It may have swam off, but that doesn't really = survival. Most fish that are going to die actually will die after they have been back in the water a few minutes. It's called delayed mortality. Another thing to remember is that fish was in the river for one reason, to spawn. A 30 minute battle takes away a lot of stored enegry that they need to make their migration and to spawn successfuly. Since they are not feeding in fresh water, they rely on stored proteins and energy from their salt water life. During the battle it probably used up a lot of this energy, thus greatly decreasing it's chances of spawning. The fact that you put in on the rocks, wrapped it in a net, and had it out of the water for a minute, is even worse. I hope you don't treat all the fish you catch like this.