Surely there must be one, an adjective to describe a lake that which is well-stocked with fish?
Teeming.
Sounds familiar eh?
I don’t know of such a lake, but a location where fish are plentiful is a “hot spot”. I do remember hearing people say that a lake is “loaded with fish”. There’s nothing better than finding a hot spot. You have to catch’em as fast you can before others find out about it. Lotsa fun.
Does a word exist of the form “piscigenous” i.e. fish producing?
My. Well. I suppose that would do. 
Although the possibility of something fish-specific remains interesting…
Thanks.
I second “teeming.”
bountiful
Another vote for “teeming.”
You have 2 in your OP: bountiful and well-stocked.
I’d call it SECRET! As in “I ain’t telling nobody!”
