The impact of artificial lights and anthropogenic noise on Loggerheads (Caretta caretta) and Green Turtles (Chelonia mydas), assessed at index nesting beaches in Turkey and Mexico

Thomas Magyar

Abstract

 

Coastal development is considered to be a significant hazard that endangers sea turtles by destroying nesting space. If uncontrolled, it will result in decline and local extinction, or will prevent recovery of these endangered species. Light pollution (excess of artificial light at night) is known to affect the site choice of sea turtle females and the seaward orientation of hatchlings at nesting beaches. The results of this are decreased nesting efforts and increased hatchling mortality (WITHERINGTON, 1992a, 1997). It is expected that light pollution will further increase worldwide, with a 10% increase per year, in areas where no counter-measures are taken. Therefore, for sea turtle conservation, identification and designation of critical habitats and development of mitigation measures at nesting beaches are crucial. Light pollution maps specifying sea turtle index nesting sites on a global scale remain unavailable. This thesis investigates light pollution at sea turtle nesting sites of global importance, with a focus on the Mediterranean. To this end, the Top ten global nesting sites (nesting females/year) for two species, the Loggerhead (Caretta caretta) and the Green Turtle (Chelonia mydas), were identified from the literature and entered into a geo-database using Arc View GIS. Nesting site coordinates were intersected with light pollution maps, showing the propagation of light at sea level in 1996/1997 (CINZANO et al., 2001a). On a global scale, nesting beaches in Japan, SE- USA and the eastern Mediterranean are located within light pollution hotspots near urban areas (Figs. 3.3 - 3.6). Mediterranean nesting sites are of global importance for C. caretta populations and have regional importance for C. mydas. Newer nesting data (1990-2004), which were available for index nesting sites here, were compared with light pollution maps (Figs. 3.9 - 3.13, 3.15, 3.16). In the Mediterranean, 76% of the C. caretta and 79% of the C. mydas index nesting sites were affected by light pollution, which was a > 10% increase in natural light levels at night. Twenty-eight percent (C. caretta) and 43% (C. mydas) of the index sites were located within light pollution hotspots, which are up to 3 times brighter than natural sky brightness at night (Figs. 3.8, 3.14). However, no quantitative data on light pollution measured on the ground were available for sea turtle nesting beaches in the Mediterranean or any other index sites, apart from nesting sites in the United States. Therefore fieldwork was done at Belek, which is the most important C. caretta index nesting site in Turkey and one of the largest in the entire Mediterranean. Despite its importance for this species, Belek is affected by mass tourism and coastal development, also resulting in light pollution. Within a beach section of 16.1 kilometres, I identified a minimum of 57 polychromatic Metal Halide (MH) lamps and 16 High Pressure Sodium Vapour (HPS) lamps close to the egg-laying zone (Figs. 4.11 – 4.16). These were the main contributors to increased Illuminance, mainly in front of hotels. At Belek, high resolution data on light pollution measured on the ground was used to investigate the effects on sea turtle nesting density, hatchling disorientation, and hatchling mortality. Statistical evaluation confirmed that light pollution by MH and HPS lamps affected the nest site selection of C. caretta females, which tried to avoid illuminated places and shifted from hotel zones to the few remaining darker, undeveloped areas. Seaward orientation of hatchlings was disturbed and disorientation was also high in darker areas due to higher nest density and light propagation from adjacent hotel zones. Hatchling disorientation and mortality were positively correlated (Table 4.20), highlighting the need for effective mitigation measures at Belek for species conservation. Because polychromatic Metal Halide lamps were identified to be the key problem at Belek, a set of hatchling orientation experiments was performed, investigating the effect of dichroic filters attached in front of a Tungsten Halogen lamp with similar spectral properties as the Metal Halide lamps in the field. These experiments were conducted in Xcacel, Mexico, because this nesting site provided better experimental conditions, less light pollution, and higher hatchling sample sizes compared with the Mediterranean site. In addition, a second species (C. mydas) could be included in the experiments. Each of five distinct dichroic filters tested cut off a proportion of the Tungsten Halogen lamp’s emitted spectrum between 350 and 850 nm (Figs. 5.8). My experimental findings suggest that the filter cutting off short wavelengths below 520 nm elicited aversion to yellow light (xanthophobia) in C. caretta hatchlings, thus reducing disorientation. This mitigating effect was smaller in C. mydas. The impact of sounds on sea turtle hatchlings has not been thoroughly investigated in behavioural experiments. Electrophysiological data for juvenile C. mydas (RIDGWAY et al., 1969) confirm that sea turtles are capable of perceiving low frequency aerial sounds. In a first step, I investigated ambient sounds at Belek to test for possible effects on sea turtle hatchlings. Surf/wave sounds and anthropogenic noise were identified to be the predominant sounds in this coastal area. Some noise sources had definite peaks in the low frequency range < 1000 Hz, similar to surf sound (Table 4.15). My sound measurements were compared with literature data on sea turtle sound perception (Fig. 4.18). This comparison revealed that sea turtles in the egg-laying zone at 20 m from the shore are physically able to perceive low frequency aerial sounds at moderate sound pressure levels ( 50 dB SPL re 20 μPa). Based on these outcomes I tested hatchlings’ orientation behaviour under experimental conditions in Mexico. Low frequency test sounds (Figs. 5.6) were used as stimuli on C. caretta and C. mydas hatchlings. It could neither be proven that these test sounds were significantly repelling hatchlings, nor if the hatchlings were orienting towards the sounds. However, a high proportion of inactive hatchlings were observed in most sound simulation experiments except in the control groups (Fig. 5.12 a, b). This led to the conclusion that, in total darkness, sounds may cause inhibition of crawling in hatchlings. Inhibition was not observed in the experiments using lights exclusively (Fig. 5.13 a, b). I also tested the effect of wave/surf sound recordings versus light stimuli on hatchlings simultaneously. As with testing light stimuli only, I found that C. caretta and C. mydas hatchlings significantly oriented towards short visible wavelength light stimuli (< 520 nm) but both species exhibited some degree of xanthophobia (Fig. 5.14 a, b). Overall my investigation confirms that light stimuli have a predominant effect on sea turtle hatchlings compared with sounds. Wave/surf sounds probably do not provide an orientation cue to hatchlings. In contrast, indicated by the consistently high proportion of inactive hatchlings only in the sound treatment groups, the inhibition of natural behavioural patterns by artificial sounds may be possible. Based on the outcomes of my studies I drafted recommendations for the Mediterranean sea turtle index nesting site at Belek. Primarily, this is the overall reduction of light pollution. This should be achieved by abandoning new building projects within the coastal zone and by rebuilding light barriers such as dunes and vegetation. Further proposals are the regulation of night-time lighting at hotels, the replacement of polychromatic lights and the application of colour filters, which I found adequate to reduce hatchling disorientation. Reducing high-intensity noise after nightfall, in particular the regulation of traffic and relocation of dance-floors from the beach during the nesting season is also recommended here. Mitigation measures should also be implemented for other Mediterranean nesting beaches affected by coastal development (Table 6.1). Such measures have already been successfully realized in the US, and should be an important part of European conservation efforts for sea turtles. The legal framework for these measures is provided by numerous European and national agreements protecting sea turtles, such as the Berne Convention and the Convention on Migratory Species.

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© Universitäts- und Landesbibliothek Bonn | Veröffentlicht: 02.06.2009