Elucidating climate, density, and oil spill effects on Kemp’s ridley sea turtle somatic growth rates

Matthew David Ramirez, Oregon State University, Fisheries and Wildlife, Corvallis, OR, United States, Larisa Avens, NOAA NMFS, Southeast Fisheries Science Center, Beaufort Laboratory, Beaufort, NC, United States, Lisa R. Goshe, NOAA NMFS, Southeast Fisheries Science Center, Beaufort Laboratory, United States, Melissa L. Snover, Population Ecology Services, American Samoa, Melissa Cook, NOAA Southeast Fisheries Science Center, Pascagoula, MS, United States, Heather Haas, NOAA NMFS, Northeast Fisheries Science Center, United States and Selina S Heppell, Oregon State University, Department of Fisheries and Wildlife, Corvallis, OR, United States
Somatic growth variation manifests from the cumulative effects of a suite of biological, ecological, and environmental processes and can have profound effects on individual fitness and species population dynamics. Nevertheless, logistical limitations have greatly limited our ability to identify specific environmental factors influential to the somatic growth rates of marine top predators. Using a 20-year dataset of somatic growth generated through skeletochronology, we evaluated the effect of multiple region-wide environmental stressors—the Deepwater Horizon oil spill, climate change, and population density—on age- and region-specific Kemp’s ridley sea turtle (Lepidochelys kempii) somatic growth rates. We used temporal analyses to identify structural shifts in the growth time series and fit von Bertalanffy growth curves to explicitly compare somatic growth rates before and after the Deepwater Horizon oil spill. General Additive Models and cross-correlation analyses were used to examine relationships between growth rates, population abundance measures, and time-lagged climate indices, and to partition variance explained by theses alternate environmental covariates. We identified significant, multi-year reductions in mean somatic growth rates beginning in 2012 for Age 0 and Age 2–5 turtles stranded in both the U.S. Gulf of Mexico and Atlantic Ocean. Von Bertalanffy growth parameters differed before and after the Deepwater Horizon oil spill. Regional climate indices were moderately to strongly correlated with mean Age 0 growth rates with a 2-yr lag (cross-correlation = –0.57 to 0.60), whereas population abundance exhibited significant relationships with mean Age 2–5 growth rates. In integrative analyses, the 2012 growth shift, which we theorize may be related to long-term deleterious effects of the Deepwater Horizon oil spill, was identified as the greatest predictor of somatic growth variation. However, we cannot rule out the potential additive influence of climate and population abundance on somatic growth rates for certain age classes. Continued collection and study of sea turtle humerus bones is needed to further identify mechanisms underpinning the observed growth patterns.