I know the answer and I don't get Quizmonkey's clue either. I don't know fish either so I just looked at lists of fish and found one that fit the letter clue and then looked at pictures of the fish to make sure.
First thing I think of when I hear this fish is a certain cloud formation: look up "(fish name) sky" and you'll get it. Sorry, thought it was a very common phrase!!
Sussed the dam too - I was looking at a very similar one on the same river, not named after a pres. Apologies for sounding so thick!