To elaborate on my observation: aprox 30 years ago in the summer you would see thousands of seagulls on Kalaloch. Perhaps 50-100 in eye view at all times. Over the last few years I've gone to Kalaloch in summer you are lucky to see more than 2 at a time.
Seagulls still exist obviously, but there numbers are drastically smaller. As they are basically rats with wings, perhaps it's just better trash management, but the observation still stands.
You said something far more alarming and also flat out wrong - that's what I'm refuting. This 2 or 10 or whatever tiny amount of birds at a time thing you are claiming to experience...doesn't match my experience whatsoever. I can provide more photographs if you don't believe me.
I can't speak to the pacific northwest 30 years ago so I'll take your word for it that there were more birds on the coast then.
Sorry you don't believe me.
Here's my own photo evidence from South Beach in July 2017. https://photos.app.goo.gl/JzporX8XmFFswgSu8
I didn't go in 2018, but 2016 and 2017 I was there for about a week. same empty beaches both trips.
You being there in Nov may be why you see seagulls, I never have been there in winter.
I don't have any photos handy from the 1980's to scan, but it truly is a stark difference.
I used to live in Olympia and would visit Kalaloch/Ruby Beach on a monthly basis - even camped there a couple weekends ago. There are seagulls(and Bald Eagles, and even Pelicans) all around there to this day. I even have photographic evidence(shot on a medium format toy camera right in front of Kalaloch Lodge earlier this year): https://i.imgur.com/hMwDuXs.jpg
I'm not sure what these birds' patterns are wrt to collecting food but a cursory dig in my photos shows that I've captured pictures of anywhere from a handful to hundreds of birds on this strip of coast on different occasions over the course of the last year and a half or so.