Loggerhead turtle nesting on Sal island 317
urbanized and new developments are planned (Taylor & Cozens, 2010). Loggerhead turtles nest on beaches around the island, but with the majority of nesting in the south (Taylor & Cozens, 2010).
Methods
Monitoring of nesting activities Loggerhead turtles nest in Cape Verde during the warmer months of the year (June–October; Marco et al., 2012). We divided nesting beaches into sections (Supplementary Table 2) and conducted regular beach surveys during 2008–2017 to monitor nesting activities. To detect the first nesting activity of the season, we conducted morning sur- veys starting at sunrise daily, from early June onwards. After recording the first nesting activity, we carried out night surveys (20.00–06.00) on all nesting beaches until the end of the nesting season. During surveys, observers walked along the high-water
line and looked for turtles and turtle tracks on the beach. If a turtle was encountered, it was observed from a distance to assess whether or not she would nest. If turtle tracks were present, the observer looked for signs of nesting activity (e.g. presence of a body pit and sand spraying marks; Dodd, 1988) to establish if a clutch had been laid. Any tracks encountered during a survey were crossed out in the sand to prevent double-counting. Turtle activities on the beaches were recorded as nesting
when an egg clutch was laid, as so-called false crawls, during which a turtle emerged onto the beach and subsequently returned to the sea without attempting to dig an egg cham- ber, or as aborted nests, whereby a turtle emerged onto the beach, attempted to dig one or more egg chambers but returned to the sea without depositing a clutch of eggs. Any activities that could not be confirmed as nesting, false crawls or aborted nests were recorded as unconfirmed activities. We always carriedoutmorningsurveys (startingat sunrise)
afternight surveys toensure allnestingactivitieswere recorded for each
night.Monitoring of the nesting beaches continued for 10 consecutive nesting seasons during 2008–2017.
Index beaches and surveys
Wewere able to expand monitoring efforts during the study period as a result of increased funding. In the first years of monitoring (2008–2011), we conducted night surveys on three beaches, but extended this to six beaches during the final years (2014–2017) of the study period (Supplementary Table 2). Monitoring efforts were constant at two beaches, Algodoeiro and Costa Fragata (Fig. 1b), throughout the entire study period. These two beaches were used as index beaches. In addition to regular morning and night surveys at the monitored beaches, we conducted additional surveys
fortnightly during the nesting season. During these surveys all beaches that were not regularly monitored were surveyed in 1 day, with all encountered tracks being inspected, re- corded and marked. These surveys started at 06.00 and ended when all beaches had been surveyed. To estimate the total number of clutches laid annually
since 2008 we had to consider the clutches deposited at those beach sections for which no data were collected dur- ing some years. For a conservative estimate, we divided the number of clutches laid at a given beach section in a given year by the number of clutches laid on the index beaches in the same year. We then used the minimum value of that ratio across all years to estimate the minimum number of clutches laid at a section in a year in which no data were collected. For example, no data were collected in 2008 for the Praia Chano beach section (Supplementary Table 2). During the years for which data were collected, this beach harboured 2–8% of the number of nests recorded on the index beaches, so we estimated that in 2008 this section contained at least 2% of the number of nests recorded on the index beaches in that year. We assumed that the ratio of nests to activities was constant throughout the study per- iod and corrected the number of nests reported in years with outlying nests-to-activities ratios (Supplementary Table 3). We followed the methodology of Mazaris et al. (2017)to
calculate the annual rate of change in nest numbers. We used a linear regression model to examine whether there was a significant trend in nest numbers during the study period, and the non-parametric Mann–Kendall tau rank correlation test for additional confirmation.
Global assessment of large loggerhead turtle rookeries
We used the most recent IUCN assessment for loggerhead turtles (Casale & Tucker, 2017) to identify the major logger- head rookeries globally. For each site we searched the pub- lished literature to see if more recent data were available and checked the reference lists of relevant articles to locate additional sites documented since the IUCN assessment. We only included sites for which .100 nests had been reported annually. Nest numbers reported had mostly been counted directly or estimated from recorded turtle tracks. Where studies reported the annual number of nesting females we used clutch frequencies reported for those sites to estimate annual nest numbers.When local clutch frequen- cies were not available for a site we used clutch frequencies reported for the nearest rookery (Supplementary Table 4).
Results
Nesting season and activities We recorded a total of 59,041 turtle activities on Sal island during 2008–2017. Of these, 21,938 (37%) were nests, 23,285
Oryx, 2020, 54(3), 315–322 © 2019 Fauna & Flora International doi:10.1017/S0030605318001497
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