Growth and ontogeny of sea turtles using skeletochronology: Methods, validation and application to conservation

There are limits to the size of an individual that can be supported by a particular habitat. Organisms that span a large size range during their ontogeny often need to alter their life style in order to maintain optimal growth rates and minimize predation. Understanding the timing of ontogenetic habitat shifts is fundamental to the study of a species' life history and population dynamics. This information is especially critical to the conservation and management of threatened and endangered species, such as the loggerhead (Caretta caretta) and Kemp's ridley (Lepidechelys kempi) sea turtle. This dissertation investigates the application of skeletal growth marks (skeletochronology) to age estimation and growth rate analysis for loggerhead and Kemp's ridley sea turtles and the application of this technique in combination with stable isotope analyses of bone to identify ontogenetic shifts.; I validated the annual deposition of growth marks in known-age loggerhead and Kemp's ridleys. The timing of the formation of the lines of arrested growth in Kemp's occurred in the spring for animals that strand dead along the middle and south U.S. Atlantic coast. For both Kemp's and loggerheads, I found a proportional allometry between bone growth (humeri dimensions) and somatic growth (straight carapace length), indicating that size-at-age and growth rates can be estimated from dimensions of early growth marks. I analyzed stable isotope ratios within the bone tissue to track diet shifts that occur in conjunction with ontogenetic habitat shifts. For loggerheads, the transition to benthic habitats was accompanied with an increase in growth rates. In Kemp's, I found evidence of a potential secondary ontogenetic shift from analyses of the growth marks that resulted in increased growth rates. The duration of the pelagic stage in loggerheads was found to be much longer than has previously been thought although the estimated age to maturity was still consistent with current estimates due to the indication of higher growth rates in the benthic stage. The results highlighted the use of skeletochronology as a valuable tool for the rapid assessment of growth rates in species where similar data can only be painstakingly gathered over long time periods.