The secret to successful coho fishing is to understand the fish itself. Coho are very skiddish in nature, and are notorious for developing a condition called "lock-jaw". Although lighting conditions, mentioned above, may not be the only thing that can put down a school of coho, it is a "risk-factor". Other risk factors include angler pressure, water conditions, absence of cover, as well as the presence of other fish (e.g. Chinook). So it makes sense to target these fish accordingly. For us, it means using as "stealthy" of a presentation as possible, including the use of fluorocarbon leaders, small hooks, small weights, and clear / sensitive floats.
People often ask my what I'm using and I'll show them. However, I find it more valuable when I ask to see their gear - when it's 15 lb maxima ultragreen leader and a 2/0 hook with an enormous piece of green wool, I can give them an answer as to why they stand little chance of hooking a coho.
When it all adds up, coho end up being the most challenging species of salmon to catch in LM rivers like the Vedder/Chilliwack. However, under the right conditions, coho fishing can be "lights-out" (excuse the pun). I focus on the "little things" that add up to the lock-jaw condition... LOL.