Anyone know what this is?

[Born2late]

My brother took these pictures today off the bridge up by Oso. The two people in the water are fish and game biologists. He said they put that blue bucket in there and built the rocks up to make a riffle. Does anybody know what they would be doing? I was thinking maybe it’s some way to trap salmon fry if they come in to the bucket to rest because it’s out of the current and then they could tag them. Any thoughts?