Police are investigating allegations a jet skier tried to ram a pod of dolphins off the Gwynedd coast.
A member of the public reported the incident off Tywyn on Sunday.
Sgt Rob Taylor from the North Wales Police rural crime unit said this was "very concerning."
"Why on earth someone would do something like this is beyond me. It is almost like a game to them. Behaviour like this is disgusting and will not be tolerated, he said.
Police do not believe the man was local and said the witness was able to take down the registration plate of the person's car.
Sgt Taylor added: "Not only did he try and hit dolphins, he also aimed for gannets as well."