If you are trolling on a boat outside the Fraser River mouth, you will encounter the odd coho salmon while catching chinook and sockeye salmon at the same time. If you are spincasting with a spoon in the tidal portion of the Fraser River, then chances are you will encounter more coho salmon than sockeye or chinook salmon during this time of the year. If you are barfishing with a spin-n-glow in the non-tidal portion of the Fraser River, then you will most likely catch more chinook and coho salmon than sockeye salmon. If you are flossing with a bouncing betty with long leader, then chances are you will hook a lot of sockeye salmon with the odd coho salmon in between.
To answer your question, you cannot completely eliminate the chance of encountering a coho salmon while fishing for other salmon species on the Fraser River, but you can certainly reduce the mortality rate caused by recreational fishermen. Coho salmon have a tendency to be deeply hooked when fishing with roe or other natural bait, especially in still water found in the tidal portion of the Fraser River. Mortality study done on bait fishing in the Lower Fraser River demonstrated that it has a huge impact on released coho salmon, therefore a bait ban is implemented during this time of the year.
Secondly, to reduce release mortality further, correct fish identification and proper fish release need to be practiced. If the released fish is kept in the water, unhooked and released quickly without to much handling, then it has a very good chance in surviving its journey. Anglers need to bring their fish in, identify it while it remains in the water instead of dragging it onto the beach while walking backward.
I have put together this information in a more organized manner on the following page:
http://www.fishingwithrod.com/fishy_news/100908-3.html