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Ontogenetic niche theory predicts that as organisms grow they make size-specific changes in habitat use and diet to optimize growth and survival. An understanding of the variation in timing of habitat shifts and fidelity to those habitats is critical for population dynamics modeling and evaluation of conservation strategies, especially for species whose population vital rates are sensitive to changes in growth and survival of critical life stages, such as loggerhead sea turtles. For this study, we sequentially analyzed the annual humerus bone growth increments of 84 juvenile loggerhead sea turtles for stable isotopes (δ13C, δ15N) to reconstruct the diet and habitat use histories of turtles, characterize intrapopulation variation in resource use, and evaluate the somatic growth dynamics surrounding an oceanic-to-neritic ontogenetic shift.

In short, we were able to demonstrate that sequential isotopic sampling of sea turtle bone samples can elucidate alternate life history patterns in juvenile Northwest Atlantic loggerhead sea turtles, and can be used to generate estimates of size and age of transition between oceanic and neritic habitats. In addition, we demonstrated that, in general, there is a spike in growth around the time of this transition, but that this spike sometimes occurs in years before turtles transition between habitats. This counters what is predicted by ontogenetic niche theory and suggests other factors may more strongly influence sea turtle growth rates. Lastly, we observed similarities in mean size at transition, size-at-age relationships, and mean growth increment-specific growth rates between turtles exhibiting alternative life history patterns, which suggests it is unlikely that alternative life history patterns in this species affect time to sexual maturity in this species.

The primary collaborators for this project included: Selina Heppell (OSU), Larisa Avens (NOAA SEFSC, Beaufort Lab), Jeffrey Seminoff (NOAA SWFSC), and Lisa Goshe (NOAA SEFSC, Beaufort Lab).

For full description of this work see:

Ramirez, M.D. 2015. Sequential isotopic analysis to characterize ontogenetic shifts and growth dynamics of loggerhead sea turtles (Caretta caretta). Master thesis, Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR, USA

Ramirez, M.D., L. Avens, J.A. Seminoff, L.R. Goshe, and S.S. Heppell. 2015. Patterns of loggerhead turtle ontogenetic shifts revealed through isotopic analysis of annual skeletal growth increments. Ecosphere, 6: art 244. doi: 10.1890/ES15-00255.1.

Ramirez, M.D., L. Avens, J.A. Seminoff, L.R. Goshe, and S.S. Heppell. 2017. Growth dynamics of loggerhead sea turtles undergoing ontogenetic habitat shifts. Oecologia. doi:10.1007/s00442-017-3832-5.

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