Okay, here's a wild butt uneducated theory.
In our area there is almost zero bull kelp. I know of only two very small beds in a two hour boating radius around us. This means the bait fish and game fish have no suspended structure to hang out in so the only place they can find protection from light/current is bottom structure. When the tides are running the only place I find fish winter/summer is 120-180' and the bait is all in a thin layer close to the bottom. In the summer on slacks I find fish shallow as the bait balls go shallow to feed.
The oldtimers in the area say the inside strait used to be thick with kelp beds which provided spawning material for the herring as well as structure. Not sure why there is no kelp anymore other than the no seas otters, so tonnes of urchins, so no kelp theory.
It's a theory but I wouldn't bet money on it....