Oceanic dispersal characterizes the early juvenile life‐stages of numerous marine species of conservation concern. This early stage may be a ‘critical period’ for many species, playing an overriding role in population dynamics. Often, relatively little information is available on their distribution during this period, limiting the effectiveness of efforts to understand environmental and anthropogenic impacts on these species. Here we present a simple model to predict annual variation in the distribution and abundance of oceanic‐stage juvenile sea turtles based on species’ reproductive output, movement and mortality. We simulated dispersal of 25 cohorts (1993–2017) of oceanic‐stage juveniles by tracking the movements of virtual hatchling sea turtles released in a hindcast ocean circulation model. We then used estimates of annual hatchling production from Kemp's ridley Lepidochelys kempii (n = 3), green Chelonia mydas (n = 8) and loggerhead Caretta caretta (n = 5) nesting areas in the northwestern Atlantic (inclusive of the Gulf of Mexico, Caribbean Sea and eastern seaboard of the U.S.) and their stage‐specific mortality rates to weight dispersal predictions. The model's predictions indicate spatial heterogeneity in turtle distribution across their marine range, identify locations of increasing turtle abundance (notably along the U.S. coast), and provide valuable context for temporal variation in the stranding of young sea turtles across the Gulf of Mexico. Further effort to collect demographic, distribution and behavioral data that refine, complement and extend the utility of this modeling approach for sea turtles and other dispersive marine taxa is warranted. Finally, generating these spatially‐explicit predictions of turtle abundance required extensive international collaboration among scientists; our findings indicate that continued conservation of these sea turtle populations and the management of the numerous anthropogenic activities that operate in the northwestern Atlantic Ocean will require similar international coordination.

Previous studies of the olive ridley Lepidochelys olivacea population structure in the tropical eastern Pacific have indicated the existence of a single panmictic population ranging from Costa Rica to Mexico. This information has been used to design specific management measures to conserve primary nesting beaches in Mexico. However, little is known about olive ridleys in the Baja California Peninsula, their northernmost reproductive limit, where recent observations have shown differences in nesting female behaviour and size of hatchlings relative to other continental rookeries. We used mtDNA control region sequences from 137 turtles from five continental and four peninsular nesting sites to determine whether such differences correspond to a genetic distinction of Baja California olive ridleys or to phenotypic plasticity associated with the extreme environmental nesting conditions of this region. We found that genetic diversity in peninsular turtles was significantly lower than in continental nesting colonies. Analysis of molecular variance revealed a significant population structure (Phi ST = 0.048, P = 0.006) with the inclusion of peninsular samples. Our results: (i) suggest that the observed phenotypic variation may be associated with genetic differentiation and reproductive isolation; (ii) support the recent colonization of the eastern Pacific by Lepidochelys; (iii) reveal genetic signatures of historical expansion and colonization events; and (iv) significantly challenge the notion of a single genetic and conservation unit of olive ridleys in the eastern Pacific. We conclude that conservation measures for olive ridleys in Mexico should be revised to grant peninsular beaches special attention.

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Concentrations of polychlorinated biphenyls (PCBs) as well as the expression patterns of cytochrome P450 (CYP) enzymes and glutathione-S-transferase (GST) activities were measured in livers of loggerhead (Caretta caretta), green (Chelonia mydas), and olive ridley (Lepidocheyls olivacea) sea turtles from the Baja California peninsula of Mexico. The mean concentrations of total PCBs were 18.1, 10.5, and 15.2 ng/g wet weight (ww) respectively for the three species and PCB 153 was the dominant congener in all samples. Total PCB concentrations were dominated by penta- and hexa-chlorinated biphenyls. The mean estimated TEQs were 42.8, 22.9, and 10.4 pg/g (ww) for loggerhead, green, and olive ridley, respectively, and more than 70% was accounted for by non-ortho PCBs. Western blots revealed the presence of hepatic microsomal proteins that cross-reacted with anti-CYP2K1 and anti-CYP3A27 antibodies but not with anti-CYP1A antibody. There were no significant differences in GST activities between species. Grouping congeners based on structure–activity relationships for CYP isoenzymes suggested limited activity of CYP1A contribution to PCB biotransformation in sea turtles. These results suggest potential accumulation of PCBs that are CYP1A substrates and provide evidence for biotransformation capacity, which differs from known animal models, highlighting the need for further studies in reptiles, particularly those threatened with extinction.

Monthly in-water monitoring was conducted at 5 foraging areas off the Baja California Peninsula, Mexico, from 2001 to 2006 to assess black turtle population status. Turtles were captured with entanglement nets, size (straight carapace length, SCL) and weight of each individual were recorded and turtles were tagged on both rear flippers. A total of 1238 turtles were captured, involving 6309 h of effort. Of this total, 937 ind. were marked and 155 were recaptured. SCL ranged from 30 to 95.5 cm; immature turtles accounted for > 94% of the total catch and were dominant at all sites. However, size class distribution varied among sites, and turtles from the Gulf of California were significantly larger than those caught in the Pacific. Catch per unit of effort (defined as the number of turtles caught per 100 m of net soaking for 24 h) varied from 1.79 at Bahía Magdalena to 17.35 at Punta Abreojos. Mean (± SD) annual growth rate for black turtles was 2.27 ± 0.71 cm yr-1 , with the lowest and highest growth rates in Bahía Magdalena (1.37 ± 0.71 cm yr-1) and Laguna Ojo de Liebre (3.05 ± 2.23 cm yr-1), respectively. Lack of recaptures from adjacent sites and multiple recaptures of specimens at the site of origin over several years indicate a low connectivity among foraging areas and a high site fidelity. Hence, strengthening local protection is imperative to the recovery of black sea turtle populations.