Loggerhead turtles in the western and central Mediterranean Sea: habitat use, stock composition and sex ratios

Abstract

The loggerhead turtle (Caretta caretta) is the most common and best-studied sea turtle species in the Mediterranean Sea. Thanks to more than two decades of conservation efforts, population trends are now stable or even increasing which allowed the downgrading of the Mediterranean sub-population from “endangered” to “least concern” in the 2015 Red List Assessment of the IUCN. However, these results are mostly due to the effective protection of Mediterranean nesting grounds. The lack of basic knowledge on sea turtle biology and the human-turtle-environment interactions still limits our ability to accurately assess the conservations status of the species and design effective management strategies and conservation responses to the anthropogenic threats. This reflects the logistic challenges of studying sea turtles in the open ocean due to their solitary nature, migratory behaviour and longevity. This PhD thesis aimed at advancing basic biological knowledge on the loggerhead turtle by answering four sub-objectives. First, I determined the density of the loggerhead turtle humerus and examined the relationship between bone density and turtle size with the aim to contribute to a better understanding of bone density patterns and the possible functional relation to the diving behaviour and foraging strategy adopted during the different phases of the species’ life cycle. The study showed that the loggerhead turtle possess denser bones than the pelagic leatherback turtle but bone ballastin is not as extreme as in some slow swimming, shallow diving marine tetrapods. Bone density increases significantly with turtle size, which appears to be functionally correlated to the ontogenetic changes in the marine aquatic habits of the species. Further research is necessary to understand if the relation between bone density and turtle size highlighted by this study may offer an alternative to the enumeration of lines of arrested growth in skeletochronological studies to infer turtle age, as found in some cetacean species. Then, I determined juvenile and adult sex ratios of the foraging aggregation in the south Tyrrhenian Sea and how they relate with the highly female skewed hatchling sex ratios produced at Mediterranean nesting beaches. The study showed that a female biased sex ratio is maintained in Mediterranean juveniles although less pronounced than in hatchlings. The proportion of females decreases further in the adult portion of the foraging aggregation although sampling bias problems associated with adult breeding migrations and homing behaviour may have affected the result. For this reason, I suggested that juvenile sex ratios may offer a more viable estimate of the 3 population’s functional sex ratio provided that long time series, large sample sizes and the most rigorous criteria for sex identification are used. This study contributed to improve significantly our understanding of the dynamics of loggerhead turtle sex ratios in the Mediterranean Sea and identify the south Tyrrhenian foraging ground as a possible index area to detect biologically significant sex ratio shifts in the Mediterranean sub-population thanks to the availability of long time series. A third research line aimed to determine the connections among rookeries and foraging grounds and the parameters that shape them. The loggerhead turtle is a highly migratory species that use widely separated and ecologically disparate habitats throughout its life. Individuals from reproductively independent Management Units (MU) usually mix in foraging habitats. The capacity to link these turtles back to their rookery of origin is crucial for our ability to manage the species effectively. I investigated the demographic composition of the juvenile and adult cohorts foraging in the south Tyrrhenian Sea and on the African continental shelf off south Tunisia. Juveniles of Atlantic origin were found to be significantly less abundant in the latter and differences in the relative contribution of Mediterranean MUs to the juvenile aggregations foraging at these two locations were also detected. Surface currents and hatchling imprinting on possible suitable neritic habitats during the early oceanic developmental stage were suggested as important factors shaping the composition of juvenile aggregations in the Mediterranean Sea. The study evidenced also weak but significant differentiation between juvenile and adult cohorts on the African continental shelf, with the latter showing a much stronger connectivity with the nearby Libyan MU which may be explained by the strong homing behaviour of adult turtles. Overall, this study demonstrates how anthropogenic threats in the Tyrrhenian Sea and on the African continental shelf differentially affect the Mediterranean and Atlantic sub-populations and provides information that is essential for developing effective management plans of the species. Finally, I investigated how climate warming is affecting loggerhead turtle nesting in the western Mediterranean and what is the likely future scenario. Climate warming is a major challenge to Earth’ biota that forces species to adapt in situ or move to a different location in order to follow shifts in their thermal niche. Many different marine taxa have already shifted their distributions to endure changes in ambient temperature. In particular, marine ectotherms, such as the loggerhead turtle, are expected to colonise northern habitats as they become suitable for colonists due to the ongoing climate warming. This study showed that recently, the species has begun to nest 4 steadily beyond the northern edge of the species’ range in the Mediterranean basin. This range expansion was associated with a significant warming of spring and summer sea surface temperature (SST) that offers a wider thermal window suitable for nesting. Moreover, the analysis of incubation and nest parameters indicated that environmental conditions have improved lately supporting the predictions of good habitat suitability for nesting in this area under a climate warming scenario. However, post-hatchlings departing from this location experience low winter SST that may affect their survival and thus hamper the stabilization of the site by self-recruitment. The different dynamics of winter and spring SST responses to climate change may result in a conundrum where the number of exploratory females and hatching success will increase with climate warming but the stabilization of this northern nesting site will still be hampered by high mortality rates of post-hatchlings due to the persistence of low winter temperatures. This study highlighted the importance of quantifying and understanding the interplay between dispersal and environmental changes at all life stages for predicting loggerhead turtle range expansion with climate warming. Overall this PhD thesis contributed to improve our understanding of loggerhead turtle biology and provided information that are important for the future development of effective conservation and management strategies of the species in the Mediterranean Sea.