the combination of the thin mono & 1/4, 3/8 ounce jig allows the jig to drop fast when the rod tip is lowered or dropped really that gives the jig it’s falling & rising motion that the fish attack. Not really possible with fly line. At least not the same vertical rising/falling steady motion. And that’s the trick it has to be steady at the same time as your reeling it back towards you. So the jig is not only moving up & down but swimming back to shore. Very tough to achieve with a fly line unless you’re in deeper water like from a boat. You can mimic this technique on rivers with deeper pools but then the jig fly is a bugger to cast. One thing I would try is using the sequin in front of the fly to get erratic motion. The Danish sea trout guys use this plastic cone thing in front of the fly to get some very nice wiggly motion. I want to get some myself & try it on Salmon. Try a cone head fly on maybe a sink tip line, intermediate lines, or a type 3-4 full sink line. A fly tied with rabbit, marabou or even flash might work. It’s got to be wriggly & have movement. Also try some scent on the fly like herring, anchovy, shrimp etc. works better on rabbit fly’s. With coho I always say bring everything you got including the kitchen sink. You never know what will work on certain day or hour. Up here on the Snohomish River the guys bring everything in their tackle box cause no one knows what they will bite on — when the river is open of course. Hey at least you have fish there & if’s open for fishing.