Something to remember is that the speed indicated on your GPS is speed over ground (SOG), not the speed through the water (STW). When trolling for salmon it's the speed through the water that makes your lure/bait act correctly. For instance, when using flashers like Pro Troll or Qcove the recommended STW is about 2.5 mph. So if you're trolling down hill (with the tidal current) your SOG could be greater. Conversely, if you're trolling against the current your SOG could be less than 1 mph, but your lure/bait is spinning like crazy.
A lot of fishermen use the angle of their downrigger cable as a guide for the proper speed. The rule of thumb is about a 45 degree angle. However, that too will depend on the size of the downrigger ball and how deep you're fishing. The best thing to do is to hold the lure in the water next to the boat and adjust your speed until it's working correctly and then maintain that speed. If you're not getting bit, try trolling in a S pattern and see if you get bit on the inside or outside rod during turns. That will give you a hint as to whether you need to speed up or slow down.
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