I confess, I still use bait
So Ii'll give my insight, since my first experiences fishing for stelhead was in the bait arena. I'd say everyone is right on when they talk about uses of a ciircle hook. Ii've used them actually, and thy are great for BAIT. Not sure aboout a fly though.
Now with feeding. I've heard many things over the years I personally feel it depends on what fish you're fishing for and how long it's ben in the river. Say your salmon. They'll readily feed on egs and sanddshrimp in flux zone and parts of tribs if fresh from salt (usually bearing sealice). I've caught many kings and silvers with food in their stomachs, and caught them using bait (tha's if the're fresh). Now, same fish, sealice gone, and colors showing, then you need to tick them off. Spinners and plugs always give a good HARD strike. From my experience, same is true so far for stelhead, winterruns that is. You get one fresh from thesalt, and it'll hit eggs and sandshrimp. Caught a 22lb buck on the Nooch in Mmarchh on sandshrimmp. Iit engulfed the thinng. Luckily it was a brat and a keeper. It actualy had some eggs and some misc. shrimmp already in it's stomach (partialy digested, so cuodl't tel what it was for sure). Summerruns, I've heard conflicting reports abouut them too. But Ii've caught many with stomachs filled. The coolest thing I ever saw was about 20 years ago. Watched a stelhead slam and at a mouse thta hd fallen into the river. Had also caught a few with mice in hei stomachs (this was back in the day we kept our Nates). A favorite fly is a mouse pattern tossed next to th bank like a hopper. Have scored a couple steelhead this way. Pluus th old favorite periwinkles (stonnefly larvae I do believe). Was fishing the lower Tilton in WA for trout, whe my rod went berserk. Was fishing periwinkles. Landed a beautiful 12lb hen. I'm lucky sh didnt bust my 4# maxima. She had a belly full of them. As you can tell, if I have a keeper, I always dissect the stomach when I'm cleaning them.
I'm not a scientist, but these are y observations. Thats why alot of my tidal river/trib mouth flies are similar to aquatic insects/creatures. But my upriver fliies are big, flashy, osetimes gaudy things to tick them off or make thm realy curious. Just my thoughts.