We could provide you with that information. It's primarily the New England Aquarium that does a lot of this tracking. Sometimes an animal is essentially declared dead because it hasn't been observed for a number of years. The level of surveillance in Canadian and U.S. waters is sufficient to justify a scientific assumption that the animal is deceased after a certain amount of time.
These are obviously estimates. We don't see every animal every year. It's not a hard number like a census, but it is a fairly accurate assessment.