There is a long long tradition of fishing in the dark, not that I have done it much! Basically there is almost always some ambient light particularly if the sky is clear and the moon is out. What's more fish and salmon and trout in particular are used to looking up to find their food which will be backlit at night by such ambient light. Same holds true in very low water visibility. Some food items also have bioluminescent properties and fish will key on that. Bait has scent properties that fish will find. Nighttime sturgeon fishing was popular until  fishing was restricted to daylight hours. The lateral lines of fish are also sensitive enough to direct fish to food items that send out even small pressure variations and fish can find in conditions of low to nearly no visibility.

'91, 2001 and 2003 all exceeded 2011. Total run size in 2003 was over 24 million. https://psc1.shinyapps.io/salmon-run-size-app/

Looks like there was a good segment of the run that came in according to the average timing. If you look at page 12 of the briefing notes for Sep 16th you can see the difference between 2011 and this year plus the 'long right tail' of this years return

https://www.psc.org/wp-admin/admin-ajax.php?juwpfisadmin=false&action=wpfd&task=file.download&wpfd_category_id=721&wpfd_file_id=19385&token=&preview=1

Pretty decent numbers but probably won't break the 2011 run size record when more than 20+ Million were accounted for.  Lots of them in the Vedder now.

17 million pink salmon in season estimate

in the latest report

There are other types of fishing beside float fishing where it's all done by feel (ie spoons/spinners) and the fish can certainly see/sense your lure in lower light conditions

Then there is fly fishing for coho in frog water. They take those flies with gusto in pitch dark. Eh, I miss those days, but I've given up on the early start. No matter how early you get to the river, there will already be people in your chosen spot. I'm not giving up on my beauty sleep anymore. It's banker's hours for me now.

It seems to vary from river to river from any searches but nothing seems flow specific.

FWIW, it's not obvious but there's a glossary of definitions page linked on the region 2 salmon regulations page.

https://www.pac.dfo-mpo.gc.ca/fm-gp/rec/definitions-eng.html

To be fair, the regs page is potentially confusing because some water/species entries specifically say "one hour before sunrise to one hour after sunset", while all the others don't say anything. It implies some might be different (why point it out in only certain waters) when it fact they are all the same.

1 hour before/after but in reality the fishing starts when you're just able to start seeing your float.
People who start casting at the 1 hour before mark is just stupid imo, when you cant see your float and the fish cant see your presentation.

There are other types of fishing beside float fishing where it's all done by feel (ie spoons/spinners) and the fish can certainly see/sense your lure in lower light conditions

1 hour before/after but in reality the fishing starts when you're just able to start seeing your float.
People who start casting at the 1 hour before mark is just stupid imo, when you cant see your float and the fish cant see your presentation.
95/100 times from my experience you're just wasting bait (if you're using it), but to be fair that still leaves you with a ~5% chance.
I have found the longer the people wait to make the first cast, the better the fishing is, but in a busy spot that time is almost always consistent when it is right as you are able to see your float.

way back in the 1970's we fished the capilano very early in the morning like 1:30am until 5am for coho's. Caught many limits in the dark. Crawling over the rocks using flashlights to see where you were going and also to see what you had hooked. Zero issues with fish biting back then. Like Roeman says, it's all about poaching. The rule only came into affect after sockeye snagging started and guys were going out near midnight and staying until early in the morning to be able to go home with 2 limits

Its got nothing to do with seeing your float or wasting bait.  Its to help reduce poaching. 

Was out last night on the dropping ride and managed 2 to the net lost a couple and had some good hits. Still fish streaming through.