Seems pretty straight forward? Don't target sockeye!!! While in the salt I'm not sure what can be done differently but in the river as long as you are not BB and specifically targeting pinks/coho/sockeye you are good to go...
Who says they are protecting coho?
Seems they are giving a opportunity (when many things are closed) to fish for Chinook. Doesn't appear they are worried about the pink and coho salmon but very concerned about the sockeye. Open to catch and release of coho and more so pink salmon and the numbers may very well increase on the river I'm guessing. I'm sure the coho mortality rates are going to be quite low hence the opening.
On a side not the north arm appears blue on one of the DFO map... Zoom in and it's not but why the use green =salt, blue=fresh, and... Blue out line, lol. Almost fooled me as I know better but you'd think they'd use a different colour as barrow spots appear to be fresh water, lol. Got to love DFO's ways of doing things, lol.