A spectacle known as a “boiling sea” caused by bluefin tuna going into a feeding frenzy has been captured on film.
The phenomenon happened due to large numbers of the Atlantic bluefin tuna hunting together, Alex Purdie from Alderney Wildlife Trust said.
A video recorded on 8 September off Alderney showed the water churned into a white circle of breaking waves while hundreds of seabirds gathered above.
“Witnessing a shoal of tuna feeding is an unforgettable experience – our own oceanic equivalent of a pride of lions hunting wildebeest,” Mr Purdie added.
The marine biologist said the footage was captured by a drone flying at a safe distance of at least 50m (164ft).
He explained that tuna had returned to Alderney’s waters in recent years, and the footage marked one of the first times such a frenzy had been filmed from above.
Tuna feeding in these numbers was once a common sight before fishing decimated their populations and it was “remarkable to see our seas beginning to recover”, he said.
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).