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Old 07-09-2009, 11:18 PM
Blake Blake is offline
Join Date: Mar 2001
Posts: 11,049
There are several species of dolphin that live exclusively in fresh water, and several more that are marine but where some populations live their entire lives in rivers. The there are animals like belugas that are marine but where many individuals spend about half their lives in fresh water. Then we get the other whales that at least occasionally come into fresh water.

Whether there are issues with going from salt to fresh water is a matte of some debate. Certainly many baleen whales that have spent prolonged periods in fresh water seem to develop skin infections, but this may be because only sick animals spend prolonged periods in fresh water.

What we can say is that for the toothed whales the transition from marine to freshwater lifestyle has occurred so many times that there would appear to be no significant barrier. So it seems more likely that the baleen whales don't live in fresh water because there isn't the food to support them.