Gillnets are very size selective, that is why the can work. Or fail. I understand that back in the 30s and 40s Chinook gear for rivers like the Skagit was over 8". Smaller fish, smaller web, like Smalma says.

There was an interesting situation of the Skagit in the 80s wherein coho needed protection. So, commercial fisheries for chum fished larger gear. It was over 6, maybe 6.5. Anyway, the netters (I and NI) got lots of chum and few, as in one or two, coho. Then, December came and management switched to steelhead. Smaller mesh and all sorts of coho were now showing up.

Bill Ricker noted that gill nets, sometimes, removed the smaller fish and left the larger. This occurred, in the fisheries he looked at, where sockeye were targeted and chum incidental. Small chum taken, what's left was larger.

There are a lot of balls in play and managers are often hard pressed to keep an eye on more than a few. Plus, the data is all "in the Past". It will take a couple of years, maybe a cycle or two, to see a trend well and then you layer onto that PDO and such.