Um, maybe worrying about "ballot stuffing" really doesn't matter
that much. If "sperm whale" spikes off the chart for votes but the general chatter is for "herring", perhaps the vote is skewed. But in general I think you'll find the results will track general chatter and putting a vote validation control in place isn't worth the effort. After all, I'm sure there're other user input streams with their wish lists, etc.
I'd be very surprised if there weren't some definite themes to requests. My guesses are "dangerous" fish (sharks to morays to sea snakes to...?), "pretty" fish (various colors, markings, etc. depending on whether someone's experience is keeping fish, diving, or cruising public aquariums), "odd" fish (frogfish? stargazers? flying gounards?), and while we're at it, how about vertibrates like... mammals (how hard can a seal be?
).