Climate change and sea turtles: a 150-year reconstruction of incubation temperatures at a major marine turtle rookery

Sea turtles show temperature dependent sex determination. Using an empirical relationship between sand and air temperature, we reconstructed the nest temperatures since 1855 at Ascension Island, a major green turtle (Chelonia mydas) rookery. Our results show that inter‐beach thermal variations, previously ascribed to the albedo of the sand, which varies hugely from one beach to another, have persisted for the last century. Reconstructed nest temperatures varied by only 0.5 °C on individual beaches over the course of the nesting season, while the temperature difference between two key nesting beaches was always around 3 °C. Hence inter‐beach thermal variations are the main factor causing a large range of incubation temperatures at this rookery. There was a general warming trend for nests, with a mean increase in reconstructed nest temperatures for different months of between 0.36 and 0.49 °C for the last 100 years.     

Density-dependent effects on hatching success of the olive ridley turtle, Lepidochelys olivacea

Historically, the olive ridley arribada at Playa Nancite, Costa Rica, was one of the largest olive ridley arribadas in the eastern Pacific with 70,000 nesting females in a year. Recently the Nancite arribada drastically declined. We hypothesized that the population decline at Playa Nancite could have been due to low hatching success as a result of the high density of nests on the beach, such that recruitment to the population was insufficient to balance losses. To test this hypothesis, we examined density-dependent effects on hatching success and their underlying mechanisms by experimentally manipulating nest densities in experimental plots on the nesting beach. We set up four nest-density treatments in five experimental blocks. We measured effects of density on hatching success, CO(2) and O(2) concentrations and temperature both within nests and in sand adjacent to nests frequently during incubation. Experimental nest densities affected hatching success with the highest density having the lowest hatching success. Higher nest density led to lower O(2) levels and higher CO(2) levels in the nest with greater changes in the latter part of the incubation. Highest temperatures occurred in high-density areas. Temperatures were lower in sand surrounding the nest than in the nest. Effects of density on temperature, CO(2) and O(2) were confirmed at a naturally high-density nesting beach, Playa La Flor, Nicaragua. Long-term failure in production of hatchlings due to historic high densities may have contributed to the decline of arribadas on Playa Nancite. Thus, density-dependent population control would have operated at the embryonic life stage in this population of olive ridley turtles.     

Past, current and future thermal profiles of green turtle nesting grounds: Implications from climate change

Sex determination and hatching success in sea turtles is temperature dependent and as a result global warming poses a threat to sea turtles. Warmer sand temperatures may skew sea turtle population′s sex ratios towards predominantly females and decrease hatching success. Therefore, understanding the rates at which sand temperatures are likely to increase as climate change progresses is warranted. We recorded sand temperature and used historical sea surface and air temperature to model past and to predict future sand temperature under various scenarios of global warming at key sea turtle nesting grounds (n = 7) used by the northern Great Barrier Reef (nGBR) green turtle, Chelonia mydas, population. Reconstructed temperatures from 1990 to the present suggest that sand temperatures at the nesting sites studied have not changed significantly during the last 18 years. Current thermal profile at the nesting grounds suggests a bias towards female hatchling production into this population. Inter-beach thermal variance was observed at some nesting grounds with open areas in the sand dune at northern facing beaches having the warmest incubating environments. Our model projections suggest that a near complete feminization of hatchling output into this population will occur by 2070 under an extreme scenario of climate change (A1T emission scenario). Importantly, we found that some nesting grounds will still produce male hatchlings, under the most extreme scenario of climate change, this finding differs from predictions for other locations. Information from this study provides a better understanding of possible future changes in hatching success and sex ratios at each site and identifies important male producing regions. This allowed us to suggest strategies that can be used at a local scale to offset some of the impacts of warmer incubating temperatures to sea turtles.     

Microclimate modelling of beach sand temperatures reveals high spatial and temporal variation at sea turtle rookeries

The continual development of ecological models and availability of high-resolution gridded climate surfaces have stimulated studies that link climate variables to functional traits of organisms. A primary constraint of these studies is the ability to reliably predict the microclimate that an organism experiences using macroscale climate inputs. This is particularly important in regions where access to empirical information is limited. Here, we contrast correlative models based on both ambient and sea surface temperatures to mechanistic modelling approaches to predict beach sand temperatures at depths relevant to sea turtle nesting. We show that mechanistic models are congruent with correlative models at predicting sand temperatures. We used these predictions to explore thermal variation across 46 mainland and island beaches that span the geographical range of sea turtle nesting in Western Australia. Using high resolution gridded climate surfaces and site-specific soil reflectance, we predict almost 9 °C variation in average annual temperatures between beaches, and nearly 10 °C variation in average temperatures during turtle nesting seasons. Validation of models demonstrated that predictions were typically within 2 °C of observations and, although most sites had high correlations (r² > 0.7), predictive capacity varied between sites. An advantage of the mechanistic model demonstrated here is that it can be used to explore the impacts of climate change on sea turtle nesting beach temperatures as, unlike correlative models, it can be forced with novel combinations of environmental variables.     

Antigua revisited: the impact of climate change on sand and nest temperatures at a hawksbill turtle (Eretmochelys imbricata) nesting beach

Whether a turtle embryo develops into a male or a female depends, as with many other reptiles, on the temperature during incubation of the eggs. With sea turtles, warm temperatures produce 100% females. Therefore, global warming has the potential to drastically alter their sex ratios. Air temperatures on Antigua have increased by 0.7°C over the last 35 years. Measurements in both the sand and the clutches laid by hawksbill turtles (Eretmochelys imbricata) at Pasture Bay, Antigua, show that for important parts of the nesting season temperatures are already above the level producing 50% of each sex (pivotal level). Comparisons are made to sand temperature measurements taken on this beach in 1989 and 1990. It is estimated that fewer males were produced in 2003 than in the previous years. Recommendations are made for close monitoring of the fertility of eggs and for research on any turtles nesting at cooler times of year.     

Do female green turtles (Chelonia mydas) exhibit reproductive seasonality in a year-round nesting rookery?

In circumglobal populations of sea turtles, little nesting activity occurs during cold seasons when the sand temperature on a nesting beach is inappropriate for successful egg incubation. However, it is not known whether reproductive seasonality also occurs in year-round nesting rookeries where the sand temperature may be suitable throughout the year. Therefore, we examined the hypothesis that female turtles exhibit reproductive seasonality even in a year-round nesting population. To determine whether the time of nesting of individuals fluctuates between successive nesting seasons, a year-round nesting rookery of green turtles Chelonia mydas at Huyong Island, Thailand, was patrolled on foot every night for 3190 consecutive days, and nesting females were identified using microchips and metal tags. The date of first nesting within a season for an individual (nesting date) was considered the nesting season of each individual. We identified 94 females and 463 nests (including 47 unidentified nests) during the survey. Nesting dates were distributed throughout the year. Nineteen turtles remigrated to the nesting beach, and the nesting dates of the remigrated females occurred in the same seasons. This indicates that females have a fixed individual nesting season even in a year-round nesting rookery. The year-round nesting of green turtles may be attributed to a wide variation in the nesting seasons of individuals. Satellite tracking revealed that the mean travelling duration of post-nesting migrations in this population was 13.5±6.1 days. The proximity of locations for reproduction and feeding may also play an important role in sustaining year-round nesting in this population.     

Investigating the potential impacts of climate change on a marine turtle population

Recent increases in global temperatures have affected the phenology and survival of many species of plants and animals. We investigated a case study of the effects of potential climate change on a thermally sensitive species, the loggerhead sea turtle, at a breeding location at the northerly extent of the range of regular nesting in the United States. In addition to the physical limits imposed by temperature on this ectothermic species, sea turtle primary sex ratio is determined by the temperature experienced by eggs during the middle third of incubation. We recorded sand temperatures and used historical air temperatures (ATs) at Bald Head Island, NC, to examine past and predict future sex ratios under scenarios of warming. There were no significant temporal trends in primary sex ratio evident in recent years and estimated mean annual sex ratio was 58% female. Similarly, there were no temporal trends in phenology but earlier nesting and longer nesting seasons were correlated with warmer sea surface temperature. We modelled the effects of incremental increases in mean AT of up to 7.5°C, the maximum predicted increase under modelled scenarios, which would lead to 100% female hatchling production and lethally high incubation temperatures, causing reduction in hatchling production. Populations of turtles in more southern parts of the United States are currently highly female biased and are likely to become ultra‐biased with as little as 1°C of warming and experience extreme levels of mortality if warming exceeds 3°C. The lack of a demonstrable increase in AT in North Carolina in recent decades coupled with primary sex ratios that are not highly biased means that the male offspring from North Carolina could play an increasingly important role in the future viability of the loggerhead turtle in the Western Atlantic.     

Vulnerability of sea turtle nesting grounds to climate change

Given the potential vulnerability of sea turtles to climate change, a growing number of studies are predicting how various climatic processes will affect their nesting grounds. However, these studies are limited by scale, because they predict how a single climatic process will affect sea turtles but processes are likely to occur simultaneously and cause cumulative effects. This study addresses the need for a structured approach to investigate how multiple climatic processes may affect a turtle population. Here, we use a vulnerability assessment framework to assess the cumulative impact of various climatic processes on the nesting grounds used by the northern Great Barrier Reef (nGBR) green turtle population. Further, we manipulate the variables from this framework to allow users to investigate how mitigating different climatic processes individually or simultaneously can influence the vulnerability of the nesting grounds. Our assessment indicates that nesting grounds closer to the equator, such as Bramble Cay and Milman Island, are the most vulnerable to climate change. In the short‐term (by 2030), sea level rise will cause the most impact on the nesting grounds used by the nGBR green turtle population. However, in the longer term, by 2070 sand temperatures will reach levels above the upper transient range and the upper thermal threshold and cause relatively more impact on the nGBR green turtle population. Thus, in the long term, a reduction of impacts from sea‐level rise may not be sufficient, as rookeries will start to experience high vulnerability values from increased temperature. Thus, in the long term, reducing the threats from increased temperature may provide a greater return in conservation investment than mitigating the impacts from other climatic processes. Indeed, our results indicate that if the impacts from increased temperature are mitigated, the vulnerability values of almost all rookeries will be reduced to low levels.     

Climate change resilience of a globally important sea turtle nesting population

Few studies have looked into climate change resilience of populations of wild animals. We use a model higher vertebrate, the green sea turtle, as its life history is fundamentally affected by climatic conditions, including temperature‐dependent sex determination and obligate use of beaches subject to sea level rise (SLR). We use empirical data from a globally important population in West Africa to assess resistance to climate change within a quantitative framework. We project 200 years of primary sex ratios (1900–2100) and create a digital elevation model of the nesting beach to estimate impacts of projected SLR. Primary sex ratio is currently almost balanced, with 52% of hatchlings produced being female. Under IPCC models, we predict: (a) an increase in the proportion of females by 2100 to 76%–93%, but cooler temperatures, both at the end of the nesting season and in shaded areas, will guarantee male hatchling production; (b) IPCC SLR scenarios will lead to 33.4%–43.0% loss of the current nesting area; (c) climate change will contribute to population growth through population feminization, with 32%–64% more nesting females expected by 2120; (d) as incubation temperatures approach lethal levels, however, the population will cease growing and start to decline. Taken together with other factors (degree of foraging plasticity, rookery size and trajectory, and prevailing threats), this nesting population should resist climate change until 2100, and the availability of spatial and temporal microrefugia indicates potential for resilience to predicted impacts, through the evolution of nest site selection or changes in nesting phenology. This represents the most comprehensive assessment to date of climate change resilience of a marine reptile using the most up‐to‐date IPCC models, appraising the impacts of temperature and SLR, integrated with additional ecological and demographic parameters. We suggest this as a framework for other populations, species and taxa.     

Effects of Beach Nourishment on Sea Turtles: Review and Research Initiatives

Beach nourishment is an engineering solution to erosion of beaches. As in any restoration project, the goals of beach nourishment are the restoration of habitat to promote survival of plants and animals and to maintain aesthetically pleasing sites for humans. Unfortunately, beach nourishment sometimes alters parameters of the natural beach, decreasing the reproductive success of sea turtles. Engineers have recognized this problem and are working to improve nourishment practices. Biologists must specify problems incurred by sea turtles as a result of beach nourishment so that they may be addressed. A review of the literature on sea turtles and beach nourishment found certain problems repeatedly identified. For nesting females, characteristics induced by nourishment can cause (1) beach compaction, which can decrease nesting success, alter nest-chamber geometry, and alter nest concealment, and (2) escarpments, which can block turtles from reaching nesting areas. For eggs and hatchlings, nourishment can decrease survivorship and affect development by altering beach characteristics such as sand compaction, gaseous environment, hydric environment, contaminant levels, nutrient availability, and thermal environment. Also, nests can be covered with excess sand if nourishment is implemented in areas with incubating eggs. The extent and implication of each problem are discussed, and future research initiatives are proposed.     

Using historical data to assess the biogeography of population recovery

Historical ecology research is valuable for assessing long‐term baselines, and is increasingly applicable to conservation and management. In this study, we describe how historical range data can inform key aspects of protected species management, including evaluating conservation status and recovery, and determining practical management units. We examine contemporary (1973–2012) and historical (1250–1950) data on nesting beach distributions for green sea turtles Chelonia mydas in the Hawaiian Islands. Green turtle populations in Hawai‘i declined until federal and international protections began in the 1970s, but over the past four decades one index population has shown encouraging increases and broader recovery has been inferred. We find that 80% of historically major nesting populations are extirpated, or have heavily reduced nesting abundances in comparison with current estimates. Furthermore, historical nesting areas were not geographically isolated, but distributed across the archipelago. In comparison, today more than 90% of green turtle nesting in Hawai‘i occurs at a single site that is vulnerable to sea level rise. This research suggests that assessing recovery without historical data on spatial patterns may overlook important ecological dynamics at the popu lation or ecosystem level, which can result in improper or inadequate conservation assessments and recovery targets.     

Reconciling Biodiversity with Fishing: A Holistic Strategy for Pacific Sea Turtle Recovery

Recovery of sea turtle populations requires addressing: multiple sources of mortality; nonmarket, diffuse benefits with costs localized on the poor; and a transboundary resource with incomplete jurisprudence, markets, and institutions. Holistic recovery strategies include: beach conservation protecting nesting females, their eggs, and critical breeding habitat to maximize hatchling production; enhanced at-sea survival of turtles on the high seas and in commercial coastal fisheries; and reduced artisanal coastal fisheries mortality of turtles. The traditional approach of focusing long-term sustained conservation efforts on the nesting beaches has by itself led to increases in several sea turtle populations. However, current conservation is inadequate to reverse declines in other cases such as the critically endangered leatherback populations in the Pacific. This article discusses policy instruments comprising a holistic recovery strategy that reconciles fishing with biodiversity conservation.     

Metabolic heating and the prediction of sex ratios for green turtles (Chelonia mydas)

We compared incubation temperatures in nests (n=32) of the green turtle (Chelonia mydas) on Ascension Island in relation to sand temperatures of control sites at nest depth. Intrabeach thermal variation was low, whereas interbeach thermal variation was high in both control and nest sites. A marked rise in temperature was recorded in nests from 30% to 40% of the way through the incubation period and attributed to metabolic heating. Over the entire incubation period, metabolic heating accounted for a mean rise in temperature of between 0.07 degrees and 2.86 degrees C within nests. During the middle third of incubation, when sex is thought to be determined, this rise in temperature ranged between 0.07 degrees and 2.61 degrees C. Metabolic heating was related to both the number of eggs laid and the total number of hatchlings/embryos produced in a clutch. For 32 clutches in which temperature was recorded, we estimate that metabolic heating accounted for a rise of up to 30% in the proportion of females produced within different clutches. Previous studies have dismissed any effect of metabolic heating on the sex ratio of marine turtle hatchlings. Our results imply that metabolic heating needs to be considered when estimating green turtle hatchling sex ratios.     

Population Decline of Loggerhead Turtles: Two Potential Scenarios for Fethiye Beach,Turkey

Based on nesting data over a 12-year period (1993–2004), this study points to a negative population trend of the loggerhead turtle population at Fethiye beach, Turkey. The number of nests fluctuated from a maximum of 186 in 1995 to a minimum of 58 in 2004. Successively smaller peaks at 3-year intervals were followed by successively smaller troughs. Two analyses—one representing a dampened oscillation, the other retaining the period and the amplitudes of the nesting cycles—predict that nest number will drop to about 40–50 by 2015, i.e. to about 22–27% of its highest value. This drop at Fethiye does not correspond with a visible increase at neighboring beaches, leading to the interpretation that the number of nesting turtles here is declining. Moreover, the carapace size of emerging adult females is apparently decreasing, as are clutch sizes. Such a potential negative trend at a key Turkish nesting beach is cause for concern, an incentive for continued study, and a call for more coordinated and effective conservation programs in this region of the Mediterranean.     

Sand and nest temperatures and an estimate of hatchling sex ratio from the Heron Island green turtle (Chelonia mydas) rookery, Southern Great Barrier Reef

Sand and nest temperatures were monitored during the 2002–2003 nesting season of the green turtle, Chelonia mydas, at Heron Island, Great Barrier Reef, Australia. Sand temperatures increased from ∼ 24°C early in the season to 27–29°C in the middle, before decreasing again. Beach orientation affected sand temperature at nest depth throughout the season; the north facing beach remained 0.7°C warmer than the east, which was 0.9°C warmer than the south, but monitored nest temperatures were similar across all beaches. Sand temperature at 100 cm depth was cooler than at 40 cm early in the season, but this reversed at the end. Nest temperatures increased 2–4°C above sand temperatures during the later half of incubation due to metabolic heating. Hatchling sex ratio inferred from nest temperature profiles indicated a strong female bias.     

The Conservation of Sea Turtles: Practices and Problems

The various techniques in common use for conservation and restorationof depleted sea turtle populations are reviewed, namely: banninginternational commerce; operating artificial hatcheries, bothin the natural beach environment and in styrofoam and othertypes of incubators; "head-starting" of hatchlings in captivity;protection of nesting females by means of beach patrols; andtranslocation of eggs or hatchlings to distant areas from whichturtles have been extirpated or to which it is desired to introducenew colonies. The difficulties of monitoring the results ofall of these techniques are discussed, and potential dangersor disadvantages of each approach are reviewed. It is concludedthat, until unequivocal data become available, turtle conservationistsshould continue to pursue common sense or logically sound restorationprograms, but should constantly re-evaluate their actions inthe light of the latest available knowledge and modify or desistfrom current approaches as necessary.     

Physiological and Ecological Aspects of Gas Exchange by Sea Turtle Eggs

The sea turtle clutch of about 100 eggs is buried deeply inthe nesting beach.The eggs exchange respiratory gases with thesurrounding beach as their metabolic activity increases throughoutthe 60 day incubation. The O₂ consumption of individual eggsthroughout incubation is less than that of avian eggs of similarmass; however, this difference may be attributed to the differencein incubation temperature and growth rate. The O₂ consumptionof the sea turtle embryo is sufficiently low and the gas conductanceof the shell sufficiently large that only small gas partialpressure gradients occur across the shell. However, the metabolicintensity of the entire clutch is quite large, and since gasmovement through the beach is restricted, increasing gas partialpressure gradients are established between the center and peripheryof the clutch and between the clutch and surrounding beach.The rate of growth and mortality of the embryos is related torespiratory gas exchange, since maximum growth and hatchlingsuccess appear to occur in respiratory environments similarto those observed in natural nests. Embryonic growth slows andmortality increases in environments in which gas exchange isreduced below naturally occurring levels. Gas exchange considerationsmay influence nest construction, clutch size and incubationtime among sea turtles.     

Lizards fail to plastically adjust nesting behavior or thermal tolerance as needed to buffer populations from climate warming

Although observations suggest the potential for phenotypic plasticity to allow adaptive responses to climate change, few experiments have assessed that potential. Modeling suggests that Sceloporus tristichus lizards will need increased nest depth, shade cover, or embryonic thermal tolerance to avoid reproductive failure resulting from climate change. To test for such plasticity, we experimentally examined how maternal temperatures affect nesting behavior and embryonic thermal sensitivity. The temperature regime that females experienced while gravid did not affect nesting behavior, but warmer temperatures at the time of nesting reduced nest depth. Additionally, embryos from heat‐stressed mothers displayed increased sensitivity to high‐temperature exposure. Simulations suggest that critically low temperatures, rather than high temperatures, historically limit development of our study population. Thus, the plasticity needed to buffer this population has not been under selection. Plasticity will likely fail to compensate for ongoing climate change when such change results in novel stressors.     

Impact of Clutch Relocation on Green Turtle Offspring

ABSTRACT For species with temperature-dependent sex determination, such as marine turtles, global climate change poses numerous threats. At the nesting beach, rising temperatures are predicted to further skew already female-biased sex ratios and increase embryonic mortality; sea-level rise and resultant coastal squeeze may leave few alternative breeding habitats in developed regions. As a result, clutch relocation, a commonly used management tool to reduce egg loss, may become necessary for safeguarding populations. Although studies have examined the impact of relocation on clutch success, few have examined the impact of this practice on the sex or phenotypic characteristics of hatchlings produced. We used a randomized block design experiment to examine effects of relocation on green turtle (Chelonia mydas) clutches. We compared hatching success, thermal conditions, and size (length and mass) of hatchlings from in situ control clutches with those subjected to 2 relocation methods, while controlling for maternal and other environmental effects. Relocated clutches did not vary significantly from control clutches in incubation temperature or inferred sex ratios during the critical middle third of incubation when sex is thought to be determined. Hatchling size was also unaffected by relocation. Both relocation methods, however, resulted in a 20% reduction in hatching success in comparison to in situ clutches. Clutch relocation is, however, likely to affect the population primary sex ratio, when clutches are relocated from sites in proximity to the sea where tidal inundation is a threat. Here, cooler conditions are likely to produce more males than are the warmer female-producing temperatures higher up the beach. For clutches at risk, relocation is a viable process and does not appear to affect hatchling size or predicted sex ratios if relocation sites are selected in areas utilized by other females. We urge caution, however, when moving clutches from potentially male-producing sites, particularly given predicted impacts of climate change on already female-biased sex ratios.     

Recovery of the South Atlantic’s largest green turtle nesting population

Although many species of marine mega-vertebrates are threatened as a result of human activity, some populations are showing promising signs of recovery following decades of protection. In this study, we report on the status of the South Atlantic’s largest green turtle (Chelonia mydas) nesting aggregation at Ascension Island, 70 years after legal protection and the cessation of commercial turtle harvesting that decimated the population. Using a monitoring dataset spanning 36 years, we modelled long-term trends in nesting activity at both a rookery level and across individual nesting beaches and beach clusters. Since monitoring began in 1977, the average number of green turtle clutches deposited annually at Ascension Island has increased sixfold, from approximately 3,700 to 23,700; a trend that has been accompanied by a significant decrease in the average size of nesting females. Interestingly, however, rates of increase in nesting activity have varied dramatically among nesting beaches, ranging from 0.4 to 6 % growth per annum. More than 97 % of this variation could be explained by distance from the main human settlement of Georgetown—the historic centre of turtle harvesting—with beaches closer to Georgetown experiencing the most rapid growth. More rapid population growth close to human centres seems counterintuitive, but may reflect the more intensive depletion of these accessible, local stocks during the harvesting era. Overall, the Ascension Island green turtle population appears to be recovering strongly, mirroring positive trends for this species across many parts of its geographic range. While not a cause for complacency, these trends are encouraging and demonstrate the capacity of marine megafauna to rebound when anthropogenic pressures are alleviated through conservation action.