Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre

Leatherback sea turtles (Dermochelys coriacea) travel thousands of kilometres between temperate feeding and tropical breeding/over-wintering grounds, with adult turtles able to pinpoint specific nesting beaches after multi-year absences. Their extensive migrations often occur in oceanic habitat where limited known sensory information is available to aid in orientation. Here, we examined the migratory orientation of adult male, adult female and subadult leatherbacks during their open-ocean movements within the North Atlantic subtropical gyre by analysing satellite-derived tracks from fifteen individuals over a 2-year period. To determine the turtles'' true headings, we corrected the reconstructed tracks for current drift and found negligible differences between current-corrected and observed tracks within the gyre. Individual leatherback headings were remarkably consistent throughout the subtropical gyre, with turtles significantly oriented to the south-southeast. Adult leatherbacks of both sexes maintained similar mean headings and showed greater orientation precision overall. The consistent headings maintained by adult and subadult leatherbacks within the gyre suggest use of a common compass sense.     

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.     

Spatio-temporal variation in the incubation duration and sex ratio of hawksbill hatchlings: Implication for future management

Climate change poses a unique threat to species with temperature dependent sex determination (TSD), such as marine turtles, where increases in temperature can result in extreme sex ratio biases. Knowledge of the primary sex ratio of populations with TSD is key for providing a baseline to inform management strategies and to accurately predict how future climate changes may affect turtle populations. However, there is a lack of robust data on offspring sex ratio at appropriate temporal and spatial scales to inform management decisions. To address this, we estimate the primary sex ratio of hawksbill hatchlings, Eretmochelys imbricata, from incubation duration of 5514 in situ nests from 10 nesting beaches from two regions in Brazil over the last 27 years. A strong female bias was estimated in all beaches, with 96% and 89% average female sex ratios produced in Bahia (BA) and Rio Grande do Norte (RN). Both inter-annual (BA, 88 to 99%; RN, 75 to 96% female) and inter-beach (BA, 92% to 97%; RN, 81% to 92% female) variability in mean offspring sex ratio was observed. These findings will guide management decisions in Brazil and provide further evidence of highly female-skew sex ratios in hawksbill turtles.     

Predicting bycatch hotspots for endangered leatherback turtles on longlines in the Pacific Ocean

Fisheries bycatch is a critical source of mortality for rapidly declining populations of leatherback turtles, Dermochelys coriacea. We integrated use-intensity distributions for 135 satellite-tracked adult turtles with longline fishing effort to estimate predicted bycatch risk over space and time in the Pacific Ocean. Areas of predicted bycatch risk did not overlap for eastern and western Pacific nesting populations, warranting their consideration as distinct management units with respect to fisheries bycatch. For western Pacific nesting populations, we identified several areas of high risk in the north and central Pacific, but greatest risk was adjacent to primary nesting beaches in tropical seas of Indo-Pacific islands, largely confined to several exclusive economic zones under the jurisdiction of national authorities. For eastern Pacific nesting populations, we identified moderate risk associated with migrations to nesting beaches, but the greatest risk was in the South Pacific Gyre, a broad pelagic zone outside national waters where management is currently lacking and may prove difficult to implement. Efforts should focus on these predicted hotspots to develop more targeted management approaches to alleviate leatherback bycatch.     

DNA barcoding of Brazilian sea turtles (Testudines)

Five out of the seven recognized species of sea turtles (Testudines) occur on the Brazilian coast. The Barcode Initiative is an effort to undertake a molecular inventory of Earth biodiversity. Cytochrome Oxidase c subunit I (COI) molecular tags for sea turtle species have not yet been described. In this study, COI sequences for the five species of sea turtles that occur in Brazil were generated. These presented widely divergent haplotypes. All observed values were on the same range as those already described for other animal groups: the overall mean distance was 8.2%, the mean distance between families (Dermochelyidae and Cheloniidae) 11.7%, the mean intraspecific divergence 0.34%, and the mean distance within Cheloniidae 6.4%, this being 19-fold higher than the mean divergence observed within species. We obtained species-specific COI barcode tags that can be used for identifying each of the marine turtle species studied.     

The ecology of overwintering among turtles: where turtles overwinter and its consequences

Turtles are a small taxon that has nevertheless attracted much attention from biologists for centuries. However, a major portion of their life cycle has received relatively little attention until recently - namely what turtles are doing, and how they are doing it, during the winter. In the northern parts of their ranges in North America, turtles may spend more than half of their lives in an overwintering state. In this review, I emphasise the ecological aspects of overwintering among turtles, and consider how overwintering stresses affect the physiology, behaviour, distributions, and life histories of various species.Sea turtles are the only group of turtles that migrate extensively, and can therefore avoid northern winters. Nevertheless, each year a number of turtles, largely juveniles, are killed when trapped by cold fronts before they move to safer waters. Evidently this risk is an acceptable trade-off for the benefits to a population of inhabiting northern developmental habitats during the summer.Terrestrial turtles pass the winter underground, either in burrows that they excavate or that are preformed. These refugia must provide protection against desiccation and lethal freezing levels. Some burrows are extensive (tortoise genus Gopherus), while others are shallow, or the turtles may simply dig into the ground to a safe depth (turtle genus Terrapene). In the latter genus, freeze tolerance may play an adaptive role.Most non-marine aquatic turtles overwinter underwater, although Clemmys (Actinemys) marmorata routinely overwinters on land when it occurs in riverine habitats, Kinosternon subrubrum often overwinters on land, and several others may overwinter terrestrially on occasion, especially in more southern climates. For northern species that overwinter underwater, there are two physiological groupings, those that are anoxia-tolerant and those that are relatively anoxia-intolerant. All species fare well physiologically in water with a high partial pressure of oxygen (PO2). A lack of anoxia tolerance limits the types of habitats that a freshwater turtle may live in, since unlike sea turtles, they cannot travel long distances to hibernate.Hatchlings of some species of turtles spend their first winter in or below the nest cavity, while hatchlings of other species in the same area, including northern areas, emerge in the autumn and presumably hibernate underwater. All hatchlings are relatively anoxia-intolerant, and there are no studies to date of where hatchling turtles that do not overwinter in or below the nest cavity spend their first winter. Equally little is known of the ontogeny of anoxia tolerance, other than that adults of all species are more anoxia-tolerant than their hatchlings, probably because of their better ossified shells, which provide adults with more buffer reserves and a larger site in which to sequester lactate. The northern limits of turtles are most likely determined by reproductive limitations (time for egg-laying, incubation, and hatching) than by the rigors of hibernation.Mortality is typically lower in turtle populations during hibernation than it is during their active periods. However, episodic mortality events do occur during hibernation, due to freezing, prolonged anoxia, or predation.     

Navigation of marine, freshwater and coastal animals: concepts and current problems

The animals from the environments focused on here share the same navigational mechanisms with terrestrial animals. However, some of them seem to rely on additional ways of detecting and/or processing navigational cues, some of which are perhaps still unknown. A classification of the mechanisms of navigation is given. This is based on the source of information that animals use to head for their targets. A selected series of phenomena of current interest is presented, starting with olfactory beaconing in oceanic birds, which allows the detection of patchily distributed food and productive areas from long distances. Animals of sandy beaches rely on an array of mechanisms of orientation, which have an adaptive value for their ecotonal system. As some species are capable of using both the moon and sun compass in orientation, attention and experiments are focused on the significance of these celestial cues in the navigational process. Two clocks of different periods, one of which would appear to regulate both the activity rhythm and the sun compass, are presumed to underlie the two compass mechanisms. The feats of global navigators in and over the oceans are especially puzzling considering their ability of homing to the natal and nesting sites after long lasting, extended wandering in the open ocean, and of pinpointing tiny, isolated oceanic islands. The existent literature allows comparison of the navigational capabilities of oceanic birds with those of sea turtles. Their performances in natural conditions seem to be similar, but sea turtles exhibit a reduced capacity of compensation for experimental relocation. Capacity of positioning based on geomagnetic parameters has been indicated for sea turtles through laboratory experiments, but this is not confirmed by the routes of magnetically disturbed turtles tracked while migrating or attempting to compensate for relocation. Also albatrosses with fitted magnets are not disturbed in their homing.     

Effects of habitat alteration caused by petrochemical activities and oil spills on the habitat use and interspecific relationships among four species of Afrotropical freshwater turtles

The large-scale effects of habitat alteration produced by oil-industry related pollution on the habitat use of four species of freshwater turtles (Pelusios castaneus, Pelusios niger, Pelomedusa subrufa, Trionyx triunguis) were studied in the River Niger Delta, southern Nigeria (West Africa) between 1996 and 2004. The numbers of turtle specimens observed during our study declined drastically in polluted sites, despite a nearly identical field effort. The number of specimens of all turtle species declined considerably at all habitat types, but complete disappearance in polluted areas was found only with regard to one habitat type for Trionyx triunguis and two habitat types for Pelomedusa subrufa. The mean values of species dominance and diversity indexes were not statistically significant between pristine and altered areas. Based on the interspecific similarity in proportional frequencies of turtle specimens found in each habitat type, a multivariate set of analyses (UPGMA) showed that the turtles were arranged in three ‘ecological’ clusters: a group formed by Pelomedusa subrufa at both polluted and unpolluted areas and Trionyx triunguis at polluted areas; (ii) a group formed by Pelusios castaneus in polluted areas and Pelusios niger in polluted areas; (iii) a group formed by Pelusios castaneus in unpolluted areas and Pelusios niger in unpolluted areas; however, this latter cluster was not very close, as the linkage distance was close to 80% of Euclidean distance. Habitat use similarity among turtles in both polluted and unpolluted study areas was evaluated by the use of two types of overlap formulas (Pianka and Czechanowski) and the use of Monte Carlo randomisations in order to control for the eventual role of chance in the actual data matrix. These data indicated that, for a pair of species (Pelusios niger vs. Pelusios castaneus), there was a statistically significant increase in the similarity of habitat use in the polluted areas vs. the unpolluted areas, and that this pattern was not dependent on the chance. Considering that these two species are ecologically and morphologically similar, we conclude that the most likely consequence at the community level is an increase in the intensity of interspecific competition for space between Pelusios niger and Pelusios castaneus in the polluted areas. Although the direction of the intensification of this competition process could not be easily predicted, it is likely that the species which is least adapted to life in main rivers and creeks may be disadvantaged over the other competitor. The general implications for habitat preservation are also discussed.     

Thermal and trophic habitats of the leatherback turtle during the nesting season in French Guiana

Understanding environmental cues determining behaviour and habitat use of species of conservation concern is crucial if one aims at implementing sustainable management of these natural resources. In this way, here, we investigate the thermal and trophic conditions encountered by the critically endangered leatherback sea turtle Dermochelys coriacea during its nesting season in French Guiana where high bycatch rates have been reported.Mean sea water temperatures obtained in situ by animal-borne recorders were 26.6 ± 0.7 °C in the water column, with all but one turtle remaining in water > 25 °C during the inter-nesting interval. In terms of prey availability, regular jellyfish stranding events were recorded during the nesting season, on a 1.25-km long section of the nesting beach. The occurrence of jellyfish was supported by benthic trawls performed on the continental shelf, with a total of 45.4 kg of jellyfish collected in 3.5 h exclusively in coastal waters 10 to 20 m deep where water transparency was between 0.8 and 3 m. This is consistent with the at sea distribution area of gravid leatherbacks during their inter-nesting intervals, as they spent almost 70% of their time diving in shallow (< 20 m deep) waters in front of the Maroni River estuary. In French Guiana, leatherback's gelatinous prey are thus present in very shallow water close to the nesting site and may be easily exploited by active gravid leatherbacks. This suggests that French Guiana female leatherbacks may be influenced by local trophic conditions and actively prospect productive areas overlapping with local fisheries ground.     

The diet composition of immature loggerheads: Insights on trophic niche, growth rates, and fisheries interactions

For immature animals, diet quality and composition influence expression of life history traits such as growth rates and ultimately life stage duration and age to maturity. Circumglobally distributed loggerhead turtles (Caretta caretta) exhibit a multi-decade immature stage that generally occupies neritic habitats and is characterized by slow growth and an omnivorous diet. Although adult nesting populations are geographically distinct, foraging areas for immature loggerheads show a high degree of mixing of individuals that originate from multiple nesting stocks. Furthermore, despite their generalist foraging ecology, immature loggerheads have been observed to supplement their natural diets with fish from fishery discards and/or caught in fishing gear. However, whether trophic opportunism results in variation in loggerhead growth rates within or among feeding areas has not been investigated. In Core Sound, North Carolina (NC), USA, immature loggerheads demonstrate highly variable size-specific growth rates, in contrast to other studies that report discernible somatic growth functions in immature marine turtles. To determine whether inter-individual variation in growth rates at this site was due to variation in diet composition, and specifically variation in consumption of fish, we analyzed carbon and nitrogen stable isotope ratios of loggerhead blood plasma and of tissue samples of putative loggerhead prey, as well as commercially important fish species. Our results indicated that growth rates were not related to trophic levels at which individual turtles fed, but rather probably reflected inter-individual variation in overwintering or foraging behavior (i.e. nearshore vs. offshore). Furthermore, loggerhead diets were highly diverse, and comprised mainly blue crabs and/or whelks, as well as small proportions of cannonball jellies. Fish were unimportant dietary components for loggerheads. Although loggerheads in NC do not appear to feed on fish catch or discards, immature turtles showed dietary preferences for prey items that are also valuable to or are commonly taken as bycatch in commercial fisheries (e.g. blue crabs and whelks, respectively) in the region. Thus, the status of these prey items/fishery stocks as well as trends in loggerhead populations should be monitored to mitigate potential competitive interactions between fisheries activities and loggerhead turtles.