I guess I say "Art" because I do believe there is a bit of skill involved to do it properly. And I also say netting a "salmon" because a lot of these fish seem to know what a net is by the time they make it up the river where we, in turn, try to net them again.
I am not professing to be one that knows everything about netting a fish, but every year it amazes me to watch, what seems to be avid fishermen, not really know what to do when it comes to netting a salmon. I will only state what I believe is a "good" way to net one, but if what you do seems to work just fine, then no need to read any further. I am only posting this as a way to help those that may be having some difficulty, as I've seen far too many fish lost do to waaaaaay too much effort put forth into trying to net a fish.
First off, like I said, although I believe even a small rainbow, or dolly would be leery of seeing a net come his way, and would try his best to avoid it. But a salmon on the other hand, has potentially been swimming and dodging nets for miles and miles and knows what to look for...or should I say look "out" for. So, when the fish seems tired, rolls to it's side, and is about ready to net, do not let the net, or should I say the mesh, or bag, into the water as the fish approaches. You don't want to give the fish anything else to see and be afraid of in its panic. This can be accomplished by just barely grasping a bit of the mesh and holding it lightly against the handle just slightly below the loop. While waiting for the fish, basically point the loop of the net at the fish holding it just above water level. I really don't see any reason to put any part of loop or net into the water at all.
Secondly, when you make your move, don't drop the loop and mesh into the water under, over, or behind the fish and try a big broad sweep to try and "scoop" it up. If you do this you are immediately fighting the current of the river against the loop and the netting which can be substantial. When you try to "scoop" up the fish this way it is usually way too slow for a net savvy fish. Instead, wait until the fish comes to the surface, hold the loop and mesh above the water until the very last moment, when, at the same time, you let the mesh loose from your fingers, and with the loop, try to "knife" the water as close to fish as possible with a very quick and easy little "jab" type scoop, where the loop just barely dips under the water and the fish. By knifing the water with the loop, there is almost no resistance, and you are basically just using the loop to make a quick "hook" around the fish surrounding him with the mesh.
After the fish is in the net, no need to really lift the net and fish high out of the water. Just lower the loop against the netting closing it off, and pull it towards shore. Many times, especially if you are to release the fish, I just like to grab the mesh itself and hold it and the fish against the shore. While still in the water, the fish will remain calmer while removing the hook.
This may sound complicated and I may not be explaining it well, but it really is quite easy. Personally I have netted even kings by myself this way much to the amazement of onlookers. Again, not because it was some great feat, but just because many have never seen it done this way. Just think of this....the less movement you make with the net, the less chance you have in loosing the fish. If done properly, usually netting a salmon can be done on the first or second try.