Salmon fishing in many BC rivers is a community event, where often times dozens of anglers will string out in a long line 20 or 30 feet apart for their shot at a salmon. During the height of salmon runs this is the norm rather than the exception, and its best you know about it before showing up. If you arrive and there are a bunch of people already fishing, first take a moment and size things up. Look for any natural breaks in the line of anglers where you could slip in to fish without interfering with others, or, alternatively, if you have a spot you would like to fish and someone is there, you'll need to respectfully wait until that person, or persons are done and have left.
Wayne, seen here finding a spot to fish amongst a long line of anglers
When a stream is very busy, which does happen, look for a spot where there is a little more room between anglers and then ask both the upstream and downstream anglers if it is OK to fish by them. If they both give you the green light, go ahead and start fishing. Do not assume, however, this will always be the case. Sometimes a buddy of theirs may be somewhere downstream chasing a large, hard running salmon, or perhaps this buddy had to step out for a few minutes, so the space you thought might be available, is in fact, not. Using courtesy and being respectful of others, I've found that even though the river may be busy, most everyone is good natured and more than willing to make room. In kind, when you are that angler in the river and someone would like to fish by you, please try to make room for them. That way everyone gets a shot at catching their salmon and having a great experience.
Being respectful of others will ensure space and room for all