The carcasses of four gray whales have been found around the San Francisco Bay Area in eight days -- an alarming string of deaths that constitutes an unusual mortality event, defined by the National Oceanic and Atmospheric Administration (NOAA) as a significant die-off of a marine species.

Marine researchers don't know the reason for the spate of whale deaths, the most recent of which occurred Thursday.

More data is needed to determine how or if the ships factor into the unusual mortality event, though, he said.

A similar die-off happened in 2019

A similar die-off occurred in 2019, when Boehm said three gray whales were found dead in one week.

It's unknown whether 2021 will see a similar die-off, or if the most recent series of deaths is related to the 2019 unusual mortality event, Boehm said.

San Francisco loves the whales that travel by twice a year, Boehm said.