When I was younger, my Grandmother would take all the rough fish, such as carp and sucker, cut them into chunks, and can them in glass jars. When it came time to prepare them she would break them up, mix them with eggs and crackers, form them into patties and fry them. The same as people do with salmon today. When used in this context, the fish had no proper name, it was simply referred to as "fish", a mix of whatever unknown species happened to be in the jar.
In that respect, I agree that critters and fishies have something in common. Both are casual terms that denote generalities rather than specifics. Good analogy Sniper.
