DOI: 10.5061/dryad.wdbrv15rc
title: Aerial drones reveal the dynamic structuring of sea turtle breeding aggregations and minimum survey effort required to capture climatic and sex-specific effects

description: Quantifying how animals use key habitats and resources for their survival allows managers to optimise conservation planning; however, obtaining representative sample sizes of wildlife distributions in both time and space is challenging, particularly in the marine environment. Here, we used unoccupied aircraft systems (UASs) to evaluate temporal and spatial variation in the distribution of loggerhead sea turtles (Caretta caretta) at two high-density breeding aggregations in the Mediterranean, and the effect of varying sample size and survey frequency. In May–June of 2017 to 2019, we conducted 69 surveys, assimilating 10,075 inwater turtle records at the two sites. Optimal survey frequency to capture the dynamics of aggregations over the breeding period was &lt;2-week intervals and &gt;500 turtles (from the combined surveys). This minimum threshold was attributed to the core-area use of female turtles shifting across surveys in relation to wind direction to access warmer nearshore waters and male presence. Males were more widely distributed within aggregations than females, particularly in May when mating encounters were high. Most males were recorded swimming and oriented parallel to shore, likely to enhance encounter rates with females. In contrast, most females were generally stationary (resting on the seabed or basking), likely to conserve energy for reproduction, with orientation appearing to shift in relation to male numbers at the breeding area. Thus, by identifying the main factors regulating the movement and distribution of animals, appropriate survey intervals can be selected for appropriate home range analyses. Our study demonstrates the versatility of UAS to capture the fine-scale dynamics of wildlife aggregations and associated factors, which is important for implementing effective conservation.
description: UAS surveys were conducted using a DJI Phantom 3 Professional™ UAS (Shenzhen, China; http://www.dji.com). For all surveys, the UAS was flown at an altitude of 60 m, providing a 100 m wide field of view. From 2017 to 2019, the UAS was flown at regular intervals (between twice a week to once every 10 days) along pre-programmed (fixed) paths at the two sites. In Laganas Bay, the UAS was typically flown up to 400 m offshore along an 8 km stretch of coastline (based on Schofield et al., 2017), with intermittent checks to confirm the absence of turtles up to 800 m offshore. At Kyparissiakos Bay, the UAS was flown up to 800 m offshore along 8 km coastline (where the core nesting activity occurs; Margaritoulis and Rees, 2001). All UAS footage was reviewed manually by at least two independent observers. We recorded the longitude and latitude of each turtle detected on footage based on the timestamps following Dujon et al. (2021), along with turtle attributes. Previous field validation at the study site confirmed that the detection of turtles was reliably consistent at the surveyed depths and across surveys (Schofield et al., 2017). Repeated stationary UAS hover surveys at both sites (n = X surveys) across the season confirmed low displacement of turtles (0.17 m/s ± 0.12; n = 56), and negligible chance of being detected across multiple transects due to the high US speed (X m/s). Male turtles were visually distinguished by the presence of a tail extending beyond the carapace (Figure 1)(Schofield et al., 2017).
description: Please refer to ReadMe file.