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Eight fur seals are known to have been shot dead and the deaths of another 11 are being treated as suspicious. The seals are believed to have died around late October or early November.
A spokesperson said DOC was first alerted to the dead seals at Ōhau Point on October 25. Six dead seals were found there and the other 13 seals were found at two sites in the Half Moon Bay area.
South Marlborough operations manager Phil Bradfield says DOC is offering the reward as it views the killing of at least eight legally protected fur seals very seriously.
"We're asking the public to help us find and convict those responsible for this heartless shooting of seals.
"The evidence we have indicates at least two firearms of different calibres were used to shoot the seals and it's possible more than one person may have shot them," he said.
The penalties for harassing, disturbing, injuring or killing a marine mammal under the Marine Mammals Protection Act 1978 are up to two years' imprisonment or a fine of up to $250,000.
Although a cause of death wasn't established for 11 of the seals, it is considered suspicious that the seals died grouped closely together.
The six dead seals in the Ōhau Point Fur Seal Sanctuary weren't closely examined to avoid disturbing pregnant females and young pups in the breeding colony.
The reward is on offer for one month, up until February 12. More information about the reward can be found on the DOC website.