Big fish. Last I knew lots of them. Bring a float tube. I've heard it's a hell March to get to the water. Can't say for sure though.
If you go to castle with the thought of catching lots and lots of big fish your setting your self up for disappointment...it's not a bad hike if you know what your doing but there are lots of stories of people never making it to the lake
If your not real experienced in big country I'd definitely recommend taking the road down to the lake
I was just on the castlelake trail a couple day ago and it all but gone...not a problem if your hunting the pummy planes with passes to trespass on the monument but for the average Joe it's a pretty good fine to get caught off trail
So I never expect to catch lots and lots of big fish anywhere. I've had some friends hike down into that lake, tell me how brutal the hike was and how good the fishing was. This was 7-10 years ago, so maybe things have changed, but at one point there was real good fishing there.
Your last paragraph confuses me.