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

Abstract

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.

Opportunism on the High Seas: Foraging Ecology of Olive Ridley Turtles in the Eastern Pacific Ocean

“…The isotopic data generated in this study adds to the growing body of work describing stable isotope baseline data for marine organisms in the eastern Pacific Ocean (Arthur et al, 2008;Olson et al, 2010;Ruiz-Cooley and Gerrodette, 2012;. A natural future direction would be to estimate marine isoscapes across taxa and trophic levels to provide a systematic framework for stable isotope ecological applications, as well as empirical studies of trophic dynamics (Somes et al, 2010;Ceriani et al, 2014;Vander Zanden et al, 2015;Magozzi et al, 2017;Kurle and McWhorter, 2017). A central repository of stable isotope data for sea turtles would support the advancement of this field and reduce duplication of efforts (Pauli, 2017).…”

Satellite tracking and stable isotope analysis highlight differential recruitment among foraging areas in green turtles

“…We broadly followed previously described methods (Ceriani et al 2012, Pajuelo et al 2012, Vander Zanden et al 2015) to predict foraging area using a discriminant function analysis (see Supplement 4), with a secondary classification method as per Zbinden et al (2011) to distinguish between 2 sites that were isotopically similar.…”

“…With normal distributions found for the 3 isotopes, and the variance among foraging areas homo genous, we employed a linear discriminant function analysis. We used nonuniform priors using the number of turtles tracked to each site from the satellite telemetry (Royle & Rubenstein 2004, Vander Zanden et al 2015) and a posterior probability of assign ment set at 80%. The discriminant analysis was evaluated using the leave-one-out cross validation method with 95.7% of turtles from the training data correctly reclassified.…”

“…Thus, isotopic approaches to infer foraging area are often validated through the isotopic composition of satellite-tracked individuals (e.g. Jaeger et al 2010, Zbinden et al 2011, Seminoff et al 2012) that can then be used to create species-specific isoscapes, such as those developed for the loggerhead turtle Caretta caretta (Ceriani et al 2014, Vander Zanden et al 2015. However, a primary caveat of integrating SIA with satellite telemetry is the effective time lag between these techniques; SIA records the isotopic re gime prior to tissue sampling, whilst satellite telemetry tracks the animal after transmitter attachment (Seminoff et al 2012).…”

δ13C and δ15N in the endangered Kemp’s ridley sea turtle Lepidochelys kempii after the Deepwater Horizon oil spill

“…C values in the 3 regions used by both species (northern, eastern and southern GoM) are not significantly different and therefore could not be differentiated (Vander Zanden et al 2015). (2) Vander Zanden et al (2015) also reported significant differences in δ 15 N between the northern GoM and the eastern and southern GoM.…”