I have an eastern garter snake. Every other day I feed him 2 minnows from a bait shop (fatheads?) in a cat food dish full of water, which is big enough for him to lay in. The last 5-6 feedings I noticed he has eaten the minnows, then promptly pooed in the water dish in which I placed the minnows. The last feeding I noticed he pooed in the water dish without eating the minnow. So I removed his food and the dish.

What is going on here? Is he protesting something?