On the original list, in addition to a few species of fish, there are also other species of manatees,orca, dolphins, whales. I have no idea why they were allowed.Im pretty sure it is just the manatee on the list and yes I dont think it should of ever been on the list but for some reason it got an exception. To your point about fish having less of an impact than manatee that is true but allowing people to add new aquatic animals will only poison your data set and make fish eventually worse than the manatee.
The most important thing here is consistency if the rules for this part of the list vary dramatically from the original then there is no point in using the old data because all its doing is skewing results.