My understanding is that they don't memorize the route. What they do seek is the specific particular smell of the water source they come from, which reflects the unique mineral mix (and probably also organics). Which is how they can not only locate the right stream but smell is also how they know it is time to move in from the ocean/bay after a rain that causes the river to rise enough for passage.I think if we had less fishing pressure in the oceans, there would be higher chances that fish could overwinter in the ocean enough to grow to that size but even right now theirs pressure towards more jacks and smaller fish.
It'll be cool to see what happens on the Elwha River in Washington since they historically had a huge run of salmon and June hogs before the dams were put in but since they've been removed and more habitat has opened up maybe they'll grow bigger again. As well as if the Klamath Dams finally stop getting delayed and are finally taken down and the 400 miles of spawning habitat behind them are opened up.
I think it is interesting to think about (and if anyone has the answer do tell) Do the fish change feeding behavior and grow bigger according to predetermined genetics or to where they are born in the river?, because I read somewhere that fish picked out of the water in one spot and moved downstream as smolt skipping various parts of the river, they have a harder time migrating back to where they were hatched, which I think implies that they know the river and learn it as they go down.
The Marmot Dam on the Sandy was removed almost 12 years ago. The upper Sandy River and it's tributaries from the junction of the Salmon River have a fishing ban on them. This includes the whole Salmon River, the Zig Zag River, Still Creek and others. The idea was to create a anadromous fish sanctuary. None of this has produced an increase in the Sandy River fish runs. Even the once famous smelt stopped showing up. That is a fine river and a lot of work has gone into habitat. Wish I knew what the problem was, but removing the dam wasn't the magic cure .