We should be celebrating the revival of the bluefin tuna – but a ravenous fishing industry, backed by government and ‘science’, is already licking its lips

Over the past three weeks, I’ve been watching one of the greatest natural spectacles on Earth, here in south Devon. At a certain station of the tide, within a few metres of the coast, the sea erupts with monsters. They can travel at 45mph. They grow to 2.5 metres (8ft 2in) in length and 600kg in weight. They herd smaller fish – saury and garfish in this case – against the surface, then accelerate into the shoal so fast that they overshoot sometimes 2 or 3 metres into the air. Bluefin tuna. They are here, on our southern coasts, right now.

When I’ve mentioned this on social media, some people refuse to believe me: you must be seeing dolphins, they say. Yes, I often see dolphins too, and it’s not hard to spot the difference. They don’t believe it because we have forgotten that our coastal waters were once among the richest on Earth. Bluefin and longfin tuna were common here. So were several species of whale, including sperm, fin, humpback and Atlantic grey, and a wide range of large sharks. Halibut the size of barn doors hunted the coastal shallows. Cod reached almost 2 metres in length, haddock nearly a metre, turbot were the size of tabletops, oysters as big as dinner plates, shoals of herring and mackerel were miles long.

George Monbiot is a Guardian columnist