Which way is Which?

Research on Photopollution and sea turtle hatchlings

Sea turtle hatchlings emerge from the nests at night and crawl directly to the sea. They are able to locate the ocean from the nest using visual cues. Orientating themselves away from darker, landward silhouettes, hatchlings move toward the open, lower, brighter seaward horizon. Once hatchlings enter the sea, they swim offshore by orienting into surface waves.

Hatchlings also use a ‘magnetic compass’ to orient offshore but before this is used it must be ‘calibrated,’ or set, by previous experience. Under laboratory conditions, calibration is accomplished either by crawling in one direction (toward a light source); or by swimming in one direction either toward a light source or into surface waves.

Results of recent scientific research indicate that to reach their migratory goals, hatchlings must respond to specific cues guiding their orientation from the nest to the sea, and away from the beach.

The presence of artificial night lighting degrades the visual environment at many nesting beaches, disrupting the ability of hatchlings to either detect or respond to the cues required for a crawl in the right direction. Research has also found that even if these turtles ultimately locate the sea from illuminated beaches, they usually swim more slowly and on less direct paths.

Instead of heading out to sea these hatchlings in Florida were attracted by the lights of a nearby apartment building. Photo: N Wu

There are 2 ways artificial lighting disrupts hatchling orientation on the beach. The turtles may either crawl towards the lights (‘misorientation’) or may be incapable of crawling in any direction (‘disorientation’). As a consequence, turtles may crawl for hours without reaching the sea and become exhausted and dehydrated. A prolonged beach crawl also increases their exposure to predators that already scour the beaches for easy prey. Each year tens of thousands of hatchlings die as a consequence of disrupted orientation by artificial lighting. A disrupted crawl might prevent the turtles from calibrating their magnetic compass or result in miscalibration so that after they enter the sea, they swim in inappropriate directions.

Mis-oriented or disoriented hatchlings are sometimes found alive on the beach, either late at night or the morning after failing to locate the sea.

Source: LORNE, Jacquelyn Kay and SALMON, Michael. Effects of exposure to artificial lighting on orientation of hatchling sea turtles on the beach and in the ocean. Endangered Species Research, Vol. 3: 23-30, 2007