First let be clear I'm neither a lawyer or a game warden but WDFW/state does establish/control fishing regulations (seasons etc.) on a number on private lands. I think we would have to look no further that small streams across the region that are flowing through private lands - they all are under state fishing regulations.

Yes there are a number of waters that are privately planted with trout all of which require a planting permit from the State. As part of the permit whether the water be permitted to be planted will be under state regulations. Typically the criteria of whether the water being planted under the permit can be excepted under the permit is whether there are "state" fish in the water. Let's look at two examples 1) Some one digs a 3 acre hole in the ground creating a pond which at the time of filling has no fish. The permit may all the owner(s) to establish the rules. 2) If on the other hand on private land a lake is created by damming a small stream which has cutthroat. In this case the lake even though being planted privately would still be under state rules; it also has "state fish".

Cobble cruiser -
In other words some one should check the planting permit. If the water in question is being planted without securing the required permit (not an uncommon situation) then by default state fishing rules

Curt