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.
Highlights
IntroductionIntroduction published maps and institutional affilDetermining the reproductive mode, mating strategies and reproductive phenology of animals provides fundamental information for understanding the reproductive capacity of a species and its inherent resilience or vulnerability to disturbance in the short or long term.The ratio of breeding males to breeding females in a population, termed the operational sex ratio (OSR), is a critical determinant of population viability and is especially important in endangered populations with environmentally-determined sex, such as sea turtles [1,2,3].operational sex ratios (OSRs) are complex and difficult to measure in sea turtle populations, because adult turtles are usually present at nesting areas only transiently [4] and counting males is logistically problematic due to their completely aquatic life history [3,5]
Surveying breeding turtles is most conducted on nesting females as they come onto the beach to nest, and operational sex ratios (OSRs) are generally estimated from offspring sex ratios [6]
Adult green sea turtles of both sexes and some of unknown sex were detected using the Survey Results boat and unmanned aerial vehicle (UAV) survey techniques with a total of 145 turtles encountered in the five boat and
Summary
Introduction published maps and institutional affilDetermining the reproductive mode, mating strategies and reproductive phenology of animals provides fundamental information for understanding the reproductive capacity of a species and its inherent resilience or vulnerability to disturbance in the short or long term.The ratio of breeding males to breeding females in a population, termed the operational sex ratio (OSR), is a critical determinant of population viability and is especially important in endangered populations with environmentally-determined sex, such as sea turtles [1,2,3].OSRs are complex and difficult to measure in sea turtle populations, because adult turtles are usually present at nesting areas only transiently [4] and counting males is logistically problematic due to their completely aquatic life history [3,5]. Determining the reproductive mode, mating strategies and reproductive phenology of animals provides fundamental information for understanding the reproductive capacity of a species and its inherent resilience or vulnerability to disturbance in the short or long term. The ratio of breeding males to breeding females in a population, termed the operational sex ratio (OSR), is a critical determinant of population viability and is especially important in endangered populations with environmentally-determined sex, such as sea turtles [1,2,3]. Surveying breeding turtles is most conducted on nesting females as they come onto the beach to nest, and OSRs are generally estimated from offspring sex ratios [6]. As all sea turtles exhibit temperature-dependent sex determination (TSD), there has been increasing concern iations
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