Current speed influences the vertical position that a fish can hold. The faster the current the less likely a fish can hold close to the surface. It must seek a place resting in the slower deeper currents; consequently, its vision window is enlarged. The slower the current the shallower a fish can hold, and its
vision window is diminished. But the faster currents have a more broken surface that obscures the fish’s vision; therefore, its outside world is more distorted in faster flows. Slower currents have calmer surfaces which are easier for the fish to see through.