Sequential isotopic analysis to characterize ontogenetic shifts and growth dynamics of loggerhead sea turtles (Caretta caretta)

Graduation date: 2015Ontogenetic niche theory predicts that as organisms grow they make size-specific changes in habitat use and diet to optimize growth and survival. A variety of factors contribute to growth and survival in different habitats, ultimately leading to variation in life history that can affect population dynamics. 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 the loggerhead sea turtle (Caretta caretta). Isotopic analysis of sequentially deposited structures, such as sea turtle humerus bone, provides a means of studying intraspecific life history variation. I 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 undergoing an oceanic-to-neritic ontogenetic shift. I also used skeletochronological methods to evaluate the growth dynamics surrounding this transition.\ud Generated isotopic transects were used to classify individuals into alternative life history pattern groups and were combined with body size and growth data obtained from skeletal analyses to evaluate differences in the duration, timing, and growth dynamics of ontogenetic shifts. Sea turtles that displayed increases in nitrogen stable isotope ratios (δ¹⁵N) greater than 3.0‰ over one or more years were presumed to have transitioned from oceanic to neritic diets and/or habitats based on oceanic and neritic prey isotopic information collected from the literature, and were classified into one of two life history pattern groups: discrete shifters (n = 23) completed this transition within year, while facultative shifters (n = 16) completed this transition in up to eight years. As differences in isotopic values between neritic and oceanic prey are most likely driven by differences in isotopic baselines, I propose the gradual increases in δ¹⁵N values within facultative shifters over multiple years is indicative of foraging in both oceanic and neritic habitats within growth years. Size-at-transition between habitats was similar between discrete shifters (55.1 ± 7.6 cm straightline carapace length, SCL) and facultative shifters (52.8 ± 6.9 cm SCL). \ud Growth variance was higher for facultative shifters versus discrete shifters. Yet, mean size at transition, size-at-age relationships, and mean increment-specific growth rates were similar between turtles with alternative life history patterns. Annual growth rates generally peaked within one year of transition (31/38 of turtles), providing support for a short-term (i.e., 1-2 year) ontogenetic shift-associated growth advantage. However, there was considerable variation in the timing of maximal growth rate among turtles with some individuals exhibiting maximal growth in years prior to the ontogenetic shift (14/38 turtles). The lack of substantial differences in the timing of transition and growth dynamics between discrete and facultative shifters likely limits the influence of these alternative life history patterns on time to sexual maturity in this species, though differences in habitat-specific survival probabilities could affect loggerhead population dynamics. This study demonstrates the value of paired isotopic and skeletal analyses to the study of long-term sea turtle life history variation and its affect on growth