Wildlife officers have noticed a disturbing trend around Perth's lakes and rivers with native turtles being trapped and left to drown.

The Department of Parks and Wildlife is asking the public to report people trapping turtles after a series of incidents in which oblong, or long-necked, turtles (Chelodina oblonga) have been found dead in nets in the past year.

Dead oblong turtles at Sherlock Park in Jane Brook.

DPaW senior wildlife officer Rick Dawson said at least 30 dead turtles had been found in nets.

"Wildlife officers believe that turtles have been caught and stored in nets for collection at a later date," Mr Dawson said.