I think you'll be hard pressed to find any statistics on how fish behave when approaching a net. I tried a couple of google searches and did find some information, so you might try that. It will probably require a lot of reading to find it, but you may find what you're looking for. If I had to take an educated guess, I'd say that they probably try to swim through it when they encounter one. I say this based on the shape of salmon and their ability (or lack of ability) to move once the head comes into contact with a gill net. If the fish "see" the net prior to contacting it, then there's probably a 50/50 chance that they will attempt to avoid it. I think once the head of a salmon makes contact with the net it's committed to either moving forward or thrashing from side to side. Either way the fish becomes gilled since salmon really can't back up like people can.