The reference was in Fisheries, I'll see if I can find it. They were obviously running a whole stream study as they quantified the smolt number.

You point out the problem, though, with how release studies are conducted. Simple survival, generally over a relatively short time, is the metric. One problem is that you can't both catch a fish and not catch it.

Say a caught hen spawns at RM 50. It spawned, Great. That same fish, is unspawned, would have spawned at RM 60. Or not. But if that is what would have happened had the fish not been caught, then the resulting fry lose 10 miles of habitat to live in. Important if rearing is space limited.