East Sussex Wildlife Rescue and Ambulance Service (WRAS) said they were contacted, along with British Divers Marine Life Rescue (BDMLR), at around 5.45pm on April 26.
East Sussex WRAS said initial reports were of a harbour porpoise in the shallow water trying to strand itself on the beach.
According to the service, members of the public tried to help the animal but as the tide reached its lowest the porpoise stranded.
A spokesperson for East Sussex WRAS said, “Medics from British Divers Marine Life Rescue were the first on the scene and kept the porpoise upright and wet out of the surf until a team from East Sussex Wildlife Rescue & Ambulance Service arrived shortly afterwards.
“Sadly the porpoise passed away on the beach.”
According to the service the adult porpoise, which was approximately 142cm in length, was not in brilliant body condition.
The animal had flat lumber muscle, meaning the porpoise had not been eating well – which East Sussex WRAS said may be a result of injury or illness.
The porpoise’s body was removed from the beach and taken to East Sussex WRAS’s casualty centre where the body will be collected by the UK Cetacean Stranding Investigation Programme based at the Zoological Society of London.
A spokesperson from East Sussex WRAS said, “Sadly cetaceans strand most frequently due to injury or disease.
“If anyone comes across a dolphin or porpoise in the water struggling or out of water on the beach, they should not attempt to re-float it back into the water.
“These creatures are mammals so they can breathe out of water. Often people make the mistake of holding them under water thinking they are fish, but this often causes them to drown.
“It is very easy for well-meaning people to do the wrong thing, so we would urge anyone interested in knowing more about what they can do if they find a stranded whale, dolphin or porpoise to attend one of BDMLR’s marine mammal medic training courses.”