Yes, that's what I'm getting at.
Assuming uniform distribution of animals moving up, if I were to release the repellent on the other side of the stream it would scare them over to this side and they would become more available for trapping. In the optimal scenario, if we were to put a pipe across that released that chemical into the water we could potentially set up a chemical barrier. It would be temporary in nature, but it might tell them to not enter this stream and to move on to the next one. Instead of using a huge amount of very expensive chemicals you could just release that and ring the alarm that this is a bad spot, so go away, if everything were to go well.