Two Hawaiian monk seals were found bludgeoned to death on a remote coast of Molokai; one was killed in mid-November and the other shortly before Christmas, according to the Associated Press.
In total, four monk seals have been found dead in the last three months.
On Jan. 2, a dead seal was found in Pilaa on the Northeast coast of Kauai. The young male had head wounds, as did the other dead seals.
"They're dying because their skulls are being smashed. So for me, that is alarming. That's setting a dangerous trend. If we don't stop it, we are going to be the cause of the death of these seals," said Walter Ritte, a community activist.
It's estimated there are about 1,100 monk seals left in existence in the world, declining at a rate of about 4 percent a year.
The National Oceanic and Atmospheric Administration (NOAA) proposed two initiatives last year to protect the seals - bringing them to the main Hawaiian islands, where there is less competition for food, and expanding federally protected zones to include parts of the main islands.
Neil Kanemoto, a local fisherman, told the AP that fishermen are unhappy with the new regulations, and may be taking out their frustration on the seals.
"[Seal deaths are] not going to end. My position is, if the feds keep pushing in this way, it's going to increase," Kanemoto said.
Under Hawaii state law, it is a felony to harm or kill monk seals. Those found guilty face up to $50,000 in fines and five years in prison.
ABC News affiliate KITV and the Associated Press contributed to this report.