If we let this situation continue as it is, we do not know how severely fish populations will be harmed in the future. By that time, our environment may have suffered a great destruction.
by that time指的就是will be harmed这个将来时间。may have suffered为may的推测性用法,推测在that time之前可能发生的情况。注意by that time决定了 may have done的时间指向。例如, By the end of next year, Tom may have gone to America. 明年年底,汤姆可能就已经去美国了。