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Tracking turtles back in time: Linking stable isotope analysis with skeletochronology to determine life history patterns in endangered sea turtles

  • Author(s): Turner Tomaszewicz, Calandra N.
  • Advisor(s): Kurle, Carolyn M
  • et al.

Of the five species of sea turtles that inhabit the east Pacific Ocean, two, the North Pacific loggerhead (Caretta caretta) and East Pacific green turtle (Chelonia mydas), experience some of the highest rates of globally documented mortality in a productive foraging hotspot near the Baja California Peninsula (BCP), Mexico. This area, the Gulf of Ulloa, overlaps with high levels of fishing, resulting in thousands of dead turtles, many that wash up on the beach of Playa San Lázaro on the BCP. It is unknown how long loggerheads inhabit distinct regions of the North Pacific, and to what degree juvenile individuals in the population demonstrate variation in their life history patterns. Similarly, it is unknown how long green turtles occupy oceanic regions of the eastern Pacific before settling into more nearshore habitats, or how they use the Gulf of Ulloa. My research determined the residency duration at different foraging grounds within the eastern Pacific, as well as basic demographic information such as age-at-settlement, age-at-maturation, and timing of ontogenetic shifts, for these two endangered sea turtle populations.

By determining life history parameters and the length of time individuals in these populations spend in distinct high-risk habitats, managers can better understand exposure to spatially explicit threats and prioritize conservation approaches. I focused particularly on duration of time spent in the high-bycatch waters of the Gulf of Ulloa. To this end, I developed a novel technique that combined skeletochronology (the study of growth and age increments in bones) with sequential stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of humerus bone growth layers. Naturally occurring stable isotope gradients exist in ocean systems such that habitats and foraging behaviors can be distinguished (i.e. nearshore vs. offshore habitats or high vs. low trophic levels). Given these isotope gradients, the combination of skeletochronology with stable isotope analysis of sequentially sampled growth layers provides a multi-year record of location, diet, size, age, and annual growth of individual turtles, allowing the reconstruction of life history and long-term habitat use patterns. The combination of these techniques allowed me to address questions that could not be answered using either technique alone.

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