Denham,

Excellent topic and very complex both biologically and politically. After a 30 year career in fisheries enforcement, not biology, I have no love for gillnets. They are an efficient means of catching fish, but they are pretty much size specific, not specie specific. Under most conditions they catch anything that happens to contact them including sea birds, ducks, crab, shrimp, occasionally smaller fish species and are very wasteful under most applications. Carcassman has covered a lot of what you want to know, and is very knowledgeable. One of my main issues with gillnets is that they snag on under water obstructions, then are cut loose or ripped off, or have drifted away as in the case of some rivers. These "Ghost Nets" then continue to fish and since monofilament has an extremely long life, these ghost nets do considerable evironmental damage for years. Check the Northwest Straits Commission website for good data. You'll be amazed at the amount of destruction that they have done.
The alternative to gillnets would be seines, beach or purse, fish traps or just bidding on hatchery salmon when they get back to the hatchery which is rife with problems of quality, etc. The gillnetters have a very powerful lobby, so that is an obstacle. Another is that they are good guys.