Radiance fluctuations induced by surface waves can enhance the appearance of underwater objects

To examine the effect of wave-induced light fluctuations on the appearance of objects to fish, we recorded the spatial and temporal fluctuations of light reflected from a diffusely reflecting target that served as a simplified proxy for the body of a fish, and of light from the water background that a fish might be viewed against. Measurements were repeated at diverse depths, viewing azimuths, distances to the substrate, and sun conditions. Two conditions that are necessary for wave-induced light fluctuations to make objects more apparent to fish were satisfied. The contrast of light fluctuations reflected from either the object or water background was higher than the minimum contrast value that is detected by fish, or, alternatively, the contrast of light fluctuations reflected from both the object and water background was higher than the minimum contrast value detected by fish, but differed from one another. Furthermore, the frequency range where most of the power of wave-induced radiance fluctuations matched the frequency range of maximum contrast sensitivity in fish. Thus, light stimuli having spatial and temporal characteristics similar to those of wave-induced light fluctuations may make objects more apparent to fish. We suggest that the frequency characteristics of the visual systems of fish were likely shaped by wave-induced light fluctuations in aquatic ecosystems.