A severed shark head stuffed with cigarette butts was found impaled on a fence at an Australian marine rescue office.
Volunteers at Shellharbour near Sydney found the head of the mako shark when they arrived for duty on Sunday morning.
Marine Rescue NSW's Bruce Mitchell said it was the "first time I've ever heard of it anywhere".
The shark was said to be fairly fresh and likely to have been caught on Saturday.
"Whoever caught the shark brought it back to the boat ramp and cleaned it, and cut the head off."
He said that the whole creature would have weighed about 70 kilogrammes (154 pounds).
"Maybe someone else came there during the night, found it and thought they'll have a bit of fun. That's one possibility," Mr Mitchell said.
He added that the volunteer organisation, which helps boaters during emergencies and other situations, had not received any threats and did not report the incident to the police.
Marine Rescue posted photos of the head on Facebook and Twitter and said the incident was a "kick in the guts for our committed volunteers who give up their time to save lives on the water".
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Others also posted their outrage on social media. One Twitter user wrote: "Takes one messed up human to do something like that."
A posting on Facebook said: "Someone with a macabre sense of humour."