Decreasing size at maturity is an unmistakable sign of over exploitation of adults prior to the onset of maturation. This happens to ANY fish population where exploitation is high. I have personally seen this in a variety of fisheries from walleye and perch in Michigan to cod in New England to Chinook salmon on the Columbia. The symptoms are the same and so is the cause. Overexploitation.

The reason is simple. Harvest does not allow the adults to reach a large size at maturity. They get harvested before they reach a large body size. As such, the spawning stock becomes primarily those fish that naturally mature at a smaller size since those that naturally mature at a larger size (thus spend more time in the size range targeted for exploitation) get caught. Repeat this pattern for 50+ years, and the spawning stock will become progressively smaller and smaller. This happens to fish stocks anywhere that exploitation is high. The solution is equally as simple. Stop fishing for these fish on their feeding grounds. Wait until they reach their terminal age/size at maturity before initiating harvest.

I don't think the gill nets in the Yukon Rv are to blame for the decrease in size at maturity. The fisheries in the ocean are more likely the culprit.