Pacific staghorn sculpin is a pretty aggressive predator and will attack any moving objects in the water as mentioned. I've caught them on spoons, spinners and flies from time to time. I've only seen one chasing the spoon to the surface and slowly sinking like what Ben has described, which was actually really funny to see and took a few seconds for me to clue in what was happening. I've also had flounders on the spoon. Actually, this morning in West Vancouver, we had a big sculpin and a big starry flounder attacking the spinner.
The more time you spend spinning in the Fraser River, the more interesting sightings you will come across. You'll start seeing pikeminnows, bull trout, cutthroat trout chasing your spoons and spinners. You'll see pinks and coho salmon do that too.