Right -- as Celeste pointed out, we have lists on the Web.
http://7seasfishing.com/cc.html shows all the custom fishes people have listed for their areas. I update it based on information I receive in SL or see in the forum.
http://7seasfishing.com/fish.html lists all the official fishes.
Both lists are moving targets so it'd be tough to have a literal checklist! You can sort the
http://7seasfishing.com/cc.html list by date so you know which entries were updated recently, at least.
If I were a fishing area owner I would definitely include a link to my particular fishing area's custom fish list either in my profile, in my signature, or both.

We can't make the forum do that automatically though.