Population recovery changes population composition at a major southern Caribbean juvenile developmental habitat for the green turtle, Chelonia mydas

Abstract

Understanding the population composition and dynamics of migratory megafauna at key developmental habitats is critical for conservation and management. The present study investigated whether differential recovery of Caribbean green turtle (Chelonia mydas) rookeries influenced population composition at a major juvenile feeding ground in the southern Caribbean (Lac Bay, Bonaire, Caribbean Netherlands) using genetic and demographic analyses. Genetic divergence indicated a strong temporal shift in population composition between 2006–2007 and 2015–2016 (ϕST = 0.101, P < 0.001). Juvenile recruitment (<75.0 cm straight carapace length; SCL) from the north-western Caribbean increased from 12% to 38% while recruitment from the eastern Caribbean region decreased from 46% to 20% between 2006–2007 and 2015–2016. Furthermore, the product of the population growth rate and adult female abundance was a significant predictor for population composition in 2015–2016. Our results may reflect early warning signals of declining reproductive output at eastern Caribbean rookeries, potential displacement effects of smaller rookeries by larger rookeries, and advocate for genetic monitoring as a useful method for monitoring trends in juvenile megafauna. Furthermore, these findings underline the need for adequate conservation of juvenile developmental habitats and a deeper understanding of the interactions between megafaunal population dynamics in different habitats.

Genetic variation among sea turtle life stages and species suggests connectivity among ocean basins

“…Model 1 estimated rookery contributions by incorporating the haplotype frequencies from each potential source and both offshore sites along with the size of each rookery (Tables S1 and S2 ). We sourced rookery sizes from the literature (Bellini et al, 2013 ; Blumenthal et al, 2021 ; Broderick et al, 2006 ; Girard et al, 2016 ; Millán‐Aguilar, 2009 ; Rodríguez‐Martínez et al, 2021 ; Seminoff et al, 2015 ; Shamblin et al, 2015 ; Shamblin, Witherington, et al, 2018 ; van der Zee et al, 2019 ; Vera & Buitrago, 2012 ) to represent nest counts as close to the sampling period as possible given recent increases in green turtle rookery sizes at many sites (Seminoff et al, 2015 ). Model 2 also included particle back‐tracking probabilities from rookeries to the sampled area as calculated by Putman et al ( 2015 ).…”

Incorporating distance metrics and temporal trends to refine mixed stock analysis

“…We removed from our final dataset haplotypes found in mixed stock aggregations but not yet described in rookeries 14 , and considered only rookeries in the northwest Atlantic and the Greater Caribbean to reduce noise from unlikely contributors 49 . Though mixed stock data published by van der Zee et al 22 uses a timeframe slightly different than the one from IRL and TRID, we included their data in our dataset evaluating variations in haplotype frequencies to assess possible variations in other sites as well.…”

“…Finally, to assess how contributions from rookeries changed over time based on mixed stocks haplotype frequencies and rookery sizes, we also built two models: MSA 5 —“old” sampling period and “historical” source size, and MSA 6 —“new” sampling period and “recent” source size. We considered samples from mixed stock BON 22 to be from comparable timeframes (2006–07 and 2015–16) to IRL and TRID. Therefore, we added haplotype frequencies from BON to our "old" and "new" sampling periods in MSA 5 and MSA 6 , and used the same haplotypic data from MSA 3 /MSA 4 for all other mixed stocks (Supplementary Table S3 ).…”

“…The green turtle ( Chelonia mydas ) is an ideal organism to evaluate how demographic variations and distance between source populations (rookeries) impact mixed stock aggregations. First, green turtle foraging aggregations are typically composed of individuals from multiple populations 10 , 14 , 22 , 23 , 26 , 29 . Second, in the past two decades, the number of nests in several green turtle rookeries in the Greater Caribbean and the western North Atlantic have increased at different rates 24 , 25 .…”

“…Second, in the past two decades, the number of nests in several green turtle rookeries in the Greater Caribbean and the western North Atlantic have increased at different rates 24 , 25 . Recently, van der Zee et al 22 suggested that changes in contributions observed in a juvenile mixed stock in Bonaire could be associated with a variation in the size of the source nesting populations. Third, green turtles leave nesting beaches as hatchlings and swim away from the coast to offshore habitats where they reside for a number of years 30 .…”