320 J.-O. Laloë et al.
FIG. 5 Location and size of the major loggerhead rookeries globally (.100 nests reported annually). Plot symbol size reflects rookery size measured in annual nest numbers. For data sources, see Supplementary Table 4.
of 25% per year is at the high end of the range reported
in other studies (ranging from c. −21%to 39% per year; Mazaris et al., 2017). The reasons behind this dramatic in- crease in nest numbers are uncertain but increases in marine turtle nest numbers have been linked to the implementation of successful conservation measures in some locations (e.g. Marcovaldi&Chaloupka, 2007). In theUS Virgin Islands in the Caribbean Sea, a large increase (c. 13% per year) in the annual number of leatherback turtle nests was explained by a major conservation effort to protect eggs and so increase the number of hatchlings entering the sea (Dutton et al., 2005). However, if increased hatchling survival is the reason underlying an increase in nesting numbers, then we would expect a delay between the start of conservation measures and the rise in nesting numbers because hatchlings take many years to reach sexual maturity and return to nest. Loggerhead turtles may only mature after 45 years (Scott et al., 2012), so the marked increase in nest numbers that we reported may reflect an increase in hatchling success prior to the instigation of targeted conservation. Alternatively, a reduction in the offtake of nesting adults
while they are ashore or a reduction in catch rates of turtles at sea may be contributing to the increasing nest numbers. Nesting females were historically hunted in Cape Verde (Marco et al., 2011) and consumption of marine turtle meat continues despite national and international laws to protect marine turtles (Marco et al., 2012). Presence of sur- veyors on the beach during night surveys effectively deters turtle hunters on Sal, with lower mortality being observed on surveyed beaches vs unsurveyed beaches (Lino et al., 2010). Protection of nesting beaches has resulted in a signifi- cant reduction in the number of turtles being killed on Sal (B. Renom & A. Taxonera, unpubl. data spanning 7 years), highlighting the value of beach protection through the implementation of night surveys and illustrating how a reduction in adult catch rate can contribute to an increase in nest numbers.
Future studies may be able to identify variables that have
contributed to the increase in nest numbers on Sal. For ex- ample, although the trends in nest numbers at the index beaches were similar, the increase was more pronounced on Costa Fragata than on Algodoeiro (Fig. 2). Biophysical differences between the two beaches, such as orientation (west vs east), exposure (sheltered vs exposed) or sand colour (dark vs light; Laloë et al., 2014) could contribute to the differences in the numbers of nests observed at these beaches. Additionally, new developments have been built along the south-western coastline, including along parts of Algodoeiro, and light pollution has increased on this index beach. Light pollution affects nest site selection in logger- head turtles (Price et al., 2018), and may have resulted in turtles preferring to nest on the more secluded beaches. Because marine turtles exhibit temperature-dependent
sex determination, with females produced at higher tem- peratures, a long-term rise in sand temperatures (over mul- tiple decades) could also have contributed to an increase in the number of female hatchlings and consequently in the number of nesting females (Laloë et al., 2014; Hays et al., 2017). Population modelling could help identify the pro- cesses contributing to the increase in nesting numbers (Chaloupka, 2002; Dutton et al., 2005; Stewart et al., 2011). Our results give cause for cautious optimism, with a siz-
able population (.1,000 nesting females) and an increasing trend in nest numbers observed on Sal. However, threats to turtles remain, as on other islands of the archipelago, and conservation management needs to continue. Threats to turtles in Cape Verde include the ongoing hunting of adult nesting females for consumption (Marco et al., 2011, 2012), bycatch of turtles in longline fisheries (Melo & Melo, 2013), predation of eggs by the ghost crab Ocypode cursor (Marco et al., 2015), coastal development on the beachfront and light pollution (Taylor & Cozens, 2010; Marco et al., 2018). Climate change can also have detrimen- tal effects on marine turtles, for example through sea level
Oryx, 2020, 54(3), 315–322 © 2019 Fauna & Flora International doi:10.1017/S0030605318001497
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