Country diary: The breathtaking sight of a tuna taking flight in British waters

Bluefin tuna … ‘Dark on top, with fins like daggers, its silver underside is lit up by the sun.’ Photograph: Ray Harrington/Devon Sea Safaris

It is a perfect autumn day for a clifftop walk: sea and sky blend into a veil of baby blue, and the water is calm and dazzling. A motorboat unzips the ocean as we squint out over the bay.

My companion points to a dark line snaking along in the water. It seems to intensify for a moment, then erupts in a silver commotion. Through binoculars I can make out the frantic leaping of a shoal of eel-like fish. “Garfish,” he says. “Now just watch.”