Coho are actually significantly more plastic than recently thought. When there is enough escapement, you get age-0 smolts. There are fry, fingerlings that rear in the estuary, and fall smolts. At least the fall smolts return either 12 months later (2 year old) and smaller than average or 24 moths later as much bigger age-3's. There are also 2 year old smolts; these can be quite large. Finally, in AK at least, there are smolts that return to freshwater in the fall once or twice, overwinter, and then go to the ocean. These would be 4 and 5 year old adults and probably fairly large.

The idiocy of marine mixed stock fisheries has been known and ignored for quite a long time. It was certainly part of the first exposure I had to salmon management in college.