We certainly do sampling of the returning stocks, including some DNA analysis. Once the sockeye get back to the river though, we're pretty certain about their origin.
I think the question is really about the situation in the ocean, where there are mixes of stocks coming back, and we need to protect the weak ones and fish the strong ones, but we can't tell them apart because they all look alike. That's when timing and location are helpful to have with a DNA analysis or coded wire tags. There are other scientific techniques to understand what fish you have encountered. That work is something we do invest in. It is incredibly important.
I take your point about looking more in the rivers, but at that point, we're pretty comfortable about what fish are there.