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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.
Our understanding of size-specific sea turtle behavior has lagged due to methodological limitations. However, stereo-video cameras (SVC) are an in-water approach that can link body-size and allow for relatively undisturbed behavioral observations. In this study, we conducted SVC dive surveys at local artificial reefs, piers, and jetties in the northern Gulf of Mexico (nGOM) from May 2019 to August 2021. Using SVCs, we measured sea turtle straight carapace length, documented behaviors, and quantified wariness by assessing minimum approach distance (MAD). In green sea turtles (Chelonia mydas), the observed MAD ranged from 0.72 to 5.99 m (mean 2.10 m ± 1.10 standard deviation (SD), n = 73). For loggerhead sea turtles (Caretta caretta), the MAD ranged between 0.93 and 3.80 m (mean 2.12 m ± 0.99 SD, n = 16). Kemp’s ridley sea turtles (Lepidochelys kempii) were similar to loggerheads, and MAD ranged from 0.78 to 3.63 m (mean 2.35 m ± 0.99 SD, n = 8). We then evaluated what biological factors could impact the MAD observed by species, but we excluded Kemp’s ridleys as the sample size was small. Using a linear mixed model and model selection based on AICc, the top ranked model for both green and loggerhead sea turtles included SCL as the most important factor influencing MAD. MAD did not vary with habitat type for either species. Our results showed that larger individuals, regardless of species, have a greater wariness response, becoming startled at greater distances than smaller individuals. The findings of our study support the use of SVC as an accessible, non-invasive tool to conduct ecologically relevant in-water surveys of sea turtles to link behavioral observations to body size.
Incidental bycatch of sea turtles from recreational fisheries is generally undocumented globally. At Gulf of Mexico fishing piers in the USA, bycatch is a source of injury and potential mortality of sea turtles. Recreational sea turtle bycatch has grown substantially over the last 6 yr, especially at the Navarre Beach Fishing Pier (NW Florida); the reasons for the increase and the impacts this has on the recovery of northern Gulf of Mexico sea turtle populations are largely unknown. In particular, the reasons why fishing piers are attracting sea turtles are not well understood or if the environmental context of other nearby habitats contributes to the bycatch. Using GIS, we analyzed potential environmental predictors of total sea turtle bycatch, across green sea turtles Chelonia mydas, loggerhead sea turtles Caretta caretta, and Kemp’s ridley sea turtles Lepidochelys kempii. We collated bycatch and environmental data from 20 Florida Gulf of Mexico fishing piers. We statistically assessed relationships using generalized additive models and selected the best fit model using the information-theoretic approach. We found that tonnage of nearby artificial reefs (based on defined home range and core use areas) and distance to nearest seagrass bed exhibit positive relationships with green sea turtle bycatch. For combined loggerhead and Kemp’s ridley bycatch, area of preserved water, distance to nearest seagrass bed, tonnage of nearby artificial reefs, and latitude of fishing piers were all predictors in the confidence set, but the shape of the relationships is variable and nonlinear. Further examination of sea turtle bycatch, occupancy of piers, and environmental factors pertaining to sea turtles will likely improve mitigation measures for recreational bycatch.
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