Ecological Archives A025-019-A1
Hannah B. Vander Zanden, Anton D. Tucker, Kristen M. Hart, Margaret M. Lamont, Ikuko Fujisaki, David S. Addison, Katherine L. Mansfield, Katrina F. Phillips, Michael B. Wunder, Gabriel J. Bowen, Mariela Pajuelo, Alan B. Bolten, and Karen A. Bjorndal. 2015. Determining origin in a migratory marine vertebrate: a novel method to integrate stable isotopes and satellite tracking. Ecological Applications 25:320–335. http://dx.doi.org/10.1890/14-0581.1
Appendix A. Detailed description of sample preparation, satellite transmitter attachment and foraging area determination, as well as assignment methods.
Sample storage and preparation
The majority of the samples (36) were air-dried prior to storage, while two were frozen at -10°C, and 22 were stored in 70% ethanol prior to processing. Ethanol does not significantly alter the stable isotope values of loggerhead skin (Barrow et al. 2008), and therefore was not expected to alter the isotopic values of scute. The frozen samples were not stored in a frost-free freezer, as the process may affect the stable isotope values in sea turtle epidermis, but freezing has generally found to have little affect on stable isotope values in many other studies (Barrow et al. 2008).
The mean C:N ratio of 322 scute samples (this study and unpublished data) was 3.3, which is less than the 3.5 ratio suggested for lipid removal or mathematical correction (Post et al. 2007). Therefore, scute samples were not lipid extracted in this study. Because lipid extraction is unlikely to affect stable isotope values, we are confident that the samples in this study are comparable to other samples in other studies that have been lipid extracted.
Satellite transmitter attachment and foraging area determination
Satellite transmitters were attached to nesting females after oviposition. A portable wooden box was lowered around the female as she departed the nesting site, and satellite transmitters were attached after the scute sample was collected through the following steps: (1) the carapace was prepared by firm abrasion with sandpaper; (2) the attachment site was cleaned with ethanol or isopropyl alcohol followed by sterile water rinses and drying; (3) slow curing epoxy was applied to adhere the platform terminal transmitter to the carapace; and (4) anti-biofouling boat paint was applied over the epoxy in some cases. A typical attachment took 1-1.5 hr until the turtle was released.
Our goal in assessing the foraging area was to determine the approximate centroid in the foraging home range once displacement from the rookery was achieved. The process to determine the centroid of the foraging area consisted of the following steps: (1) once displacement distance from the rookery reached an asymptote, the first 30 days with locations were used; (2) only location classes 3, 2, 1 were used in order to get high quality fixes and restrict observations to the best location per day; (3) if turtles were in shallow water, location classes 3, 2, 1 0, A, B were used; (4) turning angles >25 degrees, swim speeds >3 km, and points on land were excluded to get the best location per day; (5) the centroid of all filtered points (up to 30 days) was calculated.
The SGoM foraging area is located to the north of the Yucatan Peninsula of Mexico, also known as the Campeche Bank, and comprises an extensive carbonate bank. The NGoM foraging area extends as far west as we currently have data and east to Apalachee Bay in Florida and is characterized by soft sediments. The EGoM foraging area extends from Apalachee Bay southward along the western Florida coast nearly to the Keys. It is geologically very similar to the SGoM with a general progression toward carbonate sediments from north to south. The southern border of this foraging area is located slightly farther south than the border indicated by Wilkinson et al. (2009), but was placed where carbon and nitrogen isotope values in the loggerheads was most distinct. The SNWA foraging area includes the Keys and Bahamas and extends as far north as West Palm Beach, FL on the east coast of Florida. The SAB extends from West Palm Beach, FL northward to Cape Hatteras, NC. Both the SAB and SNWA align with the areas defined by Pajuelo et al. (2012). We considered the division between the EGoM and the SNWA the border defining the GoM for our purposes of delimiting turtles that foraged within or outside of the GoM.
Continuous surface assignment
Kriging uses the observed spatial autocorrelation to derive the the model weights to be used in interpolation and reduces the subjectivity of interpolation (Cressie 1993). Ordinary kriging with a stable semivariogram model and smooth search neighborhood was used to produce interpolated prediction and standard error maps (δ13C nugget = 0, δ15N nugget = 0.08, δ13C partial sill = 9.8, δ15N partial sill = 5.0, δ13C major range = 7.32, δ15N major range = 4.57, lag size = 0.61, number of lags = 12). The lag size was set to the average nearest neighbor distance calculated within ArcMAP. The nugget, sill, and major range parameters were optimized based on the calibration data set using the Geostatistical Analyst package in ArcGIS. The spatial range of interpolation was defined by the United States coastline in the NGoM. The eastern, western, and southern boundaries were defined by straight lines ranging from 0.53–0.87º beyond the maximum and minimum latitude and longitude of the foraging area centroids. Prior to assignment, the predictive surfaces were clipped by restricting the range to waters <200 m depth.
Barrow, L. M., K. A. Bjorndal, and K. J. Reich. 2008. Effects of preservation method on stable carbon and nitrogen isotope values. Physiological and Biochemical Zoology 81:688–693.
Cressie, N. A. C. 1993. Statistics for spatial data. Wiley, New York, New York, USA.
Pajuelo, M., K. A. Bjorndal, K. J. Reich, H. B. Vander Zanden, L. A. Hawkes, and A. B. Bolten. 2012. Assignment of nesting loggerhead turtles to their foraging areas in the Northwest Atlantic using stable isotopes. Ecosphere 3:art89.
Post, D. M., C. A. Layman, D. A. Arrington, G. Takimoto, J. Quattrochi, and C. G. Montana. 2007. Getting to the fat of the matter: models, methods and assumptions for dealing with lipids in stable isotope analyses. Oecologia 152:179–189.
Wilkinson, T. A., E. B. Wiken, J. B. Creel, T. F. Hourigan, T. Agardy, H. Herrmann, L. Janishevski, C. J. Madden, L. E. Morgan, and M. Padilla. 2009. Marine ecoregions of North America. Commission for Environmental Cooperation.
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