Note that YP will not be very visible unless you feed at twilight and the water is around 72 or less. They don't like being seen in bright sun, and seem to not like penetrating the warm surface layer that sets up on hot days. Otherwise they try to stay out of sight and below that warm layer. When they get larger, they get bolder, but not much.

For some reason it seems they don't like being picked off by birds when lingering near the surface. I think that explains their behavior.