Light pollution poses a significant threat for sea turtle reproduction. Strong lights deter turtles from nesting on otherwise suitable beaches. On areas with moderate light pollution, emerging hatchlings are confused by the lights and cannot find the sea. Instead, they crawl into the dunes, streets, or hotels and die of dehydration, exhaustion, or predation. Those that eventually make it to the sea have used up their limited energy and are unlikely to survive.
It is easy to wonder how much light pollution is too much for sea turtles. In short, the answer is that any light pollution is too much and must be eliminated from nesting beaches if sea turtles are to escape extinction. Understanding this answer requires an overview of how light pollution affects sea turtles.
Female turtles looking for a suitable nest location are averted by strong lights before they even crawl on the beach. The beach of Makris Gialos in Lassi and Skala beach in south-east Kefalonia are subject to intense light pollution due to their attractiveness for tourism. Occasional nests that are laid there have had good incubation success rates. Their known suitability and reports from locals that those beaches hosted many more nests just a few decades ago demonstrate that light pollution is the most likely cause of low nest numbers on these beaches.
Hatchlings are even more sensitive to light pollution. On a naturally dark night, some ambient light is present and allows us to see the silhouettes of objects around us; this light comes from the stars and the refraction of light through the atmosphere. On a beach, ambient light is brighter from the direction to the sea; vegetation and the landscape behind the beach look darker because they block ambient light. This effect is stronger for hatchlings because they are limited to the horizon and cannot look upwards. Hatchlings use this cue and crawl towards the brightest direction to find the sea.
If we go to a naturally dark location and measure the sky's brightness, we will get a reading from 0.0002 lux to 0.002 lux. The same brightness is given by a 60 watt incandescent lamp when measured at distances between 2000 and 600 metres. It is obvious that the visual cues sea turtle hatchlings use to find the sea can be so easily messed up with, making it impossible to determine a practical “safety level” for light pollution. While other natural cues can also help them find the sea, too many nesting beaches are affected by light pollution levels that are hundreds or thousands of times brighter than the naturally dark night sky. Those beaches are practically a death trap for sea turtles (figure 2).
Scientists have long advocated that no artificial light should be directly or indirectly visible from a sea turtle nesting beach. This requirement is generally accepted and appears in many regulations, including some issued by Greece's Ministry of Environment. Their violation falls under the general umbrella of damage to a protected species and is punishable by law. Nevertheless, the control of light pollution is virtually non-existent; conservation groups like ourselves are left to rely on the good will of business owners to reduce light pollution with limited success.
This does not mean that good will cases are negligible. When Wildlife Sense began its sea turtle nest protection programme, the beaches of Ai Chelis and Ammes near Kefalonia's airport were severely light polluted (fig 3). Due to the awareness raised in the first year of the project's operation, the brightest lights were not installed in the following year and the light pollution status on those beaches was reduced from severe to moderate.
One of the barriers to the control of light pollution is a poor availability of suitable and affordable instruments for the measurement of light pollution, making wide-scale data collection difficult. Satellite images have been used to assess levels of light pollution. This method is useful for scientific assessment but cannot be used as evidence against violations.
To overcome this technical barrier, Wildlife Sense spent the last year in the design and laboratory testing of a light measurement instrument that is specifically designed to measure light levels present on sea turtle nesting beaches, including ambient light in near darkness and up to intense light pollution caused by strong spotlights. This measurement device will be used during the nesting and hatching seasons of this year to map light pollution in Kefalonia's sea turtle nesting beaches and assess how this affects their suitability for nesting and for hatchlings attempting to find the sea. This will be an invaluable tool both in the scientific assessment and the public awareness of the extent of light pollution.