Expanding the coastal forager paradigm Tomaszewicz, Calandra N. Turner; Seminoff, Jeffrey A.; Avens, Larisa ...
Marine ecology. Progress series (Halstenbek),
01/2018, Letnik:
587
Journal Article
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The East Pacific green turtle Chelonia mydas population is gradually recovering, yet much remains unknown about their long-term demographics and habitat use due to their inaccessibility for study. We ...present the first detailed characterization of age-at-settlement (∼3–5 yr), age-at-maturity (∼17–30 yr), and long-term resource use patterns for these turtles by combining skeletochronology with stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of annual bone growth layers. We studied dead green turtles stranding along the Baja California Peninsula at Playa San Lázaro in Mexico, where their deaths are presumed to be a result of regional fisheries bycatch. Our stable isotope results indicate that these turtles utilize resources differently than other regional, lagoon-foraging green turtle aggregations. Based on stable isotope values from multiple years for individual turtles, we propose these green turtles are long-term pelagic foragers in the coastal shelf habitat of the Gulf of Ulloa and consume a more carnivorous diet from the epipelagic zone, likely including fishery discards, similar to a sympatric group of foraging North Pacific loggerhead turtles. Thus, green turtles use the Gulf of Ulloa as more than a transit area between benthic lagoon foraging and/or breeding locations. This unexpected and prolonged use of a pelagic foraging area could benefit the turtles by facilitating increased somatic growth, but may be of conservation concern as this area also experiences high fisheries turtle bycatch rates. Our findings expand the current paradigm of green turtle life history and habitat use by demonstrating an unexpected exploitation of habitat and prey for post-oceanic stage turtles.
Over the past 3 decades, the status of sea turtles and the need for their protection to aid population recovery have increasingly captured the interest of government agencies, non-governmental ...organisations (NGOs) and the general public worldwide. This interest has been matched by increased research attention, focusing on a wide variety of topics relating to sea turtle biology and ecology, together with the interrelations of sea turtles with the physical and natural environments. Although sea turtles have been better studied than most other marine fauna, management actions and their evaluation are often hindered by the lack of data on turtle biology, human-turtle interactions, turtle population status and threats. In an effort to inform effective sea turtle conservation a list of priority research questions was assembled based on the opinions of 35 sea turtle researchers from 13 nations working in fields related to turtle biology and/or conservation. The combined experience of the contributing researchers spanned the globe as well as many relevant disciplines involved in conservation research. An initial list of more than 200 questions gathered from respondents was condensed into 20 metaquestions and classified under 5 categories: reproductive biology, biogeography, population ecology, threats and conservation strategies.
The Pacific Coast of the Baja California Peninsula (BCP), Mexico, is a hotspot for foraging loggerhead turtles Caretta caretta originating from nesting beaches in Japan. The BCP region is also known ...for anthropogenic sea turtle mortality that numbers thousands of turtles annually. To put the conservation implications of this mortality into biological context, we conducted aerial surveys to determine the distribution and abundance of loggerhead turtles in the Gulf of Ulloa, along the BCP Pacific Coast. Each year from 2005 to 2007, we surveyed ca. 3700 km of transect lines, including areas up to 140 km offshore. During these surveys, we detected loggerhead turtles at the water's surface on 755 occasions (total of 785 loggerheads in groups of up to 7 turtles). We applied standard line-transect methods to estimate sea turtle abundance for survey data collected during good to excellent sighting conditions, which included 447 loggerhead sightings during similar to 400 km of survey effort. We derived the proportion of time that loggerheads were at the surface and visible to surveyors based on in situ dive data. The mean annual abundance of 43226 loggerhead turtles (CV = 0.51, 95% CI range = 15017 to 100444) represents the first abundance estimate for foraging North Pacific loggerheads based on robust analytical approaches. Our density estimate confirms the importance of the BCP as a major foraging area for loggerhead turtles in the North Pacific. In the context of annual mortality estimates of loggerheads near BCP, these results suggest that up to 11% of the region's loggerhead population may perish each year due to anthropogenic and/or natural threats. We calculate that up to 50% of the loggerhead turtles residing in the BCP region in any given year will die within 15 yr if current mortality rates continue. This underscores the urgent need to minimize anthropogenic and natural mortality of local loggerheads.
Green turtles Chelonia mydas are listed as Endangered on the IUCN Red List, yet in the South Pacific few conservation-relevant data are available for the species, especially relating to foraging and ...habitat use. Here, in situ observations and stable isotope analysis (δ
13C and δ
15N) were used to evaluate green turtle diet and recruitment patterns at Yadua Island and Makogai Island, Fiji. Juvenile green turtles (N = 110) were hand-captured, measured, and sampled. Stable isotope analysis was performed on skin samples and on putative prey items. ‘Resident’ turtles versus ‘recent recruits’ were classified based on their bulk skin tissue isotope values, which were compared with stable isotope values of local prey items and analyzed via cluster analysis. Green turtle diet composition was estimated using MixSIAR, a Bayesian mixing model. Recent recruits were characterized by ‘low δ
13C/high δ
15N’ values and ranged in curved carapace length (CCL) from 25.5 to 60.0 cm (mean ± SD = 48.5 ± 5.7 cm). Recruitment mostly occurred in summer. Green turtles identified as ‘residents’ had CCLs ranging from 43.5 to 89.0 cm (mean ± SD = 57.4 ± 9.0 cm) and were characterized by ‘high δ
13C/low δ
15N’ values; mixing model results indicate they fed primarily on invertebrates (40%), fishes (31%), and marine plants (29%). This study confirms the value of seagrass pastures as both an essential habitat and a primary food source for green turtles, and can serve as a baseline for evaluations of natural and anthropogenic changes in local green turtle aggregations.
The green turtle Chelonia mydas is a circumglobal species that is susceptible to overexploitation as a food resource and incidental mortality in fisheries. Efforts to recover regional green turtle ...populations have been hampered by a lack of information on their biology. In particular, turtle movements and home ranges in neritic foraging habitats are not well understood. Thus, wildlife managers cannot accurately determine the habitat needs of green turtle populations. To address these understudied aspects, we carried out the first ever investigation of green turtle home range in neritic foraging habitats of the eastern Pacific Ocean. Twelve turtles, ranging from 50.9 to 82.5 cm in straight carapace length and from 17 to 70 kg in mass, were tracked with radio and sonic telemetry for 34 to 96 d at the Bahia de los Angeles foraging area in the Gulf of California, Mexico. Home range areas determined with minimum convex polygon and fixed kernel density estimator methods ranged from 584 to 3908 ha (mean = 1662 plus or minus 324 ha) and 409 to 3231 ha (mean = 1537 plus or minus 280 ha), respectively. There was no evidence that straight carapace length, mass, sex, tracking duration or number of re-sightings influenced the size of the home range. Green turtle home ranges contained from 1 to 3 activity centers ranging from 3.8 to 642.2 ha in area (mean = 178.8 plus or minus 62.0 ha). Turtles were re-sighted in all depth-class regions (0 to 10 m to 40+ m) in the study area but were not found with equal frequency among these regions. The distribution of re-sightings among depth classes varied significantly between diurnal (05:00 to 18:59 h) and nocturnal (19:00 to 04:59 h) periods. While the greatest frequency of diurnal re-sightings occurred in the 10 to 20 m depth class, nocturnal encounters were most frequent in the 0 to 10 m depth class. This study shows that Bahia de los Angeles remains an important feeding ground; thus, underscoring the need to develop conservation strategies that address the impacts of ongoing commercial marine-algae harvests and net fisheries that threaten green turtles in this region.
The interactions of numerous abiotic and biotic factors experienced by sea turtle embryos during incubation affect their survival. In this study we determined the hatching and emergence success of ...green turtles Chelonia mydas from nests on 4 beaches on the Galapagos Islands, one of the most important rookeries for green turtles in the eastern Pacific Ocean. Mean (+ or -SD) hatching and emergence success for the 1039 nests examined were 46.0 + or - 33.4 and 45.6 + or - 33.4%, respectively. These values are relatively low compared to other green turtle populations worldwide. We evaluated the effects of beach, year, day of oviposition, carapace length and width of female, nest position, nest habitat, and nest chamber depth on hatching and emergence success with binomial generalized additive models with fixed effects. We found variation in hatching and emergence success was significant among beaches, years, day of oviposition, and nest habitat. Predation by feral pigs and beetles and destruction of earlier nests by nesting females were the most important causes of embryo mortality. Efforts to keep threats at minimum levels, particularly controlling pigs near Isabela beaches, should be considered a major conservation objective. This study highlights important differences among beaches within a rookery and emphasizes the need to continue improving management strategies to protect green turtles and their critical habitats. Quantitative information provided here can be used as a basis for long-term studies in the Galapagos and for comparison to other sea turtles rookeries.
Species’ foraging choices influence their somatic growth rates, age at maturity, and time spent in vulnerable early life stages. Thus, differences in population demographics are often attributed to ...variability either in diet type, quality, and/or quantity ingested. Knowledge of diet selection, though currently limited, can enhance our understanding of the roles of marine turtles in marine ecosystems and, at a finer scale, elucidate how nutrition and diet influence their growth and productivity. To investigate this relationship, we coupled stable isotope analysis with a diet preference index to provide insights into the selection and plasticity of juvenile green turtle Chelonia mydas diet. The study was conducted at 2 sites (Bonefish Hole and South Bimini) in Bimini, Bahamas, in 2016. Habitat surveys were conducted to gather habitat data and determine resource availability. A dichotomy in diet was found between the sites: at Bonefish Hole, turtles exhibited a more generalist omnivorous diet, selecting for sessile filter feeders and green algae, whereas turtles in South Bimini had a more specialist herbivorous diet, primarily consuming seagrasses and selecting for red algae, when available. The foraging dichotomy found in this study expands our understanding of the spatial differences in green turtle biology in the Bahamas and provides novel information for turtle foraging in Bimini. Knowledge about differences in intra-specific diet, with a focus on diet selection and potential drivers, can shed light on the factors that influence critical life history traits and ultimately inform species management.
Marine turtles have been exploited by humans since pre-history, with particular intensity in the last century, the result of which has been the depletion of most nesting populations in the world. In ...many cases these declines have been reversed thanks to a variety of effective conservation programs. Several nesting populations maintain positive growth trends, although most are probably depleted relative to historic levels, while others continue in a severely depleted state, with little or no population growth in recent decades. This mosaic of population trajectories along with demographic and life-history traits that buffer against extinction has created unique challenges for marine turtle assessments such as those by the World Conservation Union's (IUCN) Marine Turtle Specialist Group, which conducts global assessments for the IUCN Red List. While the Red Listing approach describes extinction risk, which theoretically can be useful for developing conservation priorities, the descriptors that have been assigned to marine turtles so far (e.g. Vulnerable, Endangered, Critically Endangered) state an unrealistic imminence of extinction, a problem enhanced by the fact that its global resolution fails to reflect the disparate population trends ongoing in different regions worldwide. Coupled with misuse of the Red List by governments and conservation organizations worldwide, these shortcomings have led to increased debate regarding its efficacy for marine turtles. In this paper we describe the Red Listing assessment process, the problems associated with this approach for marine turtles, as well as the overall value of Red List assessments for marine turtle conservation. We suggest that Red list assessments for marine turtles at the global scale do not accurately depict the current status of marine turtles and may have unintended consequences for their conservation. Largely the data do not exist, or are not reliable, making the use of the current criteria intractable. We discuss novel methods for conducting marine turtle assessments, such as using a wider array of the current Red List Criteria, modelling future population dynamics, and developing regional assessments and/or conservation prescriptive assessments.
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