Loggerhead turtle nesting on Sal island 319
7,500, our results indicate that the island of Sal is an impor- tant nesting site for this species. Within Cape Verde, Boa Vista is the most important nesting site, with 12,000– 20,000 nests annually (Marco et al., 2012). Recently, annual nest numbers for the whole archipelago were estimated to be 43,500 (Inforpress, 2018), indicating that the Northeast Atlantic Regional Management Unit is the third most important Regional Management Unit for the loggerhead turtle globally (Marco et al., 2012; Supplementary Table 4). Based on these estimates, Cape Verde currently hosts c. 20% of the world’s adult loggerhead turtle population. We used annual nest numbers as an indication of the
FIG. 4 Total number of loggerhead nests reported in Cape Verde during 2005–2017. Data for Sal are from this study. Data were available for Boa Vista (López-Jurado, 2007; Marco et al., 2012), Maio (Cozens et al., 2011; Martins et al., 2013), Santa Luzia (Rocha et al., 2015) and Santiago (Loureiro, 2008). Data points connected by a line originate from the same study.
Using data extending to 2015, the global annual number of
loggerhead turtle nestswas estimated to be just over 200,000, and the Northeast Atlantic Regional Management Unit (which encompasses Cape Verde) accounted for 7.5%of this value at the time (Ceriani & Meylan, 2017).
Discussion
Our key finding of a hitherto undescribed increase in nest- ing numbers for a major breeding population of a threat- ened species may have important broad implications for conservation biology. It is perhaps surprising for a group as iconic and charismatic as marine turtles that a significant trend in abundance may go unreported, considering that nesting activities are clearly evident from large, visible tracks left on beaches. This lack of reporting could be a result of marine turtles often nesting in remote or undeveloped areas, a trait shared with a range of threatened species in both terrestrial and aquatic environments. However, even when data are available they are often not reported in the public domain. For example, Mazaris et al. (2017) reported that abundance time series for many sites have not been up- dated in the last decade. This lack of recent published data highlights a need for census data to be made available, so that regional and global conservation status assessments can be based on more robust evidence. Our study makes an important contribution in this respect. The earliest census for loggerhead turtles on Sal dates
from 2008 and estimated the annual number of nests to be ,500 (Cozens et al., 2009, as cited in Marco et al., 2011). In the following year 1,071 nests were counted on the entire island during the nesting season (Lino et al., 2010). With the latest annual nesting numbers exceeding
annual number of nesting turtles at our study site. However, annual nest numbers can vary over time for several reasons (e.g. the number of clutches laid by a female can fluctuate between breeding seasons, and the distribution of repro- ductive efforts varies across different nesting sites), so annual nest numbers may not reflect overall marine turtle populations dynamics (NALWG, 2018). In addition, the number of nesting turtles is only one measure of population size and does not include all individuals in a population. Adult males are essential for population viability and may become gradually more important if hatchling production becomes increasingly female-biased as a result of tempera- ture-dependent sex determination combined with warmer incubation conditions, which can be expected with ongoing climate change (Hays et al., 2014; Esteban et al., 2016). Marine turtle population trends are routinely estimated
using annual nest numbers (e.g. Casale & Tucker, 2017), and nesting populations are often estimated by dividing the number of nests by clutch frequency. Similarly, nest numbers are often estimated by multiplying nesting popu- lation size by clutch frequency. Because clutch frequency can vary amongst populations, population-specific clutch frequency values should be used for robust population esti- mates. If a population-specific clutch frequency is not avail- able, the clutch frequency from another population can be used, but this will reduce the accuracy of the resulting esti- mate. In addition, clutch frequencies tend to be underesti- mated by foot patrols compared to newer techniques such as using satellite tags to record each time a female nests (Esteban et al., 2017). Future studies may refine mean clutch frequency estimates and hence improve information on the number of nesting turtles. Despite these methodological limitations, our key conclusion that the Cape Verde islands host a large number of nesting loggerhead turtles remains robust. As outlined in the 2015 IUCN assessment of the
loggerhead turtle’s status, more data are needed to assess the population trend for the Northeast Atlantic Regional Management Unit (Casale & Marco, 2015). Here we offer some insight, using nest numbers as a proxy for population size. We report a 15-fold increase of nest numbers over a 10-year period. The observed rate of change in nest number
Oryx, 2020, 54(3), 315–322 © 2019 Fauna & Flora International doi:10.1017/S0030605318001497
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