One answer comes from the Tuesday, Sept. 6, 1887 edition of The Daily Astorian:
• The Astorian this morning gives a list of over 100 salmon canneries on this northwestern coast. Any corrections, additions or alterations will be gladly received, our desire being to furnish a complete and accurate list.
Note: You can see the full list at bit.ly/pnwcanned. There were 39 canneries on the Columbia River, six in Grays Harbor, four in Shoalwater Bay, one on the Nehalem River, two in Tillamook, and so on.