Yup, that's pretty much exactly it. As for our regulations, Fish and Game determines when to open a region for a specific fishery, in the example we're using, i

Yup, that's pretty much exactly it. As for our regulations, Fish and Game determines when to open a region for a specific fishery, in the example we're using, it's purse seining, as you've correctly gathered. The decision to open or close is made based on "escapement" numbers, i.e. the estimated number of fish who have made it back up their origin river to spawn. In the case of hatcheries, that's easy to determine, but they've been using a sonar system named Bendix since the 70s or so to figure out the rivers.