Attackers have clubbed 23 fur seals to death, including newborn pups, at a New Zealand breeding colony re-established two decades ago after the species was nearly wiped out by hunting.
Some of the eight bludgeoned pups were just days old, said conservation minister Kate Wilkinson.
She appealed for assistance from the public to help track down the attackers.
"To go around and club 23 seals to death over a number of days is very deliberate and you have to question the state of mind of someone who can carry out such a cruel and abhorrent attack," Ms Wilkinson said in a statement.
The Ohau Point seal colony in the South Island is a popular tourism spot north of the town of Kaikoura, which is an international whale-watching centre.
The Department of Conservation said the attacks took place over as long as two weeks. The location may have delayed the discovery.
The colony stretches about 1.2 miles along the coast and is at the bottom of a steep, 100-foot cliff with no easy access, department spokesman Rory Newsam said.
Fur seal numbers are gradually increasing in New Zealand after the species was nearly exterminated by hunters in the 1800s. The medium-sized seals are not native to New Zealand.
The Ohau Point colony was only reoccupied for breeding in 1990, and about 600 fur seal pups were born there in 2004, said Bruce Robertson, a seal specialist and senior lecturer in zoology at Otago University.
He said the long-term losses for the population could be dire. The attacks killed 13 breeding females, which meant 13 pups dependent on their milk would die and fewer pups will be born next year, Mr Robertson said.