At around 9 am, volunteers from the British Divers Marine Life Rescue were called to the area after the animal was discovered sleeping near La Frégate Café.
It was taken to New Era Veterinary Hospital, where it was found to be just 17 kg – around half of what it should have weighed.
Donna de Gruchy, the BDMLR co-ordinator, said that they would have preferred to leave the animal alone but due to forecasted severe weather, this was not a feasible option.
It is thought that the pup might have arrived from the Minquiers and been washed away from its mother.
Meanwhile, Jersey could be hit by severe-gale-force winds on Friday in what could prove to be the worst storm of this autumn to date.
A deep area of low pressure is due to sweep across the Atlantic, bringing gale-force eight to severe-gale-force nine winds to the Channel Islands.
The worst of the weather is expected overnight Friday. Jersey Met forecaster Sharon Merren confirmed that it could be the windiest spell of weather so far this autumn.
‘The details could change but, as things stand, we are expecting gale-force to severe-gale-force winds later tomorrow.
‘We are also expecting a period of rain, which could be heavy, so it is not looking too great. There are some bonfire events scheduled for that night so people should keep checking the forecast.’