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
Daniel Bandy appointed CEO of Peel Thunder Football Club
Dwellingup comes alive ahead of Dwellingup 100 and Mighty Jarrah Trail Run event
Man charged, others on the run after Mandurah Forum knife incident
Man critically injured in North Dandalup crash
Kwinana Recquatic's swim school recognised nationally
Rockingham Police: Locals advised not to approach wanted man
Concerns for missing girl
Peel Thunder cruise into 2026 WAFL grand final
Two teens cautioned, one charged over brutal Falcon attack