TUNA ARE SPECIAL

The tuna family includes some of the largest fish in the sea. They have several remarkable features which fascinate biologists. They never stop swimming – if they did, they’d suffocate, because instead of using their mouth muscles to pump water onto their gills, they simply swim with open mouths. This is called ram ventilation and it’s efficient because it moves plenty of water past the gills very fast. But it does mean tuna can never rest.  Your average tuna swims 43 kilometres a day when he’s not trying.

Tuna gills are huge and very efficient and backed up by a big heart - a tuna’s heart is ten times bigger than that of other fish of the same size.  This enables oxygen to be pumped round the body in large volumes so that tuna can swim fast. The typical swimming speed of a tuna is two to three times faster that of a white fish (say, a cod) of the same size.

Tuna have a distinct and very efficient style of swimming called thunniform in which the head moves hardly at all and the power comes from the tail.

Fish are cold blooded – their body temperature is the same as their environment. But not the tuna. Whilst not being technically warm-blooded, tuna have a unique circulatory system which means they are warm bodied - typically 6 to 10 degrees C higher than water temperature. Warm muscles deliver more power in colder waters. 

Skipjack tuna is the one you can buy for pennies a tin in every supermarket. Albacore also turns up in tins, but more expensive ones, usually distinguished from skipjack by being termed white tuna. In countries such as Spain and Portugal where the cuisine carefully distinguishes each species of tuna from the others, albacore is known as “bonito del norte” and highly prized. (The name “bonito del norte” is potentially confusing because bonito is another species altogether – a cousin of the tunas). Of all the tuna, albacore has the highest fat content and its super-fatty belly flesh, marketed under the name “ventresca de bonito” fetches a tenner a tin. (All fish bellies have high fat levels, to keep their vital organs warm in cold seas.)

Bigeye (the deepest-swimming tuna has big eyes to see better at depths), yellowfin and bluefin are the three tuna most likely to turn up on your plate in a restaurant. Or were. Bluefin - overfished for decades and rated between near threatened and critically endangered on the IUCN Red List – has become an environmental cause célèbre. Many European and American western restaurants have removed it from their menus. There is much to be said about the bluefin, the stocks of which stand 95% below their level of 100 years ago. But this is not the place to say it. Here is an authoritative account.

Bigeye doesn’t get much of a look-in in Europe. Although caught in meaningful quantities in the Atlantic Ocean, virtually all is exported to Japan and to sushi restaurants in the USA. Thus, yellowfin is the tuna you’ll find in raw form (ie, not in a tin) in Europe.

GLOBAL TUNA MARKET

In 2016, The Pew Charitable Trust - an American NGO interested in marine conservation - commissioned an analysis of the global tuna market. This presented some interesting data on catch value and retail value:

*including 10-12,000 tonnes farmed across the two species

THE COLOUR OF TUNA

In terms of cuisine, the outstanding feature of tuna is its dark flesh. Fish, like mammals, have dark muscle and light muscle (think chicken leg and chicken breast). The dark and light muscles are known to biologists as red and white muscles. They create energy in different ways for different purposes (basically, red muscle provides steady endurance power - eg trans ocean migration - and white provides “burst” energy - eg escaping from a seal). The tuna has lots of red muscle – five to ten times as much as other fish. But this isn’t the muscle you eat. The part of the tuna which biologists label red muscle is virtually black and tastes very bitter. The consumer never sees this.  The red / brown tuna meat which has become so popular in the west over the last 20 years is technically-speaking the tuna’s white muscle… except it’s red (when fresh). The redness comes from myoglobin, a protein which stores and processes oxygen. Myoglobin is the same stuff that makes beef red. Exactly why tuna should have all this myoglobin in its red muscle seems to be a mystery that scientists have yet to solve. But we should be grateful because it gives tuna a distinct and attractive taste, more akin to meat than fish.

Red - just cut	Brown - two hours later	Unnaturally red - gassed

Perfectly fresh tuna is red but it turns brown within hours, or within weeks if frozen (unless "superfrozen - see below). This is due to a chemical reaction between the air and the myoglobin. This does not affect the taste, but it clearly reduces the visual appeal. However, this red to brown reaction can be arrested or even reversed by immersing the fish in carbon monoxide, which produces another chemical reaction. The tuna will then stay red (initially an unnatural verging on-neon red, if the treatment has been too enthusiastic) for a fortnight– long after it has become unfit to eat.  Carbon-monoxide treatment was commercialised 30 years ago as a means of making tuna look fresher for longer. The practice was banned by the European Union in 2008 but persists in some regions.

In its place, alternative antioxidants which are on the right side of EU legislation have been introduced to keep tuna red.

Good toro comes from large fish and ideally bluefin.  There is a limited circulation of bluefin toro in the UK.  In terms of sashimi from tuna, The Fish Society only sells akami from yellowfin tuna. You’ll find this on the maguro page on our website.

Tuna, being oily, is ideally kept frozen at a lower temperature than white fish. Whereas white fish is happy at -22C, frozen tuna is ideally kept at -60C. The very best frozen sashimi tuna is caught on vessels equipped to freeze the fish quickly down to -60C and is then kept at that temperature all the way to the sushi bar. This requires an expensive investment in specialised vehicles and powerful cold stores and in electricity to power them. Such tuna is known as “superfrozen”. It is the main form in which tuna is sold in Japan and a couple of European companies also distribute tuna in this form. The Fish Society now offers superfrozen tuna.

SUSTAINABILITY

Yellowfin tuna is clearly overfished.

Control of tuna-fishing is very difficult because the fish migrate so far including across the high seas where no country can even attempt to prevent bad fishing practice. Such management as there is rests with Regional Fishery Management Organisations such as ICCAT, the International Commission for the Conservation of Atlantic Tunas (it also looks after swordfish and similar species). These organisations depend on countries being willing to co-operate and within countries, their fishing fleets to co-operate with their governments.  ICCAT has a number of policies, such as a catch limit (currently 110,000 tonnes a year for Atlantic yellowfin), restrictions on the use of certain fishing techniques, observers on five per cent of boats, and registration of vessels targeting yellowfin.  Frankly, none of them are very convincing, but the Commission appears to be trying harder than ten years ago when it earned the epithet, The Commission to Catch All Tuna.

Perhaps more significantly, consumer opinion in western countries is slowly being mobilised, partly by Sir Richard Attenborough and the environmental lobby and increasingly by consumer-focused marine protection organisations, such as the Marine Stewardship Council. At the Fish Society we buy most of our fresh tuna from pole and line fishing fleets in Oman and the Maldives.

Fortunately, the tuna is fantastically fertile. A mature yellowfin tuna produces upwards of 20 million eggs a year. Only a few get past the first few weeks, but still that’s enough to assure its survival. But survival is not enough. There is probably enough common sense around, abetted by the sorry example of the bluefin, to ensure that yellowfin will not suffer the same fate.