Regional environmental drivers of Kemp’s ridley sea turtle somatic growth variation

Abstract

Many environmental processes influence animal somatic growth rates. However, elucidating specific drivers of somatic growth variation has been challenging for marine megafauna. Using a 20+ year dataset of somatic growth generated through skeletochronology, we evaluated the relationship between multiple region-wide environmental factors—the Deepwater Horizon (DWH) oil spill, increasing population density, and climate variability—and age- and region-specific Kemp’s ridley sea turtle (Lepidochelys kempii) somatic growth. We observed significant, multi-year reductions in mean oceanic (age 0) and small neritic (age 2–5) juvenile growth rates beginning in 2012 for turtles stranded along the U.S. Gulf of Mexico (GoM) and Atlantic Coasts, which resulted in a reduction in mean size-at-age. We hypothesize that this growth decline is related to long-term deleterious effects of the DWH oil spill on neritic and oceanic food webs in the GoM. Additionally, regional climate indices were strongly correlated with oceanic juvenile growth with a 2-year lag (cross-correlation = –0.57 to 0.60), whereas GoM small neritic juvenile growth was strongly related to population abundance metrics. Generalized additive models that included all examined environmental covariates indicated that the drivers of the 2012 growth rate decline had the strongest effect on Kemp’s ridley growth rates between 1995 and 2015, but that additive or synergistic effects of both climate variability and changing population abundance are likely for certain life stages. Continued collection of sea turtle humeri is needed to further clarify mechanisms underpinning the observed growth patterns given the coincidental timing of changes in environmental parameters examined herein.