318 J.-O. Laloë et al.
FIG. 2 Time series of the number of nests recorded on the index beaches Algodoeiro and Costa Fragata, where survey efforts were constant during 2008–2017.
(39%) false crawls, 12,706 (22%) aborted nests and 1,112 (2%) unconfirmed activities. The first clutch of the season was typically laid in mid
June (median = 17 June; interquartile range = 13 days, 9–22 June) and the last nest of the season was typically in early November (median = 3 November; interquartile range = 14 days, 27 October–10 November). The nesting season lasted 127–186 days (median = 143 days; interquartile range = 23 days, 133–156 days).
Number of nests on the index beaches
We calculated the annual number of nests for each index beach during 2008–2017 (Fig. 2). Except for the first survey year, the number of nests on Costa Fragata was higher than on Algodoeiro throughout the study period and trends at both beaches were similar. During the last 2 years of the study period the number of nests increased considerably. Over the course of the study period, nest numbers increased from 98 and 95 to 1,239 and 3,194 on Algodoeiro and Costa Fragata, respectively. In 2017, the final year of monitoring, the index beaches
accounted for c. 57% (Algodoeiro 16% and Costa Fragata 41%) of all recorded nests on Sal (Fig. 3). Six beaches had ,200 recorded nests over the course of the 10-year period and three beaches (including the two index beaches) had .1,000 nests during the study period.
Total number of nests on Sal island
In the first 3 years of beach monitoring we probably under- estimated the numbers of nests on the island. The ratio of nests to activities increased from 0.29 in 2010 to 0.38 in 2011 and remained relatively constant thereafter. The higher nests-to-activities ratio observed from 2011 onwards is probably the result of an increased ability to identify nests
FIG. 3 Numbers of loggerhead turtle nests recorded on different study beaches in 2017.
because of experience gained over several years of monitor- ing. We corrected the number of nests reported in 2008– 2010 to account for this observed change in nesting success (Supplementary Table 3). However, a change in the assess- ment of nesting versus non-nesting activities alone does not account for the overall increase in the number of nests estimated over the study period. The total number of nests on Sal increased from 506 to
7,771 during 2008–2017 (Fig. 4), mirroring the trend at the index beaches (Fig. 2), and the same significant increase is observed in the last 2 years of the study period. The annual rate of change in nest numbers was r = 25%
per year and the increase in nest numbers observed over 10 years is statistically significant (linear regression: R2 = 0.54, P=0.015;Mann–Kendall tau rank correlation test: tau = 0.56, P=0.032).
Loggerhead turtles lay on average three to five clutches
per breeding season on Boa Vista (Varo-Cruz et al., 2007). Using these values, we estimated that the number of females nesting on Sal was 100–200 in 2008 and 1,600–2,600 in 2017.
Global assessment of large loggerhead turtle rookeries
Loggerhead rookeries occur across the subtropical and temperate regions, with major nesting sites in the Atlantic, Indian and Pacific oceans and in the Caribbean and Mediterranean seas (Fig. 5). There are only two loggerhead turtle rookeries in the world in which reported nestnumbers exceed 50,000 annually: Masirah Island in Oman (c. 65,000 nests/year; Tucker et al., 2013, as cited in Casale, 2015) and Florida in the USA (c. 73,000 nests/year; Richards et al., 2011). There are few sites with .1,000 annual loggerhead turtle nests; these include Australia, Brazil, Greece, Japan, South Africa, Turkey and the USA (Fig. 5). With 7,771 nests reported in the 2017 nesting season, Sal is therefore one of the 10 largest loggerhead turtle rookeries globally (Supplementary Table 4).
Oryx, 2020, 54(3), 315–322 © 2019 Fauna & Flora International doi:10.1017/S0030605318001497
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