Heres a tip for beaver trappers that want to collect rats at the same time.
Set a large funnel trap in the shallow entrance(beaver lodge).I learned that one from Jim Gibb.
Another one I learned from Bob Stitt was to set a large funnel trap in the channel below beaver dams-pick up a ton of rats while trapping beaver.

Yes, you have to wait until just before the opener. A lot can happen in four months with droughts, flooding, disease, and predation just to name a few.