First, the Siletz is CLOSED to all fishing "from 900' below Siletz Falls" and above. This closure also includes the "North Fork Siletz, South Fork Siletz, and tributaries of these streams." The quotes are directly from the 2010 ODFW reg book.
Secondly, in addition to hatchery-stocked Cutthroat (that become Blue Backs), you may also encounter Summer Run Steelhead too. The Siletz is also the ONLY Northern Oregon Coastal stream to still have a NATIVE Summer Run.
The river is ONLY open for "adipose fin-clipped steelhead the entire year". That means that ALL NATIVE fish must be released unharmed and returned safely into the water.
In regards to trout:
Daily Bag Limit: 5 fish per day
2 daily limits in possession (maximum)
8 - inch minimum length
Only 1 trout over 20 inches may be kept per day
A Rainbow Trout over 20 inches is a Steelhead (you need a Salmon / Steelhead tag to keep them)
Salmon under 15 inches are considered trout, except Coho Salmon
Flies, Artificial Lures, or Bait are all allowed
The area above Moonshine Park is private land and owned by the Georgia-Pacific company. Sometimes there are closures, so it is recommended that you call the company before you go. Sorry, I don't have their number.
Hope that this helps. Good luck!