Loggerhead turtle nest site selection 153
deposition over 8 years in one of the largest loggerhead turtle Caretta caretta colonies: Maio Island on Cabo Verde, West Africa (Cozens et al., 2011; Dutra & Koenen, 2014; Patino- Martinez et al., 2022a). In the last decade (2012–2021) the number of nests has increased on Maio Island, with a mean of 19,415 ± SD 16,450 nests per season during 2017– 2021 (Patino-Martinez et al., 2022b). The loggerhead turtle is categorized as Vulnerable on
the IUCN Red List and its persistence is considered to be conservation-dependent (Casale & Tucker, 2017). The sub- population of Cabo Verde has been identified as genetically separate from other loggerhead turtle stocks (Monzon- Arguello et al., 2010; Wallace et al., 2010; Stiebens et al., 2013), with multiple genetic groups in the rookery (Stiebens et al., 2013; Baltazar-Soares et al., 2020). This subpopulation is categorized as Endangered based on IUCN Red List crite- rion B2 because of the continuing decline in area, extent and/or quality of its habitat (Casale &Marco, 2015). Maio Island is heterogeneous and the characteristics of
its nesting beaches vary throughout the island in terms of dimension, slope, colour, composition and temperature of the sand, providing a variety of microhabitats for nest incubation (Patino-Martinez et al., 2022b). In this study we (1) investigated the re-nesting beach consistency across the entire island and (2) evaluated the consistency of intra- beach nest site selection for three beach areas with different flood risks. The results of this study will help prioritize areas of conservation importance, optimize capture–mark– recapture programmes and evaluate the potential for adaptive responses to projected sea-level rises (Fuentes et al., 2010).
Study area
Maio Island (Fig. 1) in the Republic of Cabo Verde, West Africa, is 269km2 (24kmlong × 16kmwide) and hosts nest- ing loggerhead turtles on sandy beaches that cover 35 km of its 117.8 km of coastline. At present, the high-energy beaches of Maio Island are largely undeveloped, with low anthropo- genic impacts resulting in near-pristine habitat. The colour and temperature of the sand, the dimensions, slope and bathymetry of the beaches and the natural hatching success rates vary greatly throughout the island (Patino-Martinez et al., 2022a). The nesting season is mid June–mid November, with a peak in August.
Methods
Geographical distribution of nesting Nesting is distributed around the whole island. For the purposes of this study, the island was divided into eight geographical areas (each c. 13 km long) according to the eight points of the compass (Fig. 1). Loggerhead turtle nesting habitats (sandy beaches) available and monitored in each area were 2.4 km (in the east-north-east) to 8.0 km (in the south-south-west) long.
We monitored 63,101 loggerhead turtle nesting activities (aborted nesting attempts and successful nesting) in all eight areas over eight nesting seasons (2012–2019) during July– October. We tagged the 7,872 monitored turtles with either PIT tags on the right shoulder, metal tags on both front flip- pers or using bothmethods, following recommended sea tur- tle tagging protocols (Balazs, 1999).We recorded the location of each nesting activity of each female using a GPS, and set the maximum possible observed distance of nest scattering to c. 52 km.We did not examine inter-island nest scattering.
Intra-beach nest site selection During 2017–2019 we stud- ied the intra-beach locations of 2,769 pairs of nests of fe- males for which we recorded at least two nests (excluding aborted nesting attempts) within a nesting season. We can- not exclude the possibility that turtles could have nested between recorded events. We classified the distribution of nests according to their location in three different zones as- sociated with varying levels of flood risk: high, medium and low. The high-risk zone covers the nesting area below the high water mark, the medium-risk zone represents that be- tween the high tide line and where dune vegetation begins, and the low-risk zone corresponds to the area above the dune vegetation line extending to the back of the beach. We measured the widths of the three zones simultaneously (1 h after low tide) at six reference beaches. For the null model we calculated the expected number of nests as a function of the mean area of each zone, assuming equal density across zones.
Data analyses We aggregated all unique tag records and organized them chronologically. We combined the infor- mation regarding tagged nesting females and geographical areas or intra-beach flood risk zone to produce a from– to relation matrix for geographical area/flood risk zone re-nesting preferences. If the starting geographical area (y axis) and the destination geographical area (x axis) were the same, this implies that the female returned to the same zone in the following observed nesting event (Table 1). We calculated the per cent of re-nesting events in the same area (column R in Table 1). The remaining from–to cells represent the tendency to choose different geographical areas in two sequentially observed nesting events. The number in each cell indicates the number of re- petitions of each specific re-nesting behaviour (Table 1). To determine the re-nesting site selection consistency at both study scales (geographical area and intra-beach microhabi- tat) we ran χ2 goodness-of-fit tests for observed counts that included the analysis of specific proportions of nests per geographical area or microhabitat. We analysed successful nests and nesting attempts separately, and analysed observa- tions of re-nesting both within and between nesting sea- sons. Statistical significance was set to P,0.05 and values reported are means ± 1 SD.
Oryx, 2023, 57(2), 152–159 © The Author(s), 2022. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321001496
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