I was in Florida this summer, where there are large schools of small transparent fish swimming near the shore. These fish were swimming all around me, spaced only a few inches apart, and occasionally bumping in to me. I have heard that the best way to catch a member of a flock/school/herd, is to single one member out. Looking at the school of fish, I could see that this was almost impossible. By the time you see the individual fish clearly, the school has already surrounded you, and it fills your vision. It was impossible (for me) to pick out one fish from the school – they are constantly swerving in unison, and they all look identical. Just as you start to get a bead on one, the school swerves in unison, and you find that you are now focusing on a different fish a centimeter away from the first fish. It would be challenging enough to catch an isolated fish, but with the visual confusion of thousands of other fish, forget about it. That centimeter is all the fish needs.
To catch fish in a school like that, you need a net or strong suction. Your eyes will not help you.