While I know that gill nets are viewed as the Devil's Tools around here, it is possible to GN either the SH or late Coho and fish them reasonably separately IF there is a decent size difference. For example, on the Skagit they (I and NI) fished for chum with a larger mesh and avoided the smaller coho. It was when they switched to small steelhead mesh that Skagit coho got slammed. So, again, if there is enough size difference they can be gill netted. Or, they can be fished using traps/rec gear with mandatory release.