Abstract
Population size estimates are key parameters used in assessments to evaluate and determine a species’ conservation status. Typically, sea turtle population estimates are made from nesting beach surveys which capture only hatchling and adult female life stages and can display trends opposite of the full population. As such, in-water studies are critical to improve our understanding of sea turtle population dynamics as they can target a broader range of life stages – though they are more logistically and financially challenging to execute compared to beach-based surveys. Stereo-video camera systems (SVCS) hold promise for improving in-water assessments by removing the need to physically capture individuals and instead extract 3D measurements from video footage, thereby simplifying monitoring logistics and improving safety for the animals and surveyors. To demonstrate this potential, snorkel surveys were conducted at artificial habitats in the northeastern Gulf of Mexico (neGOM) to collect size and photo-identification data on sea turtles in situ using a SVCS. Over 29.86 survey hours, 35 sea turtles were observed across three species (Caretta caretta, Chelonia mydas, and Lepidochelys kempii) and all neritic life stages (juvenile, sub-adult, and adult) utilizing different habitats, including artificial reefs, jetties, and fishing piers. Greens straight carapace length ranged from 28.55 to 66.96 cm (n = 23, mean 43.07 cm ± 11.26 cm standard deviation; SD) and loggerheads ranged from 59.71 to 91.77 cm (n = 10, mean 74.50 cm ± 11.35 cm SD), and Kemp’s ridleys ranged from 42.23 cm to 44.98 cm (mean 43.61 cm ± 1.94 cm SD). Using a linear mixed model, we found that species and habitat type were the most important predictors of sea turtle body length distribution. Overall, this case study demonstrates the potential of SVCS surveys to enhance our understanding of the population structure of sea turtle species within the neGOM and elsewhere.
Highlights
Population assessments are crucial to determine population trends and status
The northeastern Gulf of Mexico (neGOM) is a dynamic coastal environment composed mostly of soft, sandy bottom interspersed with inlets of estuarine seagrass beds and sparse natural hard-bottom or reefs (Locker et al, 2000) used by loggerhead, green, and Kemp’s ridley sea turtles (Lamont and Iverson, 2018; Wildermann et al, 2019)
To examine factors that may be influencing size distribution, we evaluated individual body size (i.e., straight carapace length (SCL)) as a function of water temperature, species, and habitat type in a linear mixed effects model (LMM)
Summary
Population assessments are crucial to determine population trends and status (i.e., trends in size class distribution; Crouse et al, 1987; Summers et al, 2017). Incorporating demographic data, morphometric data, would lead to more effective modeling of populations and allow for researchers to estimate age at maturity, growth rates, and survival rates (Heppell et al, 2003; Casale et al, 2011). These vital rates allow researchers and conservation management agencies to determine if a population is declining or recovering and which, if any, conservation management actions are needed to aid in recovery (Bjorndal et al, 2011; Redfoot and Ehrhart, 2013). Size-frequency distributions which encompass juvenile sizes can be powerful tools to understand population recovery that may not yet be reflected in adults, who are more commonly monitored (Hilborn and Walters, 2001; Ault et al, 2008; Heppell et al, 2012)
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