Darwin police have been called to the scene of what they thought was a serious assault, only to discover the man had been hit in the head by a catfish.

Duty Superintendent Louise Jorgensen said police received a call from a "very concerned citizen" reporting they had seen a man on Vestey's Beach, near the Darwin CBD, bleeding profusely from the head after a suspected assault around 9:00pm on Monday.

Police and St John ambulance crews attended to find a 45-year-old man, who told them he had been fishing with a friend when he got a bite from a catfish.

"He actually flicked the fish into his face and the venomous back spine of the fish lodged in the side of his head," Superintendent Jorgensen told the ABC.

"While he was writhing in pain and bleeding everywhere, the friend actually took the catfish and was admiring it."

Superintendent Jorgensen said the man was treated at the scene and police officers offered him advice, coinciding with Marine Safety Week in the Northern Territory.

"I am not quite sure he appreciated it," she said.

Superintendent Jorgensen said she could not confirm the condition of the man, other than to say "he was very embarrassed".