A resident dolphin will undergo an autopsy after being found dead at the weekend.
Tom was seen floating in the Serpentine River on Friday night and was quickly recovered by wildlife officers.
"This is a dolphin we have spent endless weeks following far up the river, way upstream beyond stakehill bridge," a Facebook post from Estuary Guardians Mandurah read.
"She was an experienced river goer and did not strand.
"We do lose a few dolphins in our region each year - strandings, illness, age. We are lucky this animal was spotted and retrieved in appropriate time for necropsy so hopefully we can find out what happened to dear Tom."
It comes just two weeks after the death of young Mandurah dolphin Luca.
RELATED: Mandurah dolphin dies after third entanglement
Peel Thunder's Bo Allan becomes an Eagle
Christmas on the Murray 2024
Man charged over Henderson hit-and-run crash
Manslaughter charges dropped against man accused of Mandurah boat tragedy
Rockingham boy critically injured in Bali quad bike crash
Christmas cheers returns to Mandurah
City of Mandurah launches new community safety strategy
Winners announced for the 2024 Alcoa Peel Business Excellence Awards
Mandurah: Three girls charged with aggravated armed robbery