Boyd Lyon Sea Turtle Fund

Background

Marine turtles inhabit distinct ecosystems throughout their lifecycle; they commonly reside in defined foraging areas and migrate semi-annually to nesting beaches once they become reproductively active (Shimada et al. 2020). The identification of the different habitats used by marine turtles and the connectivity among them is key to prioritize the protection of areas needed to ensure that they fulfill their ecological roles (Troëng et al. 2005, Coffee et al. 2020). Highly migratory species such as marine turtles, depend on key environments to thrive. Thus, conservation strategies to protect these species will only be as successful as the status of the weakest link across the migratory path. Satellite telemetry has facilitated the understanding of the spatial ecology and migratory behaviour of marine turtles and provided insight into their biology, habitat use and conservation (Wallace et al. 2010). In the past, tracking nesting turtles has illuminated migratory corridors and helped to locate foraging areas (Vander Zanden et al. 2015). Despite the great value in satellite telemetry studying species’ movement, one major drawback is the high cost of transmitters, which often leads to limited sample sizes. To offset this challenge, stable isotope analysis (SIA) of common elements found in nature has been a useful tool to identify areas connected by animal movements in marine environments. Migratory movements can be tracked based on the spatial gradients in isotope values of primary producers (Vander Zanden et al. 2015). The distribution of isotopes in organic and inorganic matters can be predicted describing environmental conditions across spatial and temporal scales, creating isotopic landscapes or isoscapes. These biochemical markers are induced by the environment through trophic transfer, therefore all animals within a specified location are labelled without having to be captured and tagged (McMahon et al. 2013). These characteristics make SIA techniques more effective and cost efficient, allowing access to a larger sample size, and increasing representativeness of the studied population. Thus, conducting SIA by sampling nesting turtles can provide the opportunity to assess resource use in foraging areas prior to the breeding period (Witteveen 2009). Furthermore, comparison of isoscape predictions based on SIA from samples collected across the study area, with observational data obtained from previous mark-recapture and satellite telemetry studies, can be used to determine spatial connectivity in biogeochemical, and ecological systems. This approach is therefore well suited for the study of species that may be unavailable to researchers for significant periods of their lives (McMahon et al. 2013). Tortuguero National Park (TNP), on the northern Caribbean coast of Costa Rica, is the largest nesting beach for green sea turtles in the Caribbean Sea (Seminoff et al. 2015; Restrepo et al. 2023). Tag return data from international recaptures have identified post-nesting dispersal patterns from this population throughout Costa Rica, and 19 other countries in the region (Troëng et al. 2005). Historically, research activities at Tortuguero have been concentrated in the northern 8 km of the beach (Carr et al. 1978). Between 2000 and 2002, ten satellitetagged turtles released from this section of the beach travelled north to neritic foraging grounds off Nicaragua, Honduras, and Belize (Troëng et al. 2005). Though, flipper-tag return information provided clear evidence of females embarking on longer migratory trajectories, some routes have not yet been seen in the movement of satellite-tagged turtles (Troëng et al. 2005). The eight-kilometre geographical focus of previous studies may have biased the relative proportion of migratory trajectories observed, overweighting the importance of northern migration routes and foraging areas. The aim of this study is to evaluate migratory connectivity for Tortuguero’s green turtle population, by assessing the carbon (δ 13C) and nitrogen (δ 15N) isotopic values for putative foraging habitats across the Caribbean Sea. We will then compare these values with those of nesting females at Tortuguero to identify their respective putative foraging grounds.

 

Methods

Tortuguero Rookery

Between 2019 and 2023, we released 25 green turtles equipped with satellite transmitters from Tortuguero beach. Using a sterile scalpel, we collected a 1 cm2 skin sample from these turtles, taken from the trailing edge of the hind flipper after disinfecting the sample area. Additionally, we collected tissue samples from nesting green turtles predated by jaguars (Panthera onca) along the 29 km stretch of beach, within the limits of TNP (Arroyo-Arce and Solomon-Pérez 2015). For each sample we recorded: date of the nesting event, GPS location and minimum curved carapace length of the sampled turtle. Samples were preserved in a 96% ethanol solution and stored at room temperature until they be transported to the Center for Stable Isotopes (CSI), at the University of New Mexico for SIA. These samples were collected under the authorization of the National Committee for the Management of Biodiversity in Costa Rica (CONAGEBIO; res. R-030-2022- OT). Furthermore, all procedures were approved by The University of Queensland’s Animal Ethics Committee.

Foraging Areas

Working in collaboration with conservation groups across the Caribbean, we will collect viable samples at foraging areas in Cayman Islands, Colombia, Dominican Republic, Mexico, and Panama. Samples at each site will include blades of T. testidinum seagrass, which constitutes most of the diet for green turtles in the region (Bjorndal 1997). Also, we will collect tissue samples from resident green turtles in these pasture meadows. Most turtles will be captured implementing either netting or hand-captured techniques following protocols described by Ehrhart and Orgen (1999). Further samples will be obtained from stranded or injured turtles. All samples will be preserved in 96% alcohol solution before shipping them to the Center for Stable Isotopes (CSI) in New Mexico, U.S.

Sample Analysis

Tissue and seagrass samples will be imported to the U.S. under CITES US blanket permit from the National Oceanic and Atmospheric Administration (NOAA) and will be processed following the protocols presented by Vander Zanden et al. (2015). First, we will dry the material at 60° C for 24 hours. For the sea turtle tissue samples, lipids will be removed from the epidermis using an accelerated solvent extractor for 3 consecutive cycles. Then, we will grind the sample material into a homogenous powder which will later be packed in tin capsules. Grinded samples will be analyzed on a Delta V mass spectrometer, where they will be burned to be converted into ultrapure analyte gases to ease the process of comparing these with known references (Wassenaar 2019).

 

Expected Outcomes

Thanks to our sampling efforts we have already collected over 800 tissue samples from green turtles. Most of these are from Tortuguero, with sample collection in foraging areas to be completed throughout the year. Based on SIA from the samples collected throughout the region, we will generate an isoscape model for green turtles in the Caribbean, presenting distinct areas for values of δ13C and δ15N in seagrass habitats (McMahon et al. 2013; Vander Zanden et al. 2015). This model would be then used to assess the corresponding forging areas of green turtles nesting at Tortuguero, based on their individual SIA