Mariela Pajuelo

Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic using stable isotopes

Differential foraging area use can affect population demographics of highly migratory fauna because of differential environmental changes and anthropogenic threats among those areas. Thus, identification of foraging areas is vital for the development of effective management strategies for endangered migratory species. In this study, we assigned 375 loggerhead turtles (Caretta caretta) nesting at six locations along the east coast of the United States to their foraging areas in the Northwest Atlantic (NWA) using carbon and nitrogen stable isotope values (δ13C and δ15N). We first evaluated the epidermis δ13C and δ15N values from 60 adult loggerheads with known foraging grounds. Twenty-two females from 6 nesting beaches and 23 males from one breeding area were tracked with satellite transmitters to identify their foraging locations following breeding, and 15 adult turtles were sampled at one foraging ground. Significant trends were observed between both δ13C and δ15N values of satellite-tracked loggerheads and the latitude of the foraging grounds to which the turtles migrated, reflecting a geographic pattern in the stable isotope values. Both δ13C and δ15N values characterized three geographic areas—with distinct abiotic and biotic features—used by adult loggerheads in the NWA. Discriminant analysis assigned all 375 female loggerheads to one of the three foraging areas; 91% were assigned with probabilities of ≥80%. The proportion of nesting turtles using each foraging ground varied geographically; most turtles nesting in northern beaches (72–80%) tend to forage at higher latitudes while most turtles nesting in southern beaches (46–81%) tend to forage at lower latitudes. Stable isotopes can reveal the foraging location of loggerhead turtles in the NWA, which will allow robust analyses of foraging ground effects on demography and improve the design of management strategies for the conservation of loggerhead populations. The conclusions and methods developed in this study are also relevant for other populations of sea turtles and for other highly migratory species.

Determining origin in a migratory marine vertebrate: a novel method to integrate stable isotopes and satellite tracking

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.

Mother-egg stable isotope conversions and effects of lipid extraction and ethanol preservation on loggerhead eggs.

Loggerhead egg stable isotopic composition can be used as a proxy for mother values as a result of the strong correlation between mother and egg tissue. Also, the significant effects of lipid extraction and ethanol preservation on carbon isotope values can be accounted for using mathematical corrections.

Common metabolic constraints on dive duration in endothermic and ectothermic vertebrates

Dive duration in air-breathing vertebrates is thought to be constrained by the volume of oxygen stored in the body and the rate at which it is consumed (i.e., “oxygen store/usage hypothesis”). The body mass-dependence of dive duration among endothermic vertebrates is largely supportive of this model, but previous analyses of ectothermic vertebrates show no such body mass-dependence. Here we show that dive duration in both endotherms and ectotherms largely support the oxygen store/usage hypothesis after accounting for the well-established effects of temperature on oxygen consumption rates. Analyses of the body mass and temperature dependence of dive duration in 181 species of endothermic vertebrates and 29 species of ectothermic vertebrates show that dive duration increases as a power law with body mass, and decreases exponentially with increasing temperature. Thus, in the case of ectothermic vertebrates, changes in environmental temperature will likely impact the foraging ecology of divers.

Supplement 1. Raster format of loggerhead δ13C and δ15N isoscapes and associated standard deviation.

File List loggerhead_d13C_isoscape.txt (md5: 75d5be745e2018c1fbf639224e598736) loggerhead_d15N_isoscape.txt (md5: d76570667098245e4102e8380b588324) loggerhead_d13C_error.txt (md5: 7f8bb3cc91d34eb8ad033942bd2f93cf) loggerhead_d15N_error.txt (md5:995ce245c4e066001d6f8322e33038ef ) Description loggerhead_d13C_isoscape.txt – krigged surface of δ 13 C values from 41 calibration loggerhead sea turtles in coastal waters < 200m depth of the Gulf of Mexico and Greater Caribbean loggerhead_d15N_isoscape.txt – krigged surface of δ 15 N values from 41 calibration loggerhead sea turtles in coastal waters < 200m depth of the Gulf of Mexico and Greater Caribbean loggerhead_d13C_error.txt – standard deviation of the kriging prediction of δ 13 C values loggerhead_d15N_error.txt – standard deviation of the kriging prediction of δ 15 N values These four files are the δ 13 C and δ 15 N isoscapes and standard deviation rasters that are depicted in Fig. 1 of the main text. They are ASCII-formatted text files and contain a header to identify the structure of the file. They can be opened in ArcGIS by using the ASCII to Raster conversion tool or in R with the raster() function using the Raster package and then assigned a spatial reference within the program used. Resolution of each of the files is 0.0572 degrees.