I have no idea of how deep salmon will go, but I can tell you what I witnessed off the ORegon Coast.
During the 80's while commercial fishing, my Dad and I went 20 plus miles off of Tillamook and sent two of our deep lines down to 80 fathoms (480'), and were catching 10lbs brown bass and large chinook off of the bottom spreads 60-80 fathoms. We were using a silver canadian spoon off the canonball and plugs and/or bait on the other lines.
I only know the depth because this was so much different than the 30 fathom (180')depth we normally fished. We would normally catch chinook at the 20-30 fathom mark and silvers from the top to 20 fathom line.
Note for those not familiar with commercial fishing:
Fathom=6'
Spreads=the number of lines hooked to a commercial fishing cable. Normally we would fish one line every 12' on the main cable.
Just my experience,
V. Green
nwfish@angelfire.com