Comparison of UAV and Boat Surveys for Detecting Changes in Breeding Population Dynamics of Sea Turtles

Abstract

Surveying the breeding population of a given species can be difficult for many logistic reasons. Marine turtles are a challenging taxon for the study of reproductive ecology and breeding strategies, because turtles aggregate off-shore and males remain exclusively at sea. For successful management of sea turtle populations, determining operational sex ratios (OSRs) on a continuing basis is critical for determining long-term population viability, particularly in the context of changing hatchling sex ratios due to temperature-dependent sex determination in a warming climate. To understand how survey technique and stage of the breeding season might influence the ability to detect turtles and determine OSRs, we surveyed the presence and identified the sex of adult male and female green sea turtles (Chelonia mydas) using a boat and small commercial unoccupied/unmanned aerial vehicle (UAV), at the start (October) and peak (December) of a nesting season at an important breeding site at Heron Island, Great Barrier Reef, Australia. The ratio of males to females within the breeding ground detected by both survey methods changed from being male-biased in October to heavily female-biased in December, indicating that most males cease their reproductive effort and depart before the peak of the nesting season. Surveying with a UAV more than doubled the rate of turtles seen per minute of survey effort compared with surveying solely from the boat and allowed surveys to be conducted at times and/or places unsafe or inaccessible for boats. The sex of a slightly greater proportion of turtles seen could not be identified by observers using a UAV versus a boat, although more turtles were detected using the UAV. The departure of many males during the peak of the nesting season is likely due to an increasing biological cost of residency in the area because males encounter fewer receptive females as the season progresses and the limited foraging opportunity is insufficient to support the number of males present. Overall, we found that UAVs are an effective tool for studying important but difficult to observe aspects of sea turtle biology.

Advancing Sea Turtle Monitoring at Nesting and Near Shore Habitats with UAVs, Data Loggers, and State of the Art Technologies

“…This protocol was refined to capture fine-scale details regarding habitat use, breeding, and foraging ecology of sea turtles in nearshore marine environments. Leveraging UAVs facilitates the acquisition of time series data with high resolution, enabling the identification of temporal and spatial patterns in the in-water behavior of animals [52]. It allows for the monitoring of local foraging and breeding aggregations [53,54], as well as the documentation of responses to both natural and anthropogenic stressors and habitat attributes [55].…”

A Review of Unmanned Aerial Vehicles Usage as an Environmental Survey Tool within Tidal Stream Environments

“…Images can also be broken down into multiple subsections to increase the accuracy of manual processing [34]. These highly accurate visual datasets can provide a novel way in which to approach existing research questions related to aspects of marine top predators (life history, behaviour, abundance and distribution) as well as a permanent record for users to return to and review [35,36].…”

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

“…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). Thus, UASs have the potential to capture the fine-scale variation in the distributions of multiple individuals in synchrony in response to intrinsic (sexspecific) and extrinsic (environmental) parameters (Schofield et al, 2019;Raoult et al, 2020).…”