Density and exposure of surface-pelagic juvenile sea turtles to Deepwater Horizon oil

Abstract

The 2010 Deepwater Horizon (DWH) oil spill posed a severe threat to surface-pelagic sea turtles because the surface convergence zones, which provide vital habitat by aggregating pelagic Sargassum and other floating material, also aggregated floating oil. Following the DWH spill, turtle rescue operations between 17 May and 9 September 2010 documented 937 juvenile sea turtles in the spill area and examined 574 captured turtles. Of the captured turtles, 81% were visibly oiled. Transect searches in convergence zones found Kemp's ridleys (51% of individuals), green turtles (37%), loggerheads (7%), hawksbills (2%), and unidentified sea turtles (2%). Linetransect methods estimated the density of all surface-pelagic sea turtles in surface convergence zones to be 3.32 km −2 (95% CI = 2.82-3.88), and the density of heavily oiled turtles to be 0.24 km −2 (95% CI = 0.15-0.39). Turtle densities and the areal extent of heavy oiling probability were used to estimate total number of turtles exposed to DWH oil. We estimate approximately 402 000 surface-pelagic sea turtles were exposed, and of those, 54 800 were likely to have been heavily oiled. Our estimates formed the basis of surface-pelagic juvenile sea turtle mortality estimates included in the DWH natural resource damage assessment.

Estimating sea turtle exposures to Deepwater Horizon oil

“…2C) than did turtle sightings during aerial surveys (Fig. 2B,D) (Garrison 2015, McDonald et al 2017, DWH Trustees 2016b. The non-uniform distribution of rescue efforts in space and time is evident in a trimodal distribution of distances from the wellhead of captured and sighted turtles (Fig.…”

“…The modeling approach that we developed was used to quantify total exposures to oil and mortality of surface-pelagic turtles and neritic turtles (DWH Trustees 2016b, Garrison 2015, McDonald et al 2017. Density and abundance, as well as exposures and injuries, were quantified separately for these 2 general life stages based on differences in assessment methods and available data as described above and by the DWH Trustees (2016b), Garrison (2015), andMcDonald et al (2017). Accordingly, adaptations to our modeling approach were required to estimate the degree of oil exposure separately for these 2 groups.…”

“…For neritic phase animals, the systematic aerial surveys provided comprehensive abundance estimates (Garrison 2015), but did not characterize exposure; thus, our model was in tended to classify the level of exposure based on spatial and temporal distribution of these larger turtles relative to factors potentially associated with oil ex posure. For surfacepelagic juveniles, the rescue operations yielded direct observations of oiling that allowed assignment of turtles to different degrees of oiling (Stacy 2012), and analysis of search transect data provided density estimates for sea turtles by degree of oiling (McDonald et al 2017). Thus, we adapted our modeling approach for this life stage to define the areal extent of potential oil exposure to which these densities could be applied (i.e.…”

“…The purpose of this step was to provide areal extents to which turtle density estimates (McDonald et al 2017) could be applied to enable calculation of surfacepelagic turtle abundance, exposures, and mortality (DWH Trustees 2016b).…”

“…Turtles that were heavily fouled were considered unlikely to survive without human intervention (Stacy 2012); thus, the DWH sea turtle injury assessment prioritized estimation of the total numbers of heavily oiled turtles. In addition, the assessment also estimated the numbers of turtles that were exposed to oil to less severe degrees (Stacy 2012, McDonald et al 2017; studies of the toxic effects of oil exposure on other vertebrate taxa indicated that less extensive exposure could also lead to chronic or sublethal effects (Mitchelmore et al 2017, this Theme Section).…”