Influence of ocean currents on long-distance movement of leatherback sea turtles in the Southwest Indian Ocean

Leatherback turtles Dermochelys coriacea spend most of their life in oceanic environments, whose physical and biological characteristics are primarily forged by sea current circulation. Water mass movements can mechanically act on swimming turtles, thus determining their routes, and can differentially distribute their planktonic prey. By integrating satellite tracking data with contemporaneous remote-sensing information, we analysed the post-nesting journeys of 9 leatherbacks with respect to oceanographic surface conditions. Tracked turtles showed large variations in migration routes and in final destinations, apparently without heading for specific foraging areas. Their complex tracks spread over wide regions around South Africa. Leatherbacks were greatly influenced by the currents encountered during their movements, with their trajectories displaying curves or revolutions in the presence of (and in accordance with) rotating water masses. An impressive similarity was observed between large parts of the turtle routes and those of surface drifters tracked in the same regions. Finally, leatherbacks remained associated for long periods with specific oceanographic features, which most probably offered them profitable foraging opportunities. These results agree with previous findings in showing a strong influence of oceanic currents and mesoscale features on the movements of South African leatherbacks, and additionally identify the role of current-related features in causing the observed route variability and in determining high-quality foraging hotspots for leatherbacks moving in the ocean.

Overwintering behaviour in sea turtles : dormancy is optional

Thirteen loggerhead turtles Caretta caretta were released (10 from Naples, Italy, 2 from Monastir, Tunisia, 1 from Gallipoli, South Italy) with satellite relay data loggers (SRDL) to elucidate their overwintering behaviour. Nine turtles were successfully tracked throughout the winter, while 4 SRDLs failed to transmit after short deployment periods. Of these 9, 4 remained within 80 km of the release site, 3 travelled to a distant overwintering site, and 2 continued to move and did not remain within 80 km of a specific site. Apart from these differences, all turtles stayed near the coast and dedicated most of their time to dives lasting 3 h and longer. Maximum dive durations ranged from 270 to 480 min and were highly correlated with water temperatures, which fell below the supposed 15°C threshold for sea turtle hibernation in all overwintering sites. Median dive depths were between 4 and 24 m and were, thus, well within the mixed layer, as revealed by temperature profiles, which also were relayed by the SRDLs. No evidence was found that the turtles preferred warmer temperatures to overwinter in, because the range of temperature was very narrow on both the horizontal and the vertical scale of their movements. Despite the long resting phases and the low temperatures (minimum = 11.8°C) all turtles retained activity to some degree, at least to commute between the depth of resting and the surface to breathe. While the degree of winter dormancy is certainly affected by temperature, turtles were by no means obligatory hibernators, and their ability to move and even forage during the winter may be important for their growth and maturation rates, as well as their reproductive output.

Female–female aggression : structure of interaction and outcome in loggerhead sea turtles

Aggressive behaviour between females of the same species is not widely documented, particularly in marine vertebrates. During a 3 yr in-water survey at the temperate loggerhead sea turtle Caretta caretta breeding area of Zakynthos, Greece, female–female interactions comprised 4% of all female loggerhead sighting events (n = 60 out of 1449 events). Male–female interactions comprised an additional 4% of sighting events, while 92% were of solitary females. The structure of interactions was analysed for 58 of these sighting events, each lasting an average of 3.4 min (SD ± 1) and comprising a total of 3.1 h observation time. We found that interactions involved ritualized escalation in behaviour from passive threat displays (e.g. head–tail circling) to aggressive combat (e.g. sparring). We suggest that circling individuals evaluate opponent size, sparring individuals test opponent strength, and that the positioning of the prehensile tail signals motivational intent to either escalate or abort. The presence of intruder females triggered a passive response in 100% of events involving basking and swimming turtles (n = 19); although residents resting on the seabed only responded on 69% of occasions (n = 27), their response was almost 4 times more likely to escalate to one of aggression. Our results suggest that certain sites may be preferentially sought after and defended by sea turtles.

A review of migratory behaviour of sea turtles off southeastern Africa

The survival of sea turtles is threatened by modern fishing methods, exploitation of eggs and habitat destruction. Forming keystone species in the ocean, their extinction would disrupt the marine food chain in ways as yet unknown. The Indian Ocean has many breeding areas for sea turtles, the southernmost ones being on the Maputaland coast of KwaZulu-Natal, where loggerhead and leatherback turtles nest in large numbers thanks to long-lasting protection programmes. For the leatherback this is the only known nesting site in the entire western Indian Ocean. At the end of the reproductive season, both loggerheads and leatherbacks undertake migrations towards disparate feeding areas. To contribute to their conservation, the migratory behaviour of these animals needs to be understood. Here we review 10 years studying this behaviour using transmitters that telemeter data via satellite. It emerges that these species frequent widely dispersed areas ranging from the Atlantic Ocean to the Mozambique Channel. The migratory behaviour of leatherback and loggerhead turtles is, however, very different, probably due to their differing food requirements. While loggerhead postnesting movements have a truly migratory nature, the large-scale wanderings of leatherbacks are better described as prolonged sojourns in extended feeding areas.

Different male vs. female breeding periodicity helps mitigate offspring sex ratio skews in sea turtles

 The implications of climate change for global biodiversity may be profound with those species with little capacity for adaptation being thought to be particularly vulnerable to warming. A classic case of groups for concern are those animals exhibiting temperature-dependent sex-determination (TSD), such as sea turtles, where climate warming may produce single sex populations and hence extinction. We show that, globally, female biased hatchling sex ratios dominate sea turtle populations (exceeding 3:1 in >50% records), which, at-a-glance, reiterates concerns for extinction. However, we also demonstrate that more frequent breeding by males, empirically shown by satellite tracking 23 individuals and supported by a generalized bio-energetic life history model, generates more balanced operational sex ratios (OSRs). Hence, concerns of increasingly skewed hatchling sex ratios and reduced population viability are less acute than previously thought for sea turtles. In fact, in some scenarios skewed hatchling sex ratios in groups with TSD may be adaptive to ensure optimum OSRs.

Gonad development and hormone titres in Loggerhead Sea Turtles (Caretta caretta) in the NE Atlantic

The study proposed to describe sexual development in pelagic stage loggerhead sea turtles Caretta caretta and compare this to hatchlings and adults. It is meant as an ontogenic approach, in order to understand reproductive development and population composition and their dynamics in the pelagic environment. The study focused on the pelagic loggerheads that are found in the waters offshore Madeira Island (Portugal) in the North-eastern Atlantic and use it as a developmental habitat. The innovating character of this work relied on the lack of any description regarding the gonad ontogenesis and reproductive development for the pelagic stage in any of the 7 existing sea turtle species, all of them in danger of extinction. Three methods were used to diagnose the sex of each juvenile individual and asses the level of reproductive development: (1) laparoscopy, (2) gonad biopsy and (3) the assessment of two sex steroids circulating levels, namely testosterone and estradiol. In order to cover all life stages and compare data obtained for the juvenile stage, hatchlings and nesting female adults were sampled at the nearest nesting rookery at Boa Vista Island in the Cape Verde Archipelago. Gonads from dead hatchlings were collected for gonad histology and blood was collected from nesting females for sex steroids assessment. Laparoscopies revealed to be a valid sexing method for the juvenile stage, since gonads are morphologically differentiated at these size classes. Moreover, laparoscopy was validated using gonad histology. Gonad histology of juveniles showed that gonads are already completely differentiated into ovaries or testes at the size classes examined, but development seems to be quiescent. Males present already developed seminiferous tubules with spermatogonia lining the interior of the seminiferous tubule. Female gonads present oocytes at different development stages, but only oocytes up to stage III were observed. The maximum oocyte diameter in each individual correlated with body size, suggesting that reproductive development is an on-going process in juvenile females. The circulating levels of both testosterone and estradiol in juveniles of both sexes were very low and consistently lower than the ones observed in the nesting females from Boa Vista Island. No bimodal distribution was found for any of the sex steroids analysed and thus circulating hormone levels were not a reliable tool for sexing juvenile individuals with a non-invasive technique. The ratio testosterone:estradiol did not show a bimodal distribution either. The levels of testosterone correlated with sea surface temperature. The fact that temperatures observed during this study were below 24ºC might have hindered a differential testosterone pattern between juvenile males and females. Sex ratios for this population were generated according to laparoscopy results and compared among years and size classes. An overall sex ratio of 2 females for each male was found, but they varied among size classes but not among years. Possible causes for the sex ratios observed are discussed. This study is a contribution to our knowledge on the pelagic stage of loggerhead turtles, namely on the population structure regarding sex ratio, which is a vital tool for implementing conservation strategies.

Contribution to the chemoreception capacity of juvenile Loggerhead sea turtles (Caretta caretta, L.)

Loggerhead sea turtle juveniles (Caretta caretta), pelagic stage, are found in waters of Madeira archipelago. Pelagic turtles are in the main growth phase of their life cycle and consequently higher energy needs. However, knowledge about the ecology of pelagic loggerhead sea turtles is still quite rudimentary, mainly about the mechanisms that lead them to find food in the vast ocean. Studies with other pelagic species, such as procellariiform birds, revealed that the olfactory system play an important role for the detection of feeding areas, through the detection of concentration peaks of DMS (dimethylsulfide), a scent compound that naturally exists in the marine environment and it is related to areas of high productivity. Based on the assumption that loggerhead sea turtles use a similar mechanism, behavioural experiments were conducted in order to analyze the chemoreception capacity to DMS (airborne chemoreception - theoretically responsible for the long distance detection of areas with food patches; and aquatic chemoreception - theoretically responsible for the short distance detection of preys). The first step was to observe if pelagic loggerheads demonstrate sensitivity to DMS and the second was to verify if they really use the DMS, in natural conditions, as an airborne cue to find areas where food patches might be available. Four juveniles of loggerhead sea turtles were tested in captivity and three wild turtles in the open ocean. The results of airborne chemoreception experiments in captivity revealed that one turtle clearly demonstrated sensitivity to DMS and the sea experiments confirmed this result. However, the experiments were not conclusive on the question whether the pelagic turtles actually use the DMS as an airborne cue to detect long distance food patches. In aquatic chemoreception experiments was not observed sensitivity to DMS by the three sea turtles tested. In the classical conditioning experiment, where DMS and food were given nearly at the same time revealed that after a certain period of time, the sea turtle tested did not associated the DMS stimulus with a possible food reward. The main cause of mortality of loggerhead sea turtles in Madeira waters is due to the accidental capture (bycatch) by deep pelagic longlines fishery which the target species is the black-scabbard (Aphanopus carbo) fish. Chub mackerel (Scomber japonicus) is one of the baits used in this fishery. Aquatic chemoreception experiments were conducted in order to evaluate the attractiveness of the chub mackerel for sea turtles. For the three sea turtles tested, the results showed that in 90% of the cases the sea turtles were extremely attracted by the underwater smell of this fish.

Sea turtles and the turtle industry of the West Indies, Florida and the Gulf of Mexico...

Mode of access: Internet.

The natural history of fishes and serpents, including sea-turtles, crustaceous, and shell fishes with their medicinal uses.

Vol. 3 of his "New and accurate system of natural history."