ISTS Symposium43 Program/Agenda

Reproduction requires substantial investment of nutritional resources, and as a result, most sea turtle species do not nest annually. Reproductive investments have long thought to be financed from stored resources in sea turtles, and early studies classified them as capital breeders. A growing body of evidence suggests that sea turtles may be more flexible in their ability to rely on energy acquired during the reproductive period. However, the evidence is not always clear in establishing whether foraging occurs and if acquired nutrients are routed to the production of eggs. We addressed two objectives in a nesting population of loggerhead turtles to answer a long-standing question of where sea turtles fall on the capital vs. income breeding spectrum. First, we assessed whether loggerheads fast during the nesting season. Second, we assessed the source of nutritional allocations to the proteins in egg yolk and albumen. Samples of nesting female plasma and eggs (yolk and albumen) were collected at Wassaw Island, Georgia. Additionally, plasma was obtained from juvenile and adult loggerheads in the coastal waters off the coast of South Carolina and Georgia to represent foraging individuals. Amino acid carbon and nitrogen stable isotope analysis revealed that females were likely not feeding during the nesting season. Additionally, multivariate analysis of amino acid isotope data indicated that the yolk was synthesized while actively foraging prior to the reproductive period, but the albumen was synthesized while fasting. This approach provides a promising method for determining the nutritional status of reproduction in sea turtles.

The barnacle Chelonibia testudinaria is arguably the most common and conspicuous epibiont found on sea turtles. As C. testudinaria appears to settle in coastal habitats, this barnacle could serve as an indicator of sea turtle habitat use. The utility of C. testudinaria for this purpose is, however, limited by a lack of information on their growth rates. Here, we generated growth curves for C. testudinaria on green turtles (Chelonia mydas) by taking repeat measurements of barnacle sizes when turtles were encountered on successive nesting events in northwest Costa Rica during 2023/2024. We measured growth rates of 106 barnacles from 38 green turtles and fitted these measurements to a von Bertalanffy growth model. Our measured growth rates were double those reported for C. testudinaria on loggerhead turtles in Australia. We found a statistically significant effect of beach and turtle identity on growth rates. The models that best explained the variance in growth rates also included sea surface temperature even through this variable was not significant. Our results suggest that barnacles had settled, and thus turtles may have began occupying coastal habitats, a mean of 76 days before the host turtle laid their first clutch for that nesting season. We also provide the first measurements of barnacle detachment rates in wild sea turtles with 24.1% of barnacles detaching after a single internesting period. Our study illustrates that C. testudinaria can serve as useful indicators of habitat use in sea turtles although further studies into the factors driving barnacle growth are still required.

Prior to a reproductive event, green turtles (Chelonia mydas) must forage for more than a year to accumulate the energy needed for long-distance migrations, courtship, mating, and egg production. Although it is commonly assumed that nesting females fast, this behavior has not been confirmed. To address this gap, we investigated whether female green turtles nesting at Tortuguero, Costa Rica, fast during the nesting season. Additionally, when nesting females were not fasting, we assessed whether the diet at Tortuguero differed from that of green turtles at their main foraging ground in the Miskito Cays, Nicaragua.

In 2020, we collected 71 diet samples from green turtles legally harvested from the Miskito Cays for human consumption. In 2021 and 2022, we collected gut contents from 15 green turtles preyed upon by jaguars (Panthera onca) at Tortuguero. We measured the proportional volume of food items in the samples.

Of the 15 jaguar-killed green turtles we examined, 11 (73 %) presented food whereas 4 had empty digestive tracts. The green turtle diet at the foraging ground was composed of primarily seagrasses (>90% Thalassia testudinum) with smaller amounts of algae and animal matter. Food boli at the nesting beach were composed mainly of riparian vegetation, algae, and anthropogenic debris, especially plastics.

Our findings indicate that some green turtles nesting at Tortuguero had foraged. Although food ingested was lower in quality and smaller in quantity compared to the diet of green turtles at the foraging ground. The decreased intake at the nesting beach may be due to the lack of forage and/or the reduced coelomic space available as egg clutches occupy much of the body cavity of nesting females.

Moreover, the difference in green turtle diet composition we observed at the nesting ground can be attributed to the lack of seagrass meadows off Tortuguero. In the absence of their preferred forage, herbivorous green turtles adapt by consuming riparian vegetation that flows into the ocean through the numerous river mouths in the region.

Additionally, the major river basins discharging into the Caribbean Sea near Tortuguero carry anthropogenic debris from urban areas in Costa Rica’s Central Valley. This debris, transported with riparian vegetation, enters coastal waters where it becomes available to green turtles. Consequently, we observed elevated plastic ingestion among nesting green turtles in comparison to individuals at the Miskito Cays foraging ground where seagrass is widely available. This underscores the ecological impact of riverine pollution on endangered marine megafauna.

In conclusion, our findings provide important insights into the feeding behavior and diet composition of green turtles at major nesting and foraging grounds in the Caribbean Sea. We demonstrated that many nesting females continue to forage, albeit with decreased ingestion of seagrass and increased ingestion of low-quality riparian vegetation and plastics. These results highlight the complexity of green turtle foraging ecology and underscore the need to consider the full spectrum of ecological pressures that sea turtles face across the different habitats used to inform conservation strategies.

High and continuously rising atmospheric temperatures are altering ecosystems around the globe. On tropical and subtropical sea turtle nesting beaches, increased atmospheric temperatures are directly impacting sand temperature and, consequently, development in temperature-sensitive sea turtle eggs. Increases in incubation temperature of eggs are known to influence the sex of hatchlings (female-skewed), though also have the potential to result in developmental failure at thermal extremes (33-35°C). Leatherbacks (Dermochelys coriacea) have low hatching success (~50%) and have been shown to experience further declines in hatching success in response to higher nest temperatures; despite this, temperature mortality thresholds continue to be poorly understood for this species. In partnership with Pacuare Nature Reserve (Caribbean Coast of Costa Rica), Leatherback nest temperatures were recorded to understand 1) if this species is exceeding previously proposed thermal limits, 2) how hatching success responds to temperature, and 3) whether nest-cooling (artificial shading) is an effective intervention. During the 2022 and 2023 nesting seasons, clutches of eggs were collected from females along 6 km of beach and relocated to a central hatchery. Nests were assigned to shaded or unshaded plots and HOBO temperature loggers were installed within the egg chamber (2022, n=34; 2023, n=28), measuring temperature at 30-minute intervals throughout the incubation period. Minimum, maximum, and mean temperatures were extracted for each nest, and the relationship between temperature and hatching success was investigated using a mixed modelling approach. Differences in nest temperatures and hatching success were observed between shaded and unshaded groups, as well as interannually, with shaded nests having higher success in 2023 but not 2022. Strengthening our understanding of how hatching success might be impacted by hotter incubation temperatures driven by climate change is vital for informing future conservation decisions and scaling of interventions to future warming predictions.

Trials that aim to cool sea turtle nests are underway across the globe and usually involve nest watering or shading. An alternative, barely tested intervention, is to divide egg clutches across two nest chambers to reduce metabolic heating. In this project we coupled engineering and biological modelling approaches (Finite Element Analysis and Dynamic Energy Budget Theory) to develop computer models of sea turtle nests that account for physical and biological sources of heat in space and time. We selected three sea turtle species (green, flatback and loggerheads), each with contrasting nest shapes, nest depths, egg sizes and clutch sizes, meshed a typical nest structure for each species, and simulated the temperatures of different sized egg clutches within each nest. We ran each model assuming the nest experienced typical sand temperatures across representative beaches in Western Australia (and one on the Cocos Keeling islands) under a recent climate, and re-ran the same models assuming that the climate was either 2 or 4°C warmer. We successfully validated our model outputs against empirical temperature data collected from clutch splitting trials for flatback turtles in Western Australia, and for loggerhead turtles from the North Atlantic. We showed that a ‘clutch splitting’ manipulation is expected to have the greatest impact for the loggerhead sea turtle, which has the shallowest nest and a relatively large number of eggs. On average, the temperatures at which loggerhead embryos develop in split nests should be about 1.8°C lower under the current climate, and about 2.2 °C lower under future climates. Notably, we show that under future warmer climates, most eggs in whole (i.e. unmanipulated) loggerhead clutches would regularly experience temperatures above 34°C, which would likely be lethal, whereas most split clutches would survive and hatch as mixed sexes. Based on these promising simulations, we now have a generalised methodology that can be used to identify species and regions where clutch splitting could improve hatching success and reduce female biases. In particular, we expect this manipulation could be useful for enhancing recruitment at rookeries with consistently low hatching success due to high beach temperatures. Additionally, it could be deployed at regional scales in years when summer heatwaves are forecast.

The current apparent colonization of the western Mediterranean by the loggerhead sea turtle (Caretta caretta) provides an excellent opportunity to study the conditions under which this complex process takes place and the factors that modulate it. Many questions remain unanswered in the context of climate change and rising temperatures in the terrestrial and aquatic ecosystems that directly affect sea turtles. Aspects such as the origin of breeders, distribution, behavior, reproductive physiology and reproductive success are being studied to try to understand the process in depth, as well as to define the necessary conservation measures for the species in a likely scenario of constant change. One of the key parameters in studying the temperature-dependent reproductive biology of a species is the study of the sex production.

In order to assess the sex production within this colonization phenomenon in the western Mediterranean, we identified the sex of 618 individuals from 47 loggerhead turtle nests along the Spanish Mediterranean coast in the period between 2014 and 2023. Sex identification was carried out by histological examination of formalin-preserved tissues from embryos and dead neonates, and by endoscopic examination of post-neonatal individuals maintained in head-starting programs.

The results of a decade-long study of the sex ratio on the Spanish coast show a higher production of males (67% males vs. 33% females). Spatial and temporal analysis of nests and sex production shows a clear trend towards higher male production, with few exceptions, even in the last years of the study when the area experienced episodes of record temperatures.

The incipient colonization of the area seems to be producing a greater number of males, which could allow for a more balanced sex ratio, unlike what has been observed in the main breeding areas of the planet, where population feminization is a concern. At the same time, females are being produced that could generate recruitment to favor an effective colonization of the area.