Loggerhead turtle Caretta caretta density and abundance in Chesapeake Bay and the temperate ocean waters of the southern portion of the Mid-Atlantic Bight

Abstract

We conducted aerial surveys of sea turtles in 2011 and 2012, incorporating corrections for perception and availability bias in Chesapeake Bay and near-shore continental shelf waters of the Mid-Atlantic Bight off the US states of Virginia and Maryland. Results of these surveys and ancillary research to determine surface times for loggerhead turtles provide us with a new baseline population estimate for turtles in the region. Prior surveys were conducted in Chesapeake Bay in the mid-1980s and early 2000s, and in ocean waters in the late 1970s and early 1980s. Although comparison of density estimates not corrected for availability between prior surveys and this effort suggests that the population of sea turtles, especially loggerhead turtles, is higher than previous estimates, differences between surveys may be the result of survey methodologies and cannot be assumed to be true changes in density. Surface time for availability corrections was calculated using dive summaries from satellite telemetry on 27 loggerhead turtles tracked between 2011 and 2015. We calculated stratified seasonal availability corrections for bay and ocean waters based on assumed differences in turtle behavior and water clarity between the 2 habitats. For each habitat, we provided seasonal corrections for 3 detection depth bins (shallow, moderate, and deep) to account for differences in sub-surface detection ranges. Differences and trends toward differences among availability corrections underscore the need to better understand the many variables that affect surface time for sea turtles in temperate waters, and the effect that availability has on abundance and density estimates.

Estimating the complex patterns of survey availability for loggerhead turtles

“…Heterogeneity in availability is not uncommon for marine animals, and has been noted for mammals (Marsh and Sinclair 1989, Pollock et al 2009), sharks (Nykänen et al 2018), and particularly sea turtles (Thomson et al 2012, 2013, Barco et al 2018). Prior researchers reported differences in surface time for sea turtles based on demographics (Cardona et al 2005), environmental gradients (Thomson et al 2013, Barco et al 2018), and space and time (Barco et al 2018).…”

“…Heterogeneity in availability is not uncommon for marine animals, and has been noted for mammals (Marsh and Sinclair 1989, Pollock et al 2009), sharks (Nykänen et al 2018), and particularly sea turtles (Thomson et al 2012, 2013, Barco et al 2018). Prior researchers reported differences in surface time for sea turtles based on demographics (Cardona et al 2005), environmental gradients (Thomson et al 2013, Barco et al 2018), and space and time (Barco et al 2018). This suggests applying uniform correction factors for availability may lead to biased estimates of abundance, with the degree of bias being study‐dependent.…”

“…This suggests applying uniform correction factors for availability may lead to biased estimates of abundance, with the degree of bias being study‐dependent. For sea turtles that spend a variable amount of time underwater, this makes refining availability bias correction factors to appropriate spatiotemporal scales very important (Barco et al 2018).…”

“…Data obtained from biologgers include valuable information on behavior, which can then be used to correct density and abundance estimates for availability bias. In a marine context, knowledge about the average dive and surface duration, or the proportion of time at the surface, can aid in developing these corrections (Laake et al 1997, Barco et al 2018, Sparks and DiMatteo 2020).…”

Riders on the storm: loggerhead sea turtles detect and respond to a major hurricane in the Northwest Atlantic Ocean

“…Dive behaviors were assessed by analyzing daytime (0600–2000 EDT) dive duration and maximum dive depth for individuals that remained in their foraging ranges in order to investigate dive behavior relative to local changes in the habitat. Daytime diving was specifically chosen because changes to dive behavior could substantially impact the availability of turtles at the surface during line-transect surveys, and subsequent abundance estimates [ 40 ]. Dive duration relative to maximum dive depth was analyzed to better understand the composition of the type of dive behavior exhibited in each phase.…”