A new study says the population of false killer whales in waters close to Hawai'i appears to have dramatically declined over the past 20 years.

It's not known for sure why the animals are dropping in number. But the academic paper says the reason likely has to do with declining food supplies and how the mammals are getting caught and injured on longline fishing hooks.

The report's publication in this month's edition of Pacific Science comes weeks after environmental activists sued the federal government for allegedly failing to prevent longline fishing fleets from accidentally capturing the animals off Hawai'i.

False killer whales are a species of dolphin that grow as long as 16 feet and can weigh over 1 ton. False killer whales look like killer whales, but they're almost completely black instead of black and white.

Robin Baird, one of the study's co-authors, estimates about 120 false killer whales currently live in waters up to 60 miles off Hawai'i's coasts. Another 450 false killer whales live farther out. "What we have are a variety of independent sources of information that all suggest there's a decline," Baird said in an interview.

The data "came together to present really a much more alarming picture," he said.

Baird's paper analyzes data from several studies conducted over the past two decades.