How does artificial light at night change the fish gathering on a coral reef?

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Artificial light at night (ALAN) is a human-generated pollutant that is intensifying and spreading in marine environments, yet experimental studies of its community-level effects are generally lacking.

Coral reefs occur in inshore, shallow, clear waters and have diverse light-sensitive inhabitants, making them highly susceptible to biological disturbances. Their biodiversity and accessibility also make them model systems for gaining wider insight.

In a field experiment on a Polynesian reef system, researchers manipulated ALAN using underwater LED lights and compared local nighttime fish communities with those at control sites without ALAN. They used infrared video censuses to record baseline communities before the manipulation, then repeated the censuses after short-term exposure averaging three nights and prolonged exposure averaging 25 nights.

Short-term ALAN exposure caused no significant changes to the nighttime fish community, whereas prolonged exposure increased nighttime species richness. After prolonged exposure, species composition differed more from its baseline than at control sites.

The difference in community composition between prolonged-exposure and control sites was not apparent at the family level. Instead, it appeared in the composition of trait guilds—groups defined by shared characteristics.

Following prolonged exposure, nighttime assemblages contained more daytime-active and nighttime-active predatory species: fish-eaters, invertebrate-eaters, and plankton-eaters. This was particularly true of species that stay attached to a particular site or move within reefs.

The experimental findings indicate that coastal ALAN could cause imbalances in feeding relationships and disturbances to daily biological rhythms in local nighttime reef fish communities. Because community-wide consequences were apparent only after prolonged exposure, the researchers suggest that managing the duration of artificial lighting could potentially reduce impacts on marine ecosystems.