Assessing climate change associated sea‐level rise impacts on sea turtle nesting beaches using drones, photogrammetry and a novel GPS system

Abstract

Climate change associated sea‐level rise (SLR) is expected to have profound impacts on coastal areas, affecting many species, including sea turtles which depend on these habitats for egg incubation. Being able to accurately model beach topography using digital terrain models (DTMs) is therefore crucial to project SLR impacts and develop effective conservation strategies. Traditional survey methods are typically low‐cost with low accuracy or high‐cost with high accuracy. We present a novel combination of drone‐based photogrammetry and a low‐cost and portable real‐time kinematic (RTK) GPS to create DTMs which are highly accurate (<10 cm error) and visually realistic. This methodology is ideal for surveying coastal sites, can be broadly applied to other species and habitats, and is a relevant tool in supporting the development of Specially Protected Areas. Here, we applied this method as a case‐study to project three SLR scenarios (0.48, 0.63 and 1.20 m) and assess the future vulnerability and viability of a key nesting habitat for sympatric loggerhead (Caretta caretta) and green turtle (Chelonia mydas) at a key rookery in the Mediterranean. We combined the DTM with 5 years of nest survey data describing location and clutch depth, to identify (a) regions with highest nest densities, (b) nest elevation by species and beach, and (c) estimated proportion of nests inundated under each SLR scenario. On average, green turtles nested at higher elevations than loggerheads (1.8 m vs. 1.32 m, respectively). However, because green turtles dig deeper nests than loggerheads (0.76 m vs. 0.50 m, respectively), these were at similar risk of inundation. For a SLR of 1.2 m, we estimated a loss of 67.3% for loggerhead turtle nests and 59.1% for green turtle nests. Existing natural and artificial barriers may affect the ability of these nesting habitats to remain suitable for nesting through beach migration.

Conservation importance of previously undescribed abundance trends: increase in loggerhead turtle numbers nesting on an Atlantic island

“…Threats to turtles in Cape Verde include the ongoing hunting of adult nesting females for consumption (Marco et al, 2011, 2012), bycatch of turtles in longline fisheries (Melo & Melo, 2013), predation of eggs by the ghost crab Ocypode cursor (Marco et al, 2015), coastal development on the beachfront and light pollution (Taylor & Cozens, 2010; Marco et al, 2018). Climate change can also have detrimental effects on marine turtles, for example through sea level rise and beach erosion (Hamann et al, 2013; Varela et al, 2018), and higher temperatures can lead to female-biased sex ratios and increased nest failures (Laloë et al, 2017). At some sites in other locations, long-term increases in the number of nests at rookeries have been followed by long-term declines (Mazaris et al, 2017; NALWG, 2018), and the increases in nesting numbers that we report should therefore be used to drive continued marine turtle conservation and surveying efforts in the Cape Verde islands, to help ensure the long-term conservation of this important loggerhead turtle population.…”

Climate change resilience of a globally important sea turtle nesting population

“…The distribution of 1,559 nests, surveyed during the peak of the 2013 ( n = 407) and 2014 ( n = 1,152) nesting seasons, was used to represent the overall nesting distribution (see Patrício, Marques, et al., ), assuming no change in the spatial distribution of nesting over time. We created a digital elevation model (DEM) of the beach in Agisoft Photoscan Professional v1.3.1 (© Agisoft), using aerial photos (80% overlap, 35 m altitude) taken from a drone (Varela et al., ). During the study period, high tide at Poilão ranged from 3.2 m (neap tide) to 4.8 m (spring tide), with mean high tide (MHT) = 4.0 m ± 0.3 SD (Bubaque Island tide tables, 40 km distant, source: Hydrographic Institute of Lisbon).…”

Operational Protocols for the Use of Drones in Marine Animal Research

“…In two of the studies, the fixed-wing craft were military grade craft, not typically available to researchers, with the capacity to operate for >1 h and tens to hundreds of kilometres outside of the visual line of sight [21,65]. The published studies used a variety of altitudes, ranging from 5 m to 122 m. To monitor hatchlings on the beach and in nearshore waters, Tapilatu, et al [64] flew at an altitude of 5 m. To record beach characteristics (photogrammetry), whereas Varela, et al [66] used an altitude of 30 m.…”