Aside from damaging impact on marine environment, the tuna fattening industry which is centred on Malta – the island has the largest tuna farming capacity in the world – is probably the most wasteful type of farming that exists.

Adult bluefin tuna is caught as it migrates into the Mediterranean to spawn. It is rounded up in purse-seine nets and towed to the tuna pens, which are floating cages. The highest concentration of these tuna pens are found in four farms in Malta.

The tuna is kept in these cages for a few months, where it is fed mackerel and herring. The point of ‘fattening’ them is to increase their size to some extent (they weight gain depends on their size at capture), but most importantly to increase the fat-to-meat ratio of the tuna’s flesh to a level that is most valued in the sushi and sashimi market in Japan.

Yet the conversion ratio of tuna – the amount of feed that is needed to produce a given amount of weight gain – is very low. The Food and Agricultural Organisation (FAO) of the United Nations reports that the feed conversion ratio of the Bluefin tuna in the pens during fattening can be as high as fifteen to one and as low as forty to one, depending chiefly on the weight at capture (“large tunas do not increase mass as much as they increase the fat content”). In this range a tuna has to consume anywhere between 15kg and 40kg of fish, mostly mackerel and herring, to gain 1kg of weight.

Tuna meat sliced in steaks. (Copyright Victor Paul Borg)

In a paper published in the British Journal for Agrarian Change, Stefano Longo and Brett Clark wrote: “The metabolism of bluefin tuna requires high inputs of energy (calories) in order to increase body size and weight. Thus, the food conversion ratio of bluefin tuna is at least 20:1 or as high as 30:1, which translates into a net loss of energy during this production process.”

Tuna in Malta’s farms is mostly fed imported mackerel and herring as well as mackerel and sardines caught locally by the fishery known as tal-lampara, which consists of 13 vessels, down from 18 vessels a few years ago.

Mackerel and herring are good quality, edible fishes. They are also delicious if cooked right. So, instead of being fed to tuna, the mackerel could be eaten directly by people. You can feed a lot more people if these eat the mackerel that instead if caught in the wild to be fed to tuna.

At a time when fishing industries worldwide face the risk of collapse due to overfishing, there is something anachronistic, illogical and problematic in the tuna fattening industry. Sustainability and judicious use of resources suggests that in an era of overfishing and hunger, mackerel are best directly fed to people.