Necropsy results on 10 of about 50 seals found washed up dead showed they were bashed with a blunt object.
"What we do know was that this was a vicious act of cruelty that was absolutely criminal in nature and must be prosecuted by the authorities," said Rebecca Aldworth, executive director of the animal rights group in Canada.
The necropsy also showed that not all the animals died instantly from their injuries and that all were left to freeze.
The seals were found dead or dying by a group of veterinary students near Beach Point.
In a release, the group called the deaths a "massacre."
Humane Society International said the Department of Fisheries and Oceans was forced to euthanize many of the seal pups.
They are calling for charges under both the Fisheries Act and the Criminal Code of Canada.
The deaths were not connected to any sanctioned seal hunt so charges could be laid.
Officials from DFO are investigating.
They are encouraging anyone with information to call Crime Stoppers.
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