Mexican origins for the Texas green turtle foraging aggregation: A cautionary tale of incomplete baselines and poor marker resolution

- Distinct lineages of Chelonia mydas CM-A1.1 nest in Tamaulipas and Florida.
- The Tamaulipas rookery should be recognized as a distinct management unit.
- The Florida rookeries are not major contributors to the Texas foraging aggregation.
- Texas is important nursery habitat for juveniles from Gulf of Mexico rookeries.

The green turtle (Chelonia mydas) foraging aggregation along the Texas coast has increased dramatically in recent years, but the source populations for these turtles have not been adequately resolved. Previous mixed stock analysis (MSA) based on 490 base pair (bp) mitochondrial control region haplotypes suggested a large Florida contribution, but widespread sharing of common haplotypes among potential source populations and incomplete source population baseline data precluded precise assessment. To test the hypothesis that Texas turtles may represent proximal western Gulf of Mexico (GoM) nesting populations, we analyzed novel rookery samples from Rancho Nuevo, Tamaulipas, Mexico (RNMX) and conducted oceanic connectivity simulations. The RNMX samples yielded haplotypes CM-A1.1 and CM-A3.1 in frequencies not significantly different from those of the central eastern Florida nesting population. However, mitogenomic sequencing identified a diagnostic mitochondrial SNP (mtSNP) variant that is fixed in RNMX relative to the Florida CM-A1.1 lineage. Pairwise comparisons indicate that the Tamaulipas rookery represents a discrete population relative to those previously described in the northern Greater Caribbean, warranting recognition of a western GoM management unit (MU). Contrary to previous findings, the Florida populations were ruled out as major contributors to the Texas aggregation through screening of the mtSNP. Mixed stock analysis incorporating the mtSNP data suggested a western GoM origin for approximately 70% of the Texas foraging aggregation, with Quintana Roo contributing the majority of the remainder. Backtracking simulations within an ocean circulation model were broadly congruent with genetic results in indicating substantial probability of oceanic transport from Mexican rookeries to the Texas coast (68%) while also dismissing the possibility of transport from the eastern Florida rookeries (0%). The mixed stock analyses and backtracking simulations are consistent with previous hypotheses implicating oceanic dispersal followed by natal homing by neritic juveniles to explain juvenile green turtle distributions. In contrast to a pattern of stepping stone connectivity across the remaining northern Greater Caribbean, the Texas foraging aggregation was distinct from all others analyzed in the region, including one in the eastern GoM. This isolation highlights the significance of Texas as developmental habitat for the proposed western GoM MU and reiterates the importance of continued international cooperation to facilitate recovery of this stock. This study also underscores the importance of satisfying underlying assumptions of mixed stock analysis in order to make robust inferences of connectivity.