Dead mussels and clams coated rocks in the Pacific Northwest, their shells gaping open as if they’d been boiled. Sea stars were baked to death. Sockeye salmon swam sluggishly in an overheated Washington river, prompting wildlife officials to truck them to cooler areas.

The combination of extraordinary heat and drought that hit the Western United States and Canada over the past two weeks has killed hundreds of millions of marine animals and continues to threaten untold species in freshwater, according to a preliminary estimate and interviews with scientists.

Record-breaking temperatures hit the Pacific Northwest at the end of June, with an all-time high in British Columbia of 121 degrees. British Columbia reported at least 719 people suffered “sudden and unexpected deaths,” three times more than what would normally occur in the province during a seven-day period. The Harley Lab at the University of British Columbia is looking into the mass mortality event on the shore and trying to determine just how many animals died, how animals that survived were affected, how it happened and how likely it is to happen again.