The abundance of reef fish is strongly affected by replenishment of their populations with young-of-year that settle from the plankton. Study of the settlement process and the early post-settlement life of reef fishes has been hampered in part because of the difficulty of obtaining behavioral information about very young fish. During the daytime when observations by scuba divers are possible, the presence in the water of human observers often significantly alters behavior of the fish or their predators. And at night, when many crucial events occur, darkness usually precludes behavioral study. We have utilized in situ video technology with infrared illumination to observe and record behavior of reef fish. Tthis approach circumvents several of the major problems previously encountered. The imaging system consists of a monochrome ultra-high resolution CCD chip compact camera sensitive to infrared and visible light contained in a cylindrical waterproof housing. Camera power is supplied from a boat or from shore over a coaxial cable, which also returns the video signal. Illumination at night is provided by waterproof infrared illuminators. During daylight hours, ambient light provides ample illumination for video recording, even on overcast days. Cameras are positioned to view sea anemones or coral heads inhabited by various species of damselfish, which are abundant inhabitants of coral reefs worldwide. The exact times of settlement from the plankton, patterns and mode of movements of settlers from the water column onto the substrate, interactions with resident fish upon arrival, and visits and attacks by various predators (octopus, crabs, and fish) during the days following settlement are recorded.

Our observations have greatly enhanced our understanding of the behavior of young damselfish. Settlement occurs only at night. Settlers appear to locate sites by olfaction. Predators frequently visit corals and anemones both day and night, but virtually all deaths from predation occur at night when damselfish are inactive and sheltering in the coral or anemone, not during the daytime when they are feeding in the water column. Aggressive interactions with other fish that result in movement to the edges of the shelter cause smaller and younger individuals to become vulnerable to predation. This behavioral mechanism accounts for the density-dependent patterns of mortality we observe for young-of-year damselfish.