Tuna: big fish, big industry, little tins.
How much do you know about this stream-lined predator?
Tuna both lines supermarket shelves in unremarkable tin cans and is a culinary delight served in the most exclusive sashimi restaurants. Its popularity overlays a complex technological, ecological and political balance that allows us to continue to enjoy this fish as an everyday convenience. Here is an introduction to tuna and a few tips on sustainable choices you can make.
Tuna live the high-life in oceans world-wide. This of course means they have also taken the fancy of hungry humans the world over. There are seven species that make up most of the tuna we eat:
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Bluefin (3 species, Pacific: T. orientalis, Southern: maccoyii, Atlantic: thynnus)
… classic sushi tuna
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Skipjack (Katsuwonus pelamis)
… fills most tins of tuna
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Albacore (Thunnus alalunga)
… or long-fin tuna due to its very long pectoral (side) fins
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Bigeye (Thunnus obesus)
… he sushi substitute since bluefin population declines
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Yellowfin (Thunnus albacares)
…named for its prominent bright yellow fins
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Some species of tuna are truly massive. The Atlantic Bluefin is the largest species, measuring on average over 2m and weighing over 200kg¹ (but the largest caught on record was 678kg²). In comparison, the Skipjack is relatively small at about 80cm long¹.
Fishing Methods
About 10% of tuna is caught with long-lines³. As the name suggests this involves a very (very) long line, with hooks that hang deep below the surface. Unfortunately, a lot of other sea-life also gets snared on these hooks (up to 20% of the catch), through the rate of by-catch can be reduced with the use of special hooks and lines.
Purse-seine is used to catch 66% of the world’s tuna³. Here, a large net, guided by a number of smaller boats, encircles a school of tuna. The ropes are tightened at the bottom so that the fish can no longer escape. The by-catch is very low (only 1%), as long as the boats only target free-swimming schools of tuna and do not use fish aggregating devices (FADs) to attract sea-life.
Pole-and-line only comprises 8% of tuna caught³ but this is the most widely recommended by environmental groups⁴. Tuna are caught individually, not en masse, so there is very little by-catch and it is much harder to over-fish stocks. But don’t get too romantic, this is still big business, with pole-and-line boats catching 100,000–150,000 tonnes of Tuna each year³.
Tuna farming/ranching
Part of the tuna caught by purse-seine nets are not sold straight away, rather they are towed to sea-farms, where they are fattened up to be sold at a higher price. However, ‘full-cycle’ farming from eggs to adulthood, so we are not reliant on wild stock, still eludes us for tuna. Japanese researchers have succeeded in raising some fish to adulthood but this process is still not commercially viable⁵. The history of this industry is fascinating and dictated by the quirks of tuna’s biology (this will be the feature of a future post).
Tuna are voracious predators and require a large amount of food due to their high body temperature and super-effective but movement-reliant gill system. Here is a great you-tube clip if you want to learn more. Currently tuna are mainly fed a diet of smaller fish, such as sardines, putting additional pressure on these fish stocks. Research on non-fish based food for aquaculture is on-going and pivotal to the future of sustainable sea-food.
Off of the high-seas, how can we make a sustainable choice at the store?
Check the label
It should state the species, the location caught and the method used. Having this information means that the company has tracked the source of its fish, the boats it buys from have recorded information on their catch and this has made it the appropriate authorities to set the catch limits.
Choose skipjack (or, second-best albacore)
Fish stocks can and will change overtime but at the moment skipjack stocks in particular are not showing signs of collapse.
Choose pole-and-line (or FAD-free purse-seine)
Environmental groups advocate for pole-and-line. Purse-seine is also OK but only if they state it is FAD-free, so the by-catch was minimised.
Be wary of other claims, badges and logos
These are often essentially self-awarded, generally without evidence to back up these claims, so mean very little. The always conspicuous claims about dolphin safety are almost entirely redundant. Most tuna fishing does not occur where dolphins are and they do not report data to confirm these claims. Even the most prominant certifier, the Marine Stewardship Council (MSC), has recieved criticism from environmental groups and the WWF. It has recently come under fire for its excessive use of small-scale fishing imagery in its self-promotion, misrepresenting its largely industrial connections⁶.
What about at home?
Now that you have run and checked your tuna tins you might be tempted to make a quick tuna sandwich but what to do with the rest of the tin?
- place left-overs in new container and cover it with a tight fitting lid, wrap or a layer of water or oil — this will stop it drying out in the fridge
- the best way to preserve long-term is to cook it into a meal and freeze it for another day
- fresh fish can be frozen, just place in a layer of water preserve the texture of the fish
- if the fish is already thawed cook it before freezing again
- if storing fresh fish in the fridge protect it from air but remember it will not last much more than a day
References
¹ FAO 1983, Scombrids of the world: An annotated and illustrated catalogue of tunas, mackerels, bonitos and related species known to date, vol. 2, accessed 8 January 2021, http://www.fao.org/3/ac478e/ac478e00.htm
² International Game Fish Association, Tuna, bluefin, accessed 8 January 2021, https://igfa.org/igfa-world-records-search/?search_type=CommonName&search_term_1=Tuna,%20bluefin
³ Internatioal Seafood Sustainability Foundation, Fishing Methods: An Overview, accessed 8 January 2021, https://iss-foundation.org/about-tuna/fishing-methods/
⁴ Pelle N (2018), 6 reasons to choose pole and line tuna, accessed 8 January 2021, https://www.greenpeace.org.au/blog/6-reasons-to-choose-pole-and-line-tuna/
⁵ Tanaka, Y., Kumon, K., Higuchi, K., Eba, T., Nishi, A., Nikaido, H., & Shiozawa, S. (2018). Factors Influencing Early Survival and Growth of Laboratory‐reared Pacific Bluefin Tuna, Thunnus orientalis, Larvae. Journal of The World Aquaculture Society, 49, 484–492.
⁶ Le Manach F, Jacquet JL, Bailey M, Jouanneau C, Nouvian C (2020) Small is beautiful, but large is certified: A comparison between fisheries the Marine Stewardship Council (MSC) features in its promotional materials and MSC-certified fisheries. PLoS ONE 15(5): e0231073. https://doi.org/10.1371/journal.pone.0231073
