An unwell, aging seal resting on Great Yarmouth beach was harassed and attacked with stones on Tuesday.
Animal welfare groups and the Norfolk Police were called to the beach on Marine Parade but were unable to help the seal.
The large 7ft male seal was found on the beach at around 12pm and large crowds were seen gathering close around it.
Daniel Goldsmith, chairman of Marine Wildlife Rescue, attended the scene.
Mr Goldsmith said: "We were called by a concerned local business owner after people started to gather around the seal. We contacted the police, as we work with them as part of Operation Seabird, working to reduce the disturbance of wild animals.
"People were trying to touch it and some people were throwing stones. The police had to be very active in trying to keep crowds away. We were bewildered at people's actions, putting themselves at risk and threatening the safety of the seal, despite uniformed police officers in attendance.
"Upon arriving we realised that we needed to euthanize the seal, which would be the most humane course of action.
"We needed to do it as soon as possible, otherwise it would end up at sea and its suffering would be prolonged. The crowds that had gathered made it difficult to do this safely.
"We contacted Great Yarmouth Borough Council in order to assist with the removal of the seal after it was euthanised but they said they could not do so without advanced notice. We hoped to return later that day so the crowds would be quieter and hoped that the council would help then.
"Unfortunately they still couldn't. We returned at 4.30pm and the seal eventually went back into the sea, where it was seen floating limply but was not really swimming.
"It may take several days or weeks for a seal in this state to die and will likely turn up elsewhere along the coast in a poor state."
Great Yarmouth Borough Council said: "We received a call yesterday from a man who was on his way to Yarmouth to potentially put down a seal that had been found on Yarmouth beach.
"We were happy to assist and advised that the council could collect it from the beach once it was dead, but not before as the council does not handle live animals.
"The council advised we would be able to collect the dead animal the following morning. Normal council policy is for collection of animals within 24 to 48hrs, dependant on their location and how easy the access is to the site."
For Mr Goldsmith, it is "not best practice" to leave dead animals in situ for long periods of time if it can be avoided, as it is unpleasant for the public.
A spokesperson from the RSPCA has said relevant authorities are continuing to monitor the seal, adding: “It’s really important that members of the public stay away from the seal as getting close is likely to cause this very poorly animal a lot of distress.
"We were shocked to hear reports that some people had been throwing stones.
“The best thing people can do is to leave the seal alone - it is very important they give them a wide berth and the space that he needs to rest unstressed. For your own safety as well as the seal’s, we recommend leaving as much distance as possible from seals - but if they’re looking at you, you’re too close to them."
Norfolk Police's Operation Seabird has been helping to protect nesting seabirds, seals and other wildlife since its launch in June.
PC Chris Shelley, rural crime lead, said: “Our work with other organisations, charities and agencies to protect the seals continues, and we will be increasing patrols in hotspots.
"Seals are a protected species, and nobody should be disturbing their habitat for any reason. Our best advice is always to keep away from them.”
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