Why do we sometimes use plurals?

Obviously much to much time on my hands, I was watching a you tube video where the fisherman kept referring to the fish as herrings. It got me to thinking that for the most part the name of the fish is also it's plural (1 bluefish or 3 bluefish). However why do we say 1 porgy and 3 porgies or 1 shark and 2 sharks? Can anyone think of any other fish where the plural ends in s or es or ies?