Six sea lions have been found dead of gunshot wounds off the coast of Washington, including one that was decapitated, in the latest string of killings of the marine mammals in the region.

As NBC News reported, a total of 13 sea lions have been found dead along the Washington coast since September. Six of the sea lions were killed by gunshot wounds while another seven died of “acute trauma” believed to have been caused by humans, the report noted. One of the animals had been decapitated.

The killings have shocked wildlife officials.

“We are concerned about a number of recent reports of marine mammal deaths caused by gunshots in the greater Seattle area. All marine mammals are protected by the Marine Mammal Protection Act and OLE investigates all reported unlawful takes of sea lions,” Greg Busch, assistant director of the NOAA Fisheries’ Office of Law Enforcement, said in a statement.

Officials said some sea lions are killed every year, especially during the fishing season, but the spike this year has been unusually high, up six times the normal average. As the Seattle Times reported, there has been a dramatic increase along the coast in the number of California sea lions, which were once hunted almost to extinction for their pelts and for pet food. With the numbers back up to nearly 250,000 animals along the west coast, many have come into conflict with fisherman and sometimes killed when they try to go after salmon hooked on fishing lines. In the last 20 years, up to 700 sea lions were found dead with gunshot and stab wounds, National Geographic reported.

Officials have said they are worried that the killings may continue to increase, as November and December are the busiest fishing months.

Many people who live along the shore and watch over the lions have noted the increase in killings.

Six sea lions dead from gunshot wounds off Washington coast https://t.co/VVVmmOgj9Q via @NBCNews — Debbie (@Dangchick1) November 25, 2018

“According to those who live and work along the Elliott Bay and Duwamish waterfront, shots are being heard even more frequently this year,” Robert Lindsey of the organization Seal Sitters wrote in his Blubberblog this week. The group has helped to document some of the seal killings and works together with local authorities to track them.

Whoever is killing sea lions along the Washington coast could face serious consequences. As NBC News reported, sea lions are protected by the U.S. by the Marine Mammal Protection Act, which brings fines of up to $28,520 and up to a year in prison for each animal killed.