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Stable isotope analysis is a useful tool to track animal movements in both terrestrial and marine environments. These intrinsic markers are assimilated through the diet and may exhibit spatial gradients as a result of biogeochemical processes at the base of the food web. In the marine environment, maps to predict the spatial distribution of stable isotopes are limited, and thus determining geographic origin has been reliant upon integrating satellite telemetry and stable isotope data. Migratory sea turtles regularly move between foraging and reproductive areas. Whereas most nesting populations can be easily accessed and regularly monitored, little is known about the demographic trends in foraging populations. The purpose of the present study was to examine migration patterns of loggerhead nesting aggregations in the Gulf of Mexico (GoM), where sea turtles have been historically understudied. Two methods of geographic assignment using stable isotope values in known-origin samples from satellite telemetry were compared: (1) a nominal approach through discriminant analysis and (2) a novel continuous-surface approach using bivariate carbon and nitrogen isoscapes (isotopic landscapes) developed for this study. Tissue samples for stable isotope analysis were obtained from 60 satellite-tracked individuals at five nesting beaches within the GoM. Both methodological approaches for assignment resulted in high accuracy of foraging area determination, though each has advantages and disadvantages. The nominal approach is more appropriate when defined boundaries are necessary, but up to 42% of the individuals could not be considered in this approach. All individuals can be included in the continuous-surface approach, and individual results can be aggregated to identify geographic hotspots of foraging area use, though the accuracy rate was lower than nominal assignment. The methodological validation provides a foundation for future sea turtle studies in the region to inexpensively determine geographic origin for large numbers of untracked individuals. Regular monitoring of sea turtle nesting aggregations with stable isotope sampling can be used to fill critical data gaps regarding habitat use and migration patterns. Probabilistic assignment to origin with isoscapes has not been previously used in the marine environment, but the methods presented here could also be applied to other migratory marine species.

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Age was estimated for wild Australian freshwater crocodiles from skeletochronology of growth marks in postoccipital osteoderms. Growth marks were distinct and counted reliably in unstained calcified thin sections (60–80 µm) viewed by Nomarski interference microscopy. The periodicity of growth marks was validated directly from crocodiles of known age (up to 19 years) and from individuals sampled sequentially in different seasons. Growth marks comprised annuli deposited in winter and zones deposited in summer. Errors in age estimation for small crocodiles resulted from difficulty in assigning the growing edge as either a zone or an annulus. No osseous remodelling was noted for immature animals. Age estimates for adult females became unreliable as previous annuli were obscured by osseous remodelling, presumably resulting from calcium mobilization by egg-laying females. Old adult males continued to deposit annuli, but these were more difficult to distinguish, being more closely spaced than in young crocodiles. Skeletochronology permitted reliable (coefficient of variation, 3·4%) and accurate (±1 year) age estimation up to about 20 years. Osteoderm dimensions were strongly correlated with average snout–vent length, mass and age and have acceptable potential as an indirect means of age and size estimation for Crocodylus johnstoni.

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Approximately 40% of the world's leatherback marine turtles (Dermochelys coriacea) nest in Suriname and French Guiana. Trends in nest numbers reconstructed for the last 34 years indicate several up and down events. We undertook computer simulations to determine whether a density-dependent phenomenon might be involved because the period of high-density nesting coincides with a high level of nest destruction by nesting females. The parameters of density-dependent nest destruction were calculated for the Ya:lima:po-Awa:la beach. We show that: (1) density-dependent nest destruction occurs, but (2) it promotes a density-dependent feminization of hatchling sex ratio, and consequently (3) the global production of juveniles continues to increase in relation with the increasing number of deposited nests even for the highest densities observed at that beach. Mean annual production of female hatchlings per adult female, although density dependent, is less than two juveniles even at the lowest densities of nesting females.

We used stable isotope (δ 13 C, δ 15 N, δ 34 S) analysis in combination with satellite telemetry to evaluate the foraging areas chosen by 88 loggerhead turtles Caretta caretta nesting in southwestern Florida. Nine turtles were tracked and skin-sampled in more than one nesting season to evaluate within-individual consistency in foraging sites and stable isotope values. Turtles migrated to 5 regions: Caribbean, Florida Keys, West Florida Shelf, northern Gulf of Mexico, and Yucatan Peninsula. The stable isotope ratios across these foraging grounds ranged from −21.16 to −7.69 ‰ for δ 13 C, 3.27 to 13.99 ‰ for δ 15 N, and 1.91 to 20.64 ‰ for δ 34 S. We compared bulk skin tissue stable isotope values for all turtles by bioregion, year, body size, depth of putative foraging area, and linear distance from the closest shore; among these factors, only bioregion showed a significant effect on isotope values. There were subtle regional differences in mean δ 13 C, δ 15 N, and δ 34 S, and an apparent north-south isotopic shift aligning strongly with ocean currents adjacent to the Florida Keys. The influence of coastal topography and shifting biogeographic boundaries such as the Loop Current may cause strong ocean water mixing that results in the observed similarities in stable isotope values among regions. These results indicate that stable isotopes alone may be an inadequate tool for identifying fine-scale (<100 km) residency of sea turtles within this ocean region.