The three-week-old pup was noticed on a riverbank 15 miles from the sea by Michael and Charlotte Poole and their five-year-old daughter Tilly, who named the seal Sid.
They were taking their two dogs for a stroll when they spotted the seal struggling to climb into the grounds of Tattershall Castle, north of Sleaford.
The seal is believed to have got lost as it travelled up the river system while on the hunt for food in the River Bain.
After struggling out of the water, it began hauling itself across fields to the moated 15th-century castle, which is run by the National Trust.
Mr Poole said: “This is one walk we will always remember. It was only when we got closer that we noticed the seal in the water. It was distressed because of the weir which meant it couldn’t get any further.
“It didn’t know what to do but it somehow managed to climb on to the bank although you could tell it was struggling. It looked exhausted. Once it came out of the water towards us it went into a field and headed towards a steep-banked dyke. I blocked its path to stop it because it didn’t think it would get out. It tried to bite me and hissed at me.
Sid the seal in Lincolnshire (SWNS / Jack Evans)
“It was using its flippers to propel itself forwards. It was most bizarre. A young seal was the last thing we expected to see here.”
The family then called the RSPCA, who rescued the seal pup on Boxing Day and transported it to East Winch Wildlife Centre in King’s Lynn, Norfolk.
Mr Poole, a project manager from Coningsby, said there was “silence on the other end of the phone” when he told the RSPCA where they were.
He added: “I don’t think they really believe it. The seal must have swum up the Witham from the Boston and The Wash and then into the Bain. It must be 15 miles – and all against a flood tide. We were astonished.”
Kate Burris, the RSPCA inspector who rescued the seal, said: “It was definitely unexpected to be contacted about a seal pup at Tattershall Castle. The people who spotted him and called us couldn’t believe what they were seeing.
“The poor pup was trying to get into a small dyke when they noticed him. We think he had come down the weir and climbed up the bank.
“He’s a male grey seal and aged around three-weeks-old. He came in very underweight, at just 14kg, but is doing well. He’s very feisty and has been on fish soup until today when he started on whole fish.”
The seal will be with them for a few months because the release weight for seal pups being returned to the wild from their specialist wildlife centres is 40kg, she said.
In September, a one-month-old seal pup was noticed on a cliff in Pembrokeshire by a member of the public who got in touch with the RSPCA.
“This was quite an unusual place to find a seal as she must have travelled some distance and up a steep incline to get there,” Ellie West, RSPCA animal collection officer, said. “However she got there, she was very much lost and unsure where she was going.”
Additional reporting by SWNS
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies