More than a dozen California sea lions have washed up along the coast of Washington State in the last three months, prompting an official investigation into finding the responsible culprits.

IFLScience spoke with Seal Sitters, a volunteer organization that responds to reports of dead or stranded marine mammals, who said that the number of dead sea lions totaled 16 as of November 27. Of those necropsied, nine are confirmed to have been shot dead.

All of the animals are believed to have died by acute trauma, including human interaction (such as a shooting or boat strike) or a killer whale or shark attack. Most recently, a sea lion discovered last Sunday and another carcass found on Thanksgiving Day were confirmed to be shot with a gun, while an X-ray of a sea lion found lodged against a dock showed five shotgun pellets of two different calibers lodged into the animal’s skull.

Shootings generally increase with salmon runs occurring during the Fall spawning season, reports the Seattle Times, when both commercial and recreational anglers and marine mammals compete for catching, although the next few months are expected to see even higher numbers.

An analysis looking back at 10 years of NOAA National Marine Mammal Stranding data “indicates that the current confirmed number of shot sea lions is six times higher than the yearly average for months September to November,” wrote Seal Sitters, adding that this only reflects the number of found and reported animals. That number is expected to increase because the “high season for shot sea lions [December, January, and February) is still to come, concurrent with fish runs” when anglers and marine mammals alike compete for catching salmon.

The National Oceanic and Atmospheric Administration (NOAA) Marine Mammal Stranding Network is working in collaboration with the Washington Department of Fish and Wildlife Marine Mammal to respond to the incidents, which are in violation of federal law.

Killing a sea lion is in violation of the Marine Mammal Protection Act with crimes resulting in fines totaling more than $28,000 accompanied by up to one-year imprisonment. Anyone with information about a violation is encouraged to call 1-800-853-1964.