What are these rafts on the Green River?

There are a couple of these crazy rafts on the Green River near Tukwila Park. What are they?

Update: @PotentialIce3208 says Salmon monitoring via radio tags – https://pnwflyfishing.com/forum/threads/green-river-fish-counter.8409/

I will say until I zoomed in I thought the metal box was a barbecue grill and thought “what kind of cyberpunk party boat is this”.