I enjoy fishing those resident coho during the summer (July/early August) in the central sound (mostly MA 10).
After watching this site for a number of years I have found that there is little correlation between the winter south sound fishery and my success the following summer. Just illustrates how nomadic and unpredictable this neat and tasty fish are. Suspect that their unpredictable movements are a direct result of the movement and the timing of the that movement of their seasonal food sources. Would not be surprised that many of the coho you are expecting to find at this time of year are further to north and west.
Appreciate your post which is encouraging! It sounds like there are some resident coho just north of Marine Area 13. Resident coho which I caught last month in Marine Area 13 were also in the 12 inch range.