antibiotics at all because they are incompatible with its recirculating aquaculture model.

Welfare experts said another problem with captive tuna was how to kill large, strong fish in a humane way. Recommended tuna-killing methods include shooting or stabbing heads with a metal spike, according to the World Organisation for Animal Health.

Next Tuna said it would sell young fish before slaughter, while Attramadal said Nortuna would “use the best humane approved methods that apply at the time”, adding that a “stressed, uncomfortable fish tastes less delicious”. According to a 2009 European Food Safety Authority report, when “Atlantic bluefin tuna struggle to escape before dying, they produce considerable amounts of lactic acid … resulting in severe degradation of the flesh, [making them] unsuitable for the sushi and sashimi market”.

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