Having utilized seines I can safely say it is all about weight, gravity and numbers. If beach seining you must slowly bring the net in and I mean slowly. If you go fast the fish are jammed into the bow of the net and simply the pressure of the weight of captured fish injures the fish piled up against the web. Add current into the mix and it really gets dicey. In open water seines the same applies with the added risk of the weight of the fish lifted out of the water crushing those on the bottom to death. It does not matter if your marketing the captured fish but if your releasing fish it is a big deal. Watching videos of seiners I seriously doubt that any mortality rate is correct. Commercial harvest is driven by getting as many fish as possible in the shortest amount of time. What it is not about is slowly and carefully going about your business to insure you can release alive any unwanted catch. You do not make money with the last mentioned manner of harvest only create inaccurate data to enable commercial harvest.