Figure 1.

Reef topography and predation risk as a function of tide. The backreef at (a) low tide and (b) high tide. Deep, hard-bottom lagoons are habitable to large herbivorous and piscivorous (predatory) fishes throughout the tidal cycle. The upper surface of the reef (“reef top”), which has grown to mean low water mark in most places, is not accessible to these fishes at low tide. At high tide, the entire backreef becomes deeper, allowing all fishes, including large predators, access to all reef features. (c) Predator encounter rate (sightings/40 m²/hour; mean + s.e.m.) in the backreef at high vs. low tide each day (n = 5; paired t-test). The vast majority of predator sightings were sharks. Videos indicate that on average each 40 m² section of backreef is traversed by 4–5 sharks during each diurnal high tide cycle.