Shoals of fish that normally inhabit warm Mediterranean waters have been seen swimming in the North Sea off the Essex coast.
Angler Richard Holgate, who has fished off Walton-on-the Naze pier for decades, said it was the first time he had been able to catch anchovies.
"In 40 years fishing from the pier I have never seen them before," he said.
The Centre for Environment, Fisheries and Aquaculture at Lowestoft said some fish had moved to the North Sea waters.
Soaring temperatures in England have allowed the sea off the Essex coast to warm "four degrees above average to around 21C", said a Met Office spokeswoman, which is similar to the water temperature of the Mediterranean.
"Over the past few weeks the water has been unusually clear and we noticed the fish in good numbers in the shade below the pier," said Mr Holgate.
"They weren't in open water but in the shallows.
"Black bream, ballan wrasse and pollock are seen here but they swim around from the south-west coast but anchovies are from the Mediterranean so are highly unusual."
The Centre for Environment, Fisheries and Aquaculture (Cefas) said: "Anchovy have historically occurred in the North Sea and numbers have been increasing in the southern North Sea, including the Thames Estuary areas, since the 1990s.
"It is thought that milder winters and warmer summers, amongst other factors, are supporting the anchovy population growth.
"A research paper was published by a number of organisations including Cefas in 2012, which looked into this in detail."