Federal officials are investigating the shooting deaths of six sea lions along the Washington state coast in what experts suspect is a plot by fishermen to eliminate competition for fish. A seventh animal was found decapitated.

A total of 13 sea lions have been found dead in Puget Sound since September, according to the Seal Sitters Marine Mammal Stranding Network. Those not shot likely died from unspecified “acute trauma” in interactions with humans.

All the deaths are being investigated by the National Oceanic and Atmospheric Administration Fisheries. It’s illegal to kill or harm sea lions, which are protected by federal law and penalties that include up to a year’s imprisonment and a $25,000 fine.

“We are concerned about a number of recent reports of marine mammal deaths caused by gunshots in the greater Seattle area,“ Greg Busch, assistant director of law enforcement at NOAA Fisheries, said in a statement. “All marine mammals are protected by the Marine Mammal Protection Act.”

The number of sea lions shot is six times the annual average for this time of year, according to Seal Sitters. Attacks typically rise in December, January and February.

“Sadly, this is just the beginning of what is likely to be a very deadly fall/winter season for sea lions and seals,” Robin Lindsey wrote online in the Seal Sitters Blubber Blog. “Animals searching for food to survive and fishermen searching our fish for consumption or livelihood are on an annual, never-ending collision course.”

Marine conservation group Sea Shepherd Seattle is offering a $5,000 reward for information leading to the arrest and conviction of anyone who shot the sea lions.

Sea Shepherd is offering a $5000 reward for the arrest and conviction of the killers of these 5 sea lions recently shot between Seattle and West Seattle. Please retweet!! pic.twitter.com/KSVA0NmINy — Sea Shepherd Seattle (@SeaShepSeattle) November 22, 2018

As many as 700 sea lions were found in California with gunshot or knife wounds from 1998 to 2017, according to National Geographic. Only a few people were charged. All were fishermen.