When Atlantic bluefin tuna begin to appear in Scottish waters, you can be sure that the climate is changing. A huge bluefin tuna was caught, tagged and released off the Outer Hebrides in September, as part of a conservation programme to get a better understanding of stock density and migration patterns. Similar studies are underway in Ireland, Sweden, Denmark and Norway, where the fish have been increasingly seen in recent years. It seems that warming seas have attracted a surge of bluefin tuna to our waters. In the 1930s, the species was a common sight in the seas off Scarborough and was highly prized by big-game fishers. However, from the 1940s, the species began to decline and by the early 1990s had all but disappeared. 
The same situation has occurred in the Nordic seas. In the waters between Greenland and Norway, there was a spectacular collapse in tuna numbers in the 1960s, when the fish declined dramatically in just two years. But in the past five years, sightings of the warm-blooded fish have increased around northern coasts as sea temperatures have risen and abundant stocks of herring and mackerel have attracted the predatory species. Game-fishers have now petitioned the UK government seeking permission to catch the prized bluefin tuna. The Angling Trust want to establish a "catch and release" licensed fishery which they believe would have enormous economic benefits for areas like Cornwall. Scotland’s sport fishers will be watching developments with interest.
The bluefin is the emperor of all the tuna species. Last year, a single bluefin tuna sold for a whopping £2.5m. It was bought at auction in Tokyo’s new fish market by Kiyoshi Kimura, a Japanese sushi tycoon known as the “Tuna King”. Medium-sized and large individuals are heavily targeted for the Japanese raw-fish market, where all bluefin species are highly prized for sushi and sashimi. The Atlantic bluefin tuna has been the foundation of one of the world's most lucrative commercial fisheries for years and as a result, was for a time, heavily over-exploited. There were dire warnings that the stock was facing extinction. However, tough conservation measures and strict fishing quotas have seen a remarkable recovery in the stock.
At certain times of year, the Eastern Atlantic and Mediterranean can teem with giant bluefin tuna. They are one of the largest and fastest fish on the planet - they can weigh up to 1,500 lbs and can travel at speeds of up to 50 mph. These imposing predators grow to over 9 ft in length. They are slow maturing and live up to 40 years. But our insatiable appetite for the meaty tuna inevitably led to their over-fishing. Fishing for bluefin is strictly controlled by the International Commission for the Conservation of Atlantic Tuna (ICCAT). To fish for bluefin, a country has to become a contracting party with ICCAT. The EU is a contracting party and has distributed quota to a variety of Member States, such as France, Spain, Italy and Greece. 
Because the bluefin has started migrating northwards, Norway too has now joined ICCAT and has been awarded a small quota. However, British fishermen have no quota. UK vessels must not target bluefin tuna and if caught accidentally they must be returned to the sea, alive and unharmed to the greatest extent possible. For Britain to obtain a quota for this lucrative fishery post Brexit, they will of course have to join ICCAT and it will depend on whether the stock can withstand increased quotas. With the EU fleet facing a calamitous withdrawal from UK waters because of Brexit, they are clamouring for additional bluefin quota. The total quota for the Eastern Atlantic and Mediterranean has been increased to 36,000 tonnes for 2020, up from 32,000 last year, the lion’s share of which goes to EU Member States. But even with this hefty increase, British fishermen may still find themselves at the bottom of a long waiting list for quota.
The problem is that the increasing presence of bluefin tuna in our waters may have a detrimental impact on their ability to spawn in the Mediterranean, their most important spawning ground. That could have a serious impact on the abundance of the stock in a few years’ time. Every spring, vast shoals of bluefin tuna pass through the Straits of Gibraltar, from the Eastern Atlantic where they spend the winter, heading for the warmer waters of the Mediterranean, to spawn, each female laying millions of eggs. Hundreds of EU tuna vessels lie in wait. But EU Member States are not the only countries targeting tuna. Albania, Algeria, Libya, Egypt, Syria, Morocco, Tunisia, Turkey and even China, Taiwan, Japan and Korea, all have tuna fleets operating in the zone. Illegal spotter planes are often used to chart the shoals and modern tuna boats speed to intercept the fish, using vast purse-seine nets to scoop up entire shoals, which are then killed or herded into fattening ranches.
The annual global trade in tuna is worth an estimated £32 billion, with much of the catch going to Japan and bluefin playing a significant role. To meet growing demand, there is now a burgeoning industry in farming bluefin, where vessels are permitted to capture young, immature fish, which are then driven into large offshore cages and fattened until they are ready for slaughter, effectively ending their ability to breed and multiply. This sector has grown dramatically in the Mediterranean. It is reckoned that each summer 20,000 to 30,000 tonnes of immature tuna are caught in vast dragnets and towed to these tuna farms. Although most tuna farms act within the law, the fish's high value has caused some fisherman to exceed quota limits. 
The reappearance of bluefin tuna in Scottish waters may be hailed by some as a great new commercial opportunity, but we should be cautious. Following Brexit, Scottish fishermen will have an exciting new future to carve out that will involve sustainable and responsible catch limits across a wide range of species. Jumping on the money-spinning tuna bandwagon may once again help to place the majestic bluefin on the endangered species list.