A porpoise was found washed up at Langstone Harbour.
The beached animal, which was found by dog walker Tina Place, is thought to have died of natural causes.
The 49-year-old, of Eastern Road, said: ‘I have a holiday home on the Eastern Road and I walk my dog along the seafront.
‘I spotted what I thought was a big fish, when I got closer I thought it was a baby dolphin.
‘It was very sad to see it washed up like that.’
The mammal has now been taken away by Portsmouth City Council.
Waste and disposal manager at the council Barni Fry said: ‘A porpoise was washed up on Langstone Harbour, which had clearly died a while ago.
‘The coastguard contacted Portsmouth Natural History Museum, but because the mammal was so badly decayed they felt it wasn’t necessary to run further tests.
‘The porpoise has now been cleared from the beach.’
Experts say it had probably been dead for a long time.