If the coho return was low too wouldn't that point to the ocean temps?
Ocean survival and ocean productivity/conditions are extremely complex and to my knowledge it appears we still have a poor handle on accurately correlating the two. Add in things like the complex/dynamic life histories of steelhead and chinook, it gets very difficult to predict or explain good years from bad ones. Steelhead can return to spawn after 1-4 years at sea, so steelhead returns tend to be more stable due to the buffering effect of their varied life histories.
One tool we have and use is correlating primary production to the time of salmonid entrance to the ocean. According to a DFO contact of mine, this past years coho did indeed experience poor ocean conditions during their entrance to sea. Our coast is massive however, so we can't expect every system to experience a given trend.
Excited to see how this steely season pans out.