Atlantic bluefin tuna (ABFT) are large, carnivorous ocean fish. They used to be caught relatively rarely, mainly by sports fishermen in North America. However, around the 1950s, Japanese consumers of sushi developed more of a taste for the fish, and a large aquaculture industry developed.

Historically, ABFT have been either caught directly from the ocean, or captured while young and fattened in ‘ranches’. However, both wild fishing and ranching pose sustainability issues, since they involve taking fish from the wild. Since 2001, there have been a number of EU-funded projects to domesticate bluefin tuna, i.e. to breed them in captivity.

This is already done with other types of fish, for example salmon and tilapia, which are raised on fish farms. But it’s more difficult with ABFT: they generally don’t spawn in captivity, as they require certain specific conditions to spawn.

However, scientists have developed methods to make ABFT spawn in captivity, through manipulating light and releasing hormones into the water to stimulate egg production in the fish. This means that it’s now possible to farm these fish through ‘closed-cycle aquaculture’: that is, we can breed them in captivity so that they don’t need to be fished from the wild.

This has been seen as a win for sustainability. But what about welfare? In this report, I first offer some background on ABFT. I then examine some potential welfare issues in ABFT aquaculture.

Main takeaways:

Many larvae (young fish) in hatchery projects die. However, this is also true in the wild, and hatcheries may become better at preventing some of these deaths in future, in order to be commercially viable.
Many of the conditions in hatcheries might pose welfare issues for ABFT, but more research is needed.
The main method of slaughtering large ABFT seems relatively humane; however, the main method of slaughtering smaller ABFT seems more distressing. It’s unclear how many ABFT are slaughtered using this crueller method.

What are Atlantic bluefin tuna?

Atlantic bluefin tuna (thunnus thynnus) are native to the Atlantic Ocean and Mediterranean Sea. They are very large fish: fully mature adults are 2–2.5 m (6.6–8.2 ft) long on average and weigh around 225–250 kg (496–551 lb). Atlantic bluefin tuna (ABFT) have been called ‘tigers of the sea’ because of their size, grace, and the fact that they’re carnivorous predators.

In their natural habitat, ABFT can navigate over thousands of miles of ocean. They can dive to depths of 1000m. They eat smaller fish and other sea creatures, generally hunting in schools.

Traditional aquaculture of ABFT involves ‘ranching’. Juveniles are caught in nets when they gather to spawn, and fed and fattened in large offshore cages. When they are matured, they’re slaughtered and sold for high prices.

Domesticating ABFT

However, ranching is not sustainable, since it involves removing ABFT from the wild. Although the International Commission for the Conservation of Atlantic Tunas (ICCAT) regulates tuna fishing by setting quotas, in 2009 their scientific advisors reported that ABFT stocks were probably less than 15% their original size.^[1]

Therefore, starting in 2001, there have been several EU-funded projects to develop ‘closed-cycle’ aquaculture for ABFT: the ability to breed them in captivity. DOTT (‘Domestication of Thunnus thynnus’) was the first such project in 2001-2; this was followed by REPRODOTT (2003-2005), SELFDOTT (2008-2011), and TRANSDOTT (2012-2014).^[2]

Since then, various entities have set up ABFT hatcheries across Europe, including both public research centres and private companies. More recently, in July 2023, researchers at the Spanish Institute of Oceanography (IEO) achieved the reproduction of bluefin tuna at an inland facility, in tanks.^[3]

In 2017, Jonah Van Beijnen reported that many domestication projects were halting or slowing down production.^[4] This may be because they were failing to breed enough tuna to be commercially viable.

Welfare in hatcheries

Closed-cycle domestication projects involve keeping tuna in hatcheries. In these hatcheries, breeding tuna (broodstock) are kept in special tanks where the light is manipulated and hormones are released, to mimic the conditions in which ABFT breed in the wild. This causes them to produce eggs. After this, the eggs and young fish (larvae) are transferred to a series of tanks inside the hatchery until they are grown, at which point they are transferred to cages at sea.

Mortality of young tuna

The mortality of larvae (young tuna) in TRANSDOTT, one of the early domestication projects, was extremely high, with only 0-0.44% surviving at 30 days after hatching. This seems shockingly high, but note that larvae mortality is extremely high in the wild as well; ABFT are an r-selected species, and only 0.1% of hatched tuna make it past their first year. It’s hard to make direct comparisons here; I (Amber) wasn’t able to find information about the number of young bluefin that survive at 30 days after hatching in the wild. Generally, we should also expect domestication projects to get better at keeping larvae alive over time, since they will struggle to be profitable if most of their stock die.

Currently, there are causes of larval mortality within hatcheries that don’t exist in the wild. For example, some larvae die by crashing into tank walls. Others perish when they are moved between tanks. Still more die from sinking deaths, where they sink and crash to the tank floor. None of these risks exist in their natural environment.^[5]

One could also argue that regardless of whether more larvae die in the wild or in hatcheries, ABFT farming is bad for fish welfare purely because it involves breeding more of a species where the vast majority of young die by default.

Day-to-day welfare

It’s difficult to assess the day-to-day experience of tuna in hatcheries; partly because conditions within hatcheries are sometimes kept private, partly because the science of fish welfare is not comprehensive. However, there are reasons to suspect that closed-cycle aquaculture may pose issues for ABFT welfare, and it’s important to monitor this.

Generally, undomesticated species experience greater stress in captivity and in response to human handling than domesticated species, who have adapted to this.^[6] Since we are at the beginning of the domestication process for ABFT, we should expect the tuna to be stressed by the large degree of human contact.

A Eurogroup for Animals report lists several common welfare issues that arise when fish are farmed. Some of these may be relevant for ABFT:

Space: in captivity, ABFT are confined to spaces far smaller than in their natural habitat (where they range over thousands of miles)
Water quality
Parasites and disease

There is also some evidence that fish are distressed by noise and unaccustomed vibrations, which may be hard to avoid in farming contexts, particularly inland.^[7]

Some relevant questions:

How exactly are breeding fish made to spawn in captivity? The Eurogroup for Animals report states that there are commonly welfare issues with breeding fish: for example, invasive ‘stripping’ processes to extract sperm and eggs, or hormonal injections.^[8] Van Beijnen also reports that early domestication projects had issues with broodstock hitting the walls of their tanks.
How much space do ABFT need to flourish? In a Guardian article, Andrew Eckhardt of Next Tuna is quoted as saying ‘Our stocking density will be very low, less than 10kg [of fish weight] per cubic metre.’ Is this enough space?^[9]
Which natural behaviours are important for ABFT, and can they be appropriately fostered in captivity?

The Eurogroup report discusses tuna ranching specifically and talks about how there are likely welfare concerns.

On the more positive side, it’s in producers’ interests to ensure that their fish have good welfare and are not distressed. It’s believed that stressed fish don’t taste as good, so they don’t fetch as high a price. So if welfare improvements can be found, farmers may be willing to adopt them for commercial reasons.

Slaughter methods

The most common way of slaughtering large ABFT is ‘lupara’. This involves shooting the fish in the head while they are still underwater. Lupara seems to cause minimal suffering; usually the fish die instantly, and there is not much crowding before death. (Crowding makes the fish stressed). 70-80% of large bluefin are slaughtered this way. The remaining 20-30% of large tuna are shot from outside of the water with a shotgun.^[10]

Smaller tuna are slaughtered in a way that likely causes them more suffering; the fish are crowded before slaughter, sometimes for several hours, which causes distress. They are then killed by being hauled out of the water and spiked in the head (this is known as coring). Fish can be mis-cored, meaning that they don’t lose consciousness immediately.^[11]

I couldn’t find out what proportion of ABFT are classified as ‘small’ vs ‘large’. Since the standard methods of slaughtering smaller fish seem much more cruel than standard methods of slaughtering larger ones, this seems like an important question.

As mentioned above, it is believed that if ABFT are stressed before they are killed, this makes their flesh taste worse (because struggling releases lactic acid). This means that the industry, at least in theory, has a commercial incentive to make slaughter as painless and stress-free as possible. This raises hopes that welfare advocates and industry may be able to collaborate to develop (more) humane methods of slaughter, though in practice, welfare doesn’t seem to be a big priority for industry.

Conclusion

Since the domestication of ABFT is still in its infancy, it remains unclear what welfare issues there may be for ABFT in hatcheries. Some welfare issues we know about are the fact that small ABFT on ranches are routinely slaughtered in a way that causes distress and non-instant death; the high mortality rates of ABFT larvae in hatcheries (partially from causes that don’t exist in the wild); and the fact that ABFT are stressed by noise.

On the more positive side, many things that are bad for fish welfare are also bad for companies. Companies will struggle to be profitable if most of their fish die; stress makes the fish less palatable, and therefore less valuable. So as the industry develops, it may have internal incentives to create better welfare conditions for ABFT. It’s important to monitor the situation as this industry develops.