Incorporating Geographical Scale and Multiple Environmental Factors to Delineate the Breeding Distribution of Sea Turtles

Abstract

Temperature is often used to infer how climate influences wildlife distributions; yet, other parameters also contribute, separately and combined, with effects varying across geographical scales. Here, we used an unoccupied aircraft system to explore how environmental parameters affect the regional distribution of the terrestrial and marine breeding habitats of threatened loggerhead sea turtles (Caretta caretta). Surveys spanned four years and ~620 km coastline of western Greece, encompassing low (<10 nests/km) to high (100–500 nests/km) density nesting areas. We recorded 2395 tracks left by turtles on beaches and 1928 turtles occupying waters adjacent to these beaches. Variation in beach track and inwater turtle densities was explained by temperature, offshore prevailing wind, and physical marine and terrestrial factors combined. The highest beach-track densities (400 tracks/km) occurred on beaches with steep slopes and higher sand temperatures, sheltered from prevailing offshore winds. The highest inwater turtle densities (270 turtles/km) occurred over submerged sandbanks, with warmer sea temperatures associated with offshore wind. Most turtles (90%) occurred over nearshore submerged sandbanks within 10 km of beaches supporting the highest track densities, showing the strong linkage between optimal marine and terrestrial environments for breeding. Our findings demonstrate the utility of UASs in surveying marine megafauna and environmental data at large scales and the importance of integrating multiple factors in climate change models to predict species distributions.

Aerial Drones Reveal the Dynamic Structuring of Sea Turtle Breeding Aggregations and Minimum Survey Effort Required to Capture Climatic and Sex-Specific Effects

“…Our study showed that the distribution of sea turtles at both sites shifted in relation to wind, supporting existing studies in this study region (Schofield et al, 2007;Schofield et al, 2009;Dickson et al, 2021). Wind impacts sea turtles in different ways depending on life stage and habitat.…”

“…We showed that even one UAS survey at the peak of the breeding season could capture the overall home range of breeding sea turtles, with 50% KUDs for single-surveys similar to that generated from remote tracking datasets (Schofield et al, 2013). Furthermore, by timing surveys to coincide with peak periods of breeding area use (Limpus et al, 2003;Whiting et al, 2013), UASs could be used to conduct scoping surveys to detect habitat use inside and outside of aggregations (Dickson et al, 2021). However, as UASs only capture the distribution of wildlife within surveyed areas, combining them with other approaches, such as remote tracking of individuals (Hart and Hyrenbach, 2009;Wilmers et al, 2015), could be used to validate survey area selection and establish connectivity across surveyed sites.…”

“…This study was conducted in Laganas Bay (37°43′N, 20°52′E; Zakynthos Island) and in the southern part of Kyparissiakos Bay (37°26'N 21°37'E; Peloponnese) in the Ionian region of western Greece, Mediterranean Sea (Supplementary Figure S1). The two sites are situated about 85 km apart (Laganas to Kalo Nero), and are considered "major" (>100 nests/season) loggerhead sea turtle nesting areas in Greece, encompassing ~54% of average nests/ season in Greece and ~26% of average nests/season in the Mediterranean (Casale and Margaritoulis, 2010;Dickson et al, 2021). In the study region, mating activity peaks from late-April to mid-May, with males departing in late-May to early June; females begin nesting in late-May until early August, with females departing from early July to early August (Schofield et al, 2013).…”

“…For instance, Schofield et al (2009) showed that female loggerhead sea turtles (Caretta caretta) at a site on the edge of the species breeding range exhibited highly dynamic, but synchronised, daily movement in response to wind direction to access warm waters. Wind also has a strong effect on fine-scale sea turtle distributions in relation to beach selection (Weishampel et al, 2006;Dickson et al, 2021), optimising body temperature (Fossette et al, 2012), and foraging habitat use (Shaver et al, 2017). Recent UAS studies have shown that the relative number of male and female turtles (operational sex ratio) shifts from being highly male-biased to highly female-biased across the breeding season (Schofield et al, 2017;Yaney-Keller et al, 2021), with behavioural differences observed in response to shifts in sex ratio, and potential effects on how the two sexes distribute over time (Schofield et al, 2006).…”

“…We also assessed the effect of varying survey frequency, and the sample size of individuals detected in surveys, on evaluations of aggregation structure. The study sites include Laganas Bay (Zakynthos Island) and Kyparissiakos Bay (western Greece), both of which support in-water turtle densities of >270 individuals per km 2 (Dickson et al, 2021). We hypothesised that the aggregation structure would be highly dynamic due to both extrinsic (sea temperature and wind) and intrinsic (turtle sex) factors.…”