Light flashes and the geometry of specular fish schools.

The risk of predation presents a difficult challenge in environments that offer no physical shelter, such as the open waters of the world's seas. In the absence of hiding places, many marine fishes turn to two main anti-predator strategies: aggregation and camouflage, which, mostly, have been studied separately. Here, we consider both aspects together and examine the visual imprint of fish schools of different sizes and geometries, given that camouflage is attained by specular (mirror-like) skin texture. To do so, we developed ray-tracing simulations that model the passage of sunbeams as they go through an optically realistic aquatic environment and reflect off the skins of the fish. We find that due to frequent high-intensity specular reflections (light flashes), the marginal increase in detectability with increasing school size is significantly higher than previously estimated under the assumption of diffusive reflection. However, we also find that by increasing density and alignment the fish can mitigate the detectability of individuals, albeit at the expense of the detectability of the school as a whole. Our findings provide a new perspective on documented responses to threat by schooling pelagic fishes and underscore the importance of the optical signature of animals in structuring their behaviour.