I'm Floudering Up Here

There are various fish around the DLC - they talk and tell a joke and spit out some weapons afterwards.  Listen to 25 for the achievement.  There aren't actually 25 fish so may need to save-scum a little - but this also means you haven't got to find them all; you can just scum the one in the lodge repeatedly...

...and the easiest way to do this is just to do the fish every time you go back there through the DLC (and after finishing the DLC) and you'll get there soon enough.