Decomposition of Kemp’s ridley (Lepidochelys kempii) and green (Chelonia mydas) sea turtle carcasses and its application to backtrack modeling of beach strandings

Abstract

When a sea turtle dies, it typically sinks to the bottom, begins decomposing, and floats to the surface once sufficient internal gases have accumulated to produce positive buoyancy. This process is poorly characterized and is essential to understanding where and when sea turtles found on shore may have died. We conducted decomposition studies with detailed time-temperature histories using carcasses of cold-stunned sea turtles (22 Kemp’s ridleys Lepidochelys kempii and 15 green sea turtles Chelonia mydas) at temperatures of 14-32°C and depths of 2.2-9.5 m. We found strong depth/pressure-related effects; carcasses took longer to float when incubated at greater depths than shallower depths at similar temperatures. Furthermore, carcasses incubated at colder temperatures (~15°C) took 8 times longer to float than those at 32°C at the same depth. We applied accumulated degree hours (ADH; hourly sum of ambient temperatures a carcass experienced) to characterize environmental conditions associated with different stages of decomposition and key events, including buoyancy and sinking. A formula for temperature-correction of ADH was calculated to fit a non-linear increase in decomposition at higher temperatures. These data were then used to improve an existing backtracking model by incorporating water temperature, depth (pressure), bathymetry, and postmortem condition. Heat maps of the probable mortality locations from the model agreed well with carcass and effigy drift experiments, demonstrating the overall reliability of the enhanced model. Our method can be used to estimate at-sea locations where sea turtles found washed ashore in the northern Gulf of Mexico likely died and may help inform similar efforts in other regions.