Your fact and your speculation for the day
I learned from Wikipedia that "an estimated 12 million Chinook and Coho salmon fingerlings died when their hatcheries were destroyed" when Mt St Helens erupted in 1980.
This is the most vertebrates I can think of that were killed at once. Maybe if you count all the chickens killed due to avian flu fears as "at once," then you could come close. They're definitely in the hundred thousands.
