# Advancing Genetic Stock Identification of Leatherback Turtles From Foraging Grounds in the Southwest Atlantic: Insights From Nuclear DNA (nDNA) Analysis

## TL;DR

This study uses nuclear DNA to identify the origins of leatherback turtles in the Southwest Atlantic, finding most come from Ghana and Gabon.

## Contribution

The study improves genetic stock identification of leatherback turtles using nuclear DNA markers, resolving uncertainties from mitochondrial DNA analyses.

## Key findings

- 92% of leatherbacks in the Southwest Atlantic originated from Ghana and Gabon, with 6% from the Northwest Atlantic and 1% from the Southwest Indian Ocean.
- Nuclear DNA analysis resolved the origin of turtles with 'orphan' mtDNA haplotypes, assigning them to the Southeast Atlantic rookeries.
- One turtle previously of uncertain origin was directly assigned to the South African rookery with high probability.

## Abstract

The leatherback turtle (
Dermochelys coriacea
) undertakes extensive migrations between nesting and foraging areas, where it is exposed to threats such as fisheries bycatch, coastal development, and pollution. Although classified globally as Vulnerable by the IUCN, the Southwest Atlantic subpopulation is considered Critically Endangered, with nesting restricted to Brazil. While satellite telemetry and previous mitochondrial DNA (mtDNA) Genetic Mixed Stock Analysis (MSA) studies have indicated that leatherbacks from West African rookeries migrate to foraging grounds off Argentina and Uruguay, the potential for connectivity with rookeries from other regions remains an open question. Genetic Stock Identification (GSI) using 15 nuclear DNA (nDNA) microsatellite markers was conducted on 78 stranded or incidentally caught leatherbacks from feeding grounds off the coasts of Argentina and Uruguay. Assignment analysis results demonstrated that 92% of the foraging leatherbacks originated from Ghana and Gabon in the Southeast Atlantic (SEA), with lesser contributions from the rookeries in the Northwest Atlantic (NEA; 6%) and the Southwest Indian Ocean (SWI; 1%) rookery in South Africa, all with assignment probabilities (AP) exceeding 95%. Our findings corroborate and extend previous mtDNA studies by enhancing the precision of GSI for individuals possessing common haplotypes and by clarifying the unknown origin of individuals with ‘orphan’ mtDNA haplotypes, such as Dc7.1, which were assigned to the SEA rookeries (AP = 99%). Furthermore, we directly assigned one individual, previously of uncertain mtDNA origin (Dc9.1), to the South Africa rookery (AP = 97%), highlighting the need to consider the extension of the SWI Regional Management Unit (RMU) boundaries to Southwest Atlantic waters in future assessments. The absence of detected connectivity with Brazilian nesting populations underscores the necessity for increased sample sizes and the application of advanced molecular markers. These results advance the understanding of population connectivity across oceanic scales and emphasize the crucial role of international collaboration in conservation endeavors.

Genetic Stock Identification (GSI) of 78 stranded or incidentally caught leatherbacks from feeding grounds off the coasts of Argentina and Uruguay was genotyped with 15 microsatellite markers. Assignment analysis results indicated that 92% of leatherbacks in this region originated from Ghana and Gabon, with smaller contributions from the Northwest Atlantic (6%) and Southwest Indian Ocean (1%), all with assignment probabilities > 95%. By improving knowledge of source populations and their links to foraging areas, this study contributes informative data to future refinement of RMU boundaries and establishes a baseline for monitoring fisheries bycatch and other anthropogenic threats, contributing to the protection of critically endangered leatherback populations.