GEOSPATIAL ASSESSMENT OF ARTIFICIAL LIGHTING IMPACTS ON SEA TURTLES IN TORTUGUERO, COSTA RICA

Artificial lighting on sea turtle nesting beaches repels adult females searching for nest sites and disrupts hatchling seafinding, with potentially substantial effects on nesting activity and hatchling survival. Artificial lighting is one of a host of anthropogenic threats, including intentional harvest, bycatch in fisheries, interaction with marine debris, and habitat degradation, that have contributed to global sea turtle population declines. Because artificial light on nesting beaches has the potential to impact sea turtle life history stages that are essential to reproductive success, managing artificial lighting is a vital component of nesting beach protection. Furthermore, identifying nesting beach habitat vulnerable to the effects of artificial lighting is critical to guiding conservation efforts that aim to protect sea turtle populations.

Green turtles (Chelonia mydas) are classified as endangered globally by the International Union for Conservation of Nature (IUCN). Tortuguero Beach in Costa Rica supports the largest green turtle nesting population in the Western Hemisphere, with more than one hundred thousand green turtle nests laid on the beach every year. The Sea Turtle Conservancy (STC) has monitored and worked to protect the nesting population since 1959, and since 2004, the organization has conducted surveys to monitor artificial lighting on the beach. The primary objective of this project was to support the Sea Turtle Conservancy’s efforts by:

1. 1) Assessing the effect of artificial lighting from adjacent development on green turtle nesting in Tortuguero,
2. 2) Mapping artificial lighting on the beach in order to identify the brightest areas as appropriate targets for light pollution reduction efforts, and
3. 3) Developing a geographic information system (GIS) database to facilitate future monitoring and beach protection by STC.

Between June and August 2014, I conducted walking surveys to map the nesting beach and light sources using a Trimble Juno SB GPS unit, and I developed a GIS database that formed the basis for subsequent analyses and data visualization. I built STC’s monitoring data from 2004 through 2014 into a polygon layer of the beach subdivided into mile sections defined by mile markers erected by STC. During the new moon in June and July 2014, I conducted brightness surveys in concert with STC’s light surveys and measured brightness in units of luminance at 50-meter intervals along the beach using a Unihedron Sky Quality Meter. Using spatial data of the beach and light sources, luminance data from brightness assessments, and monitoring data from STC, I determined a mean luminance value for each mile section, examined the relationship between luminance and nesting activity, and mapped light pollution on the beach.

I found that mean luminance and the total number of green turtle emergences per mile section were significantly negatively correlated. Mean luminance exceeded the minimum threshold for light pollution in 6 of the 43 mile sections, and there were significantly fewer emergences in mile sections experiencing light pollution. Mean luminance was highest in mile sections adjacent to Tortuguero Village, where sources of artificial light were concentrated. These findings were consistent with STC’s light survey data, and mean light count and the total number of green turtle emergences per mile section from 2004 to 2014 were also significantly negatively correlated. Cumulatively, these results suggest that artificial lighting from adjacent development impacts green turtle utilization of nesting habitat and changes the spatial distribution of green turtle nesting activity on Tortuguero Beach.

These results were consistent with the findings of previous studies conducted on sea turtle nesting beaches and support the need for a turtle-friendly lighting initiative in Tortuguero. Successful management of light pollution on Tortuguero Beach will require a coordinated effort between the Sea Turtle Conservancy, the Costa Rican Electricity Institute (ICE), and local stakeholders. I made the following recommendations for addressing light pollution in Tortuguero:

1. 1) Light pollution management efforts should focus on Tortuguero Village, where the majority of light sources are found and luminance was highest.
2. 2) Light pollution management should involve both restoration of native beachfront vegetation and improvements in lighting technology.
3. 3) Future brightness surveys to document reduction of light pollution in response to STC’s efforts could focus on mile sections adjacent to Tortuguero Village. Brightness data from this study provide a baseline against which to compare future brightness levels on the beach.
4. 4) Targeted monitoring of hatchling disorientation near Tortuguero Village would provide additional insight into artificial lighting impacts on the nesting population and help identify specific lights that continue to be problematic after management actions are taken.