As an ex-commercial fisherman myself, PS gillnetter for 12 years, I question the fellow you talked to. Bycatch of steelhead, coho, kings is rare, at least by Puget Sound gillnetters. I never caught a cutt. Many factors involved, wrong time of year, mesh size, etc. We still would get something once in awhile. Not sure what type of gear your guy was fishing but for me when I got a steelhead or some other fish that I wasn't supposed to have it went home with me. The only thing I would throw overboard would be trash fish like rats and such. Spent some time sieneing and we would cook up anything like a steelhead for dinner. Still it was rare to get one even in a purse siene. I don't think steelhead mingle much with salmon.
Not sure what to think about the increase in bycatch percentages. Need to learn more. If this is indeed mostly about the Columbia River fisheries I can see where there would be more of a problem with bycatch.