Mr Purdie said the tuna rounded up the garfish they were hunting into large shoals, known as bait balls, and then charged into them.
It forced the bait balls to the surface and prevented them from escaping, he added.
“This, in turn, attracts thousands of seabirds, including gannets and herring gulls, which swoop down to feed on the fish from above,” Mr Purdie said.
He added the display was also a reminder of the ecological value of apex predators as the phenomenon gave seabirds from Alderney an opportunity to get the fish to feed their chicks.
They would otherwise have had to travel further afield.
Mr Purdie said bluefin tuna numbers were rising across the UK after being virtually absent in the 1990s, thanks to a recovery plan set up by the International Commission for the Conservation of Atlantic Tunas (ICCAT).