Men wearing camouflage clothing and with their faces covered have been spotted shooting at waterfowl on a bank of the River Don in Doncaster.
South Yorkshire Police said reports have been received of men shooting on the Fishlake side of the river.
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Officers want to hear from anyone with information about those responsible.
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A spokesman said: "Please be aware we have had a couple of incidents reported whereby men in camouflage clothing and having their face covered have been seen shooting at the waterfowl bank of the River Don - Fishlake side.
"The site is next to a public right of way and the flood plain of the River Don. The Environment agency have improved the habitat on the flood plain to encourage wildlife.
"This has been successful and birds of conservation concern are in this area. Whooper swans and Shelducks are now resident on the wetland."
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Anyone with information should call South Yorkshire Police on 101.