If the endangered species are caught by accident, they must be immediately returned to the sea. Peter Nason, 37, was volunteering at St Ive's National Coastwatch Institution when he spotted the tuna diving out of the water one after the other, putting on quite a spectacle for the onlooking crowd on October 23. According to Cornwall Live, this isn't the first time bluefin tuna have been spotted in Cornish waters. In fact, in August this year a huge fish - believed to be a bluefin tuna - was spotted near Porthcurno. This particular species of tuna is the largest in the world. These torpedo-shaped fish are built for speed and power and travel huge distances in the open ocean. They can grow to a maximum size of three meters and weigh up to 250 kilos.