A seal that had been trapped in a fishing lake since December died during a rescue attempt, the British Divers Marine Life Rescue (BDMLR) said.
The seal was first spotted in Rochford Reservoir in Essex on 12 December.
There were several attempts to rescue the seal, which was injured and had been consuming the lake's stock of fish.
The BDMLR said the seal was "darted" with an anaesthetic but "avoided capture" and died in the water.
"This is a very sad outcome from a very tricky situation," a spokesperson said.
"Rescuing any animal using anaesthetic holds significant risk, but in this case the coalition deemed it was worth taking."
Marine rescuer Simon Dennis previously said it was likely the seal made its way up the River Roach from the sea, via an inlet.
He said the harbour seal probably had "no incentive to leave" the lake as it had "found itself in a branch of Waitrose" and was "munching its way through the fish".
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The reservoir is owned by Rochford District Council but Nick North, of Marks Hall Fisheries, has been the licence holder for 11 years.
Mr North was concerned about the seal consuming the fish in the lake, which he had restocked and carried out extensive work on.
"To see all this lost due to a seal getting into a lake where it doesn't belong is ridiculous," he previously said.
The BDMLR previously said tranquilisers "do not work well with [seals]" and could result in drowning.
In a statement, the organisation said the rescue attempt was conducted by "a coalition of organisations" including BDMLR, Rochford District Council, South Essex Wildlife Hospital, police and the Environment Agency.
It said the seal "had an open wound on its rear flipper and an eye injury", and rescuers had hoped to treat the wounds and rehabilitate the seal before returning it to the sea.
"A specialist vet successfully darted the seal with a special dart gun and anaesthetic, but sadly, due to unforeseeable complications, the animal avoided capture and is believed to have passed away under the anaesthesia while in the water," the statement said.
The organisation said there was "deep disappointment and sadness that the operation was unsuccessful".
"Nevertheless, as the seal was under threat of being shot in order to remove it from the lake, it had been deemed worth trying to catch and relocate it in order to save its life," it said.
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