Ecuadorean officials are investigating the slaughter of 53 sea lions from the Galapagos Islands nature reserve, which were found with their heads caved in.
The Galapagos sea lions are not generally afraid of humans (Image: Galapagos Conservation Trust)
The dead animals included 13 pups, 25 youngsters, nine males and six females.
Galapagos National Park official Victor Carrion told AFP news agency that each was killed by "a strong blow from someone", though the motive is unknown.
They had not been injured in any other way, he said, discounting the notion they had been killed for their parts.
The animals were found in a state of decomposition on Pinta island, part of the archipelago which lies about 1,000km (600 miles) off the coast of Ecuador in the Pacific Ocean.
The Galapagos sea lions have no natural predators on land and generally do not fear humans.
Mr Carrion said no other dead animals had been discovered, but that patrols on the other islands would be increased.
He added that the sea lions were a vital link in the island's food chain and therefore any threat to them is a threat to the whole eco-system.
Sea lions are sometimes hunted for their skin, or the teeth and genitals of the male animals are removed for use as a supposed aphrodisiac in Chinese medicine.
In 2001 poachers killed 35 male sea lions in the archipelago, removing their genitals and teeth.
The islands' pristine environment is being endangered by tourism
However, according to Mr Carrion none of the animals killed in this latest attack had been mutilated and no cuts were found on their skin or limbs.
"It was a massacre whose motives the prosecutor's office must clarify," Mr Carrion told AFP.
The Galapagos Islands are known throughout the world as a home to unique flora and fauna, including exotic birds, marine iguana and giant tortoises.
The wildlife was the inspiration for British naturalist Charles Darwin's theory of natural selection.
The islands were the first place on the planet officially designated as a World Heritage site, but last year the UN Environment, Scientific and Cultural Organisation (Unesco), which administers the list of sites, declared the Galapagos "in danger".
Experts said the 19 islands and surrounding ocean were under threat from "invasive species", increased tourism and growing immigration.
In a statement, the organisation said international interest in the islands - which are Ecuador's most popular tourist attraction - was contributing to their gradual decline.