All saltwater areas are open to cutthroat fishing under catch and release regulations. You can find cuttthroat in saltwater at almost any location, year round, with late spring to fall being the most productive times. Good saltwater flies include baitfish, amphipod and euphausid shrimp patterns, but don't overlook attractors like the Reversed Spider, fish them with a floating or intermediate-sink line. The time that cutthroat enter the rivers depends a lot on the individual river. The Stilly, for instance, gets a few fish as early as mid- to late July, but it's pretty spotty before mid-August. The fishing usually holds up until the river blows out in the fall. In some places, like the mouths of the Skagit and its sloughs, there can be good fishing in April when the cutthroat (and dollies) come up to gorge on the downstream migrant humpy and/or chum fry. I've caught cutthroat as early as June in the Bogachiel and as late as February at the mouth of Pilchuck Creek. You just have to get to know your river. My number-one-favorite freshwater pattern is Mike Kinney's Reversed Spider fished on a floating line.
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PS