As many as 30 large whales have washed ashore in the Gulf of Alaska since May. The massive deaths prompted the U.S. National Oceanic and Atmospheric Administration (NOAA) to declare it as an “unusual mortality event.” The agency said in a news release that it would open a formal investigation into the mystery that is killing so many giant marine animals.

30 Whales Die Of 'Unusual Mortality Event' In Alaska

Scientists concerned over the death of whales

In the last four months, 14 humpback whales, 11 fin whales, four unidentified whale carcasses, and one gray whale were found along the coastline of Alaska. It is three times higher than the annual historic average, according to Dr Teri Rowles of the NOAA. Rowles said scientists were “very concerned” about such strandings.

Officials in British Columbia have also reported the death of six large whales, which is unusually high. The NOAA said there could be even more unreported animals. Most of the dead whales were spotted floating and beyond retrieval. Some others have been decomposed or scavenged by bears. So far, the NOAA has been able to take samples of only one whale, while Canadian authorities managed to collect samples from two.

Is Algal bloom killing whales?

The NOAA spokeswoman Julie Speegle said one possible cause of the fatalities could be harmful toxins released by a large algal bloom spanning the western coastline of North America. However, there is no conclusive evidence yet that the algal bloom was behind the massive deaths of whales.

Teri Rowles said it was a challenge to investigate the cause of these killings because of the difficulty in accessing the floating carcasses that are rarely beached. What’s more, bears feeding on the whale carcasses that washed ashore could be a threat to the safety of scientists looking to collect samples. Bears have been spotted feeding on several dead whales.

The NOAA said its investigation could take several months. Humpbacks and fin whales are endangered species. In May, marine scientist Kate Wynne said nine of the fin whale deaths were puzzling, especially because the carcasses were found in a very small area.