As the Museum prepares to commemorate its 100-year involvement with the UK Cetecean Strandings project, a dolphin and a porpoise experience different fates.
A dolphin seen swimming in the River Dee in Wales a week ago has successfully been rescued and redirected back to the sea.
The female short-beaked dolphin, Delphinus delphis, was probably chasing fish in high tides to feed on them, when it lost its bearings.
It briefly became stuck on sandbanks before being rescued by coast guards from the Royal National Lifeboat Institution (RNLI), who hoisted it into a lifeboat on a stretcher and released it back into the sea.
High tides and an abundance of fish probably pulled the dolphin off course © Environment Agency
Museum cetacean strandings officer Molly Clery said it was very lucky because if it hadn't have been rescued, it would very likely have died.
Dolphins need deep salt water to find food and this dolphin was probably struggling to find its usual prey in fresh water. It may already have become ill.
The Museum is currently involved with another reported stranding on the Thames embankment by Tower Bridge. The animal in question appears to be a harbour porpoise, Phocoena phocoena.
Unfortunately the animal has already perished, having been lost in the Thames for four or five days.
There are around 300 cases of porpoise strandings along UK coasts every year and they appear quite regularly in the Thames.
Dolphin pods swim in occasionally but tend to manage to turn around and take themselves out again. A dead humpback whale came down the Thames in March this year.
In 2006, a 5-metre long whale became stranded in the Thames and died after a massive rescue attempt. Its skeleton is now used for research at the Museum.
The Museum's strandings project is part of the UK Cetacean Strandings Investigation Programme. This year the Museum will commemorate 100 years of monitoring stranded whales and dolphins.
Museum staff collect the bodies of stranded ceteceans and take them for post-mortem examination to discover cause of death, as well as monitoring levels of disease, toxins and pollutants and parasites.
It's not always clear why animals get lost and become stranded but it is usually because of disease or noise pollution that causes distress or disorientation.
If the porpoise recently found in the Thames is in good enough condition, it may be used to provide data on how porpoise populations are changing.
A day-long Museum conference, A Century of Strandings, takes place on 20 September in the Flett Events Theatre.