Figure 4.

Sighting distance (in metres) of a spherical bait ball of fish (each being 0.1 m in length) as a function of depth and number of fish in the ball (on a log scale). (a–c) Fish with inherent contrasts of 0.25, 0.5 and 1.0, respectively. In this case (as opposed to the results shown in figure 2), the minimum contrast threshold of the viewer is not allowed to drop below 0.005, which is the lowest value measured in any animal. (Online version in colour.)