39 research outputs found
Tag loss is a minor limiting factor in sea turtle tagging programs relying on distant tag returns: the case of Mediterranean loggerhead sea turtles
No full textAs in many other species, tagging has been routinely conducted for decades in over a hundred sea turtle capture-mark-recapture (CMR) programs worldwide. Tag loss is a key limiting factor because it violates the main assumption in CMR models; however, very few estimates of tag loss exist, and we provide here a review. No published estimations of tag loss are available for the Mediterranean, in spite of intensive tagging since the 1980s. This study aims to provide an estimation of tag loss in loggerhead turtles tagged in the Mediterranean. We modeled 64 tag returns out of ca. 2200 loggerhead turtles tagged at Mediterranean foraging grounds, with mark-recapture intervals up to 11.5 years, in order to estimate a daily tag loss probability of the most used tag applied to the most common turtle species in the region. Five models were evaluated through maximum likelihood estimation. The model with the best fit described a tag loss initially high and then decreasing to a lower asymptote, which is probably due to some defective tag applications. The resulting tag loss (0.15 in the first year and 0.31 after 5 years) was comparable or even lower than those from other areas and/or species and predictions indicate that double tagging can make a turtle identifiable for a long period. Hence, in our tagging program and probably in similar ones as well, tag loss appears to be the less important of the factors affecting tag returns, and efforts in other directions are more likely to improve CMR results
Mortality induced by drifting longline hooks and branchlines in loggerhead sea turtles, estimated through observation in captivity
No full text1. Drifting longlines are considered a major threat to endangered sea turtle populations worldwide. However, for a number of reasons, the mortality rate of captured turtles is not known with any certainty. 2. Information on 409 loggerhead turtles (Caretta caretta), collected during the day-to-day activities of a turtle rescue centre in Lampedusa island, central Mediterranean, in the period 2001-2005 has been analysed. 3. Observations indicate that: (i) drifting longlines are a major cause of mortality for sea turtles in the area; (ii) in addition to the hook, the piece of line attached to it (branchline) can easily cause death if it is long enough and well-anchored; (iii) hooks and branchlines cause death in the short and long term, respectively; (iv) a turtle with a hook in the lower oesophagus/stomach has a very low chance of surviving the combined effect of hook and branchline; (v) the mortality of turtles with a hook in the mouth or higher oesophagus is probably important, though less than that of turtles with a hook in the lower oesophagus/stomach; (vi) in the study fishery, the average mortality of a turtle caught by a drifting longline is probably much higher than 30%. 4. Without specific investigations on the mortality of turtles with hooks in the mouth or higher oeasophagus, which are usually removed, the mortality induced by drifting longlines will remain unknown, preventing a full understanding of the effect on population growth and the real effectiveness of conservation measures such as use of different hooks and fishing depths, and proposals for adequate fishery management measures. 5. The number of turtles captured by drifting longlines should be drastically reduced, and because of the above uncertainty and the socio-economic importance of the fishery sector, an ecosystem-based management scheme should be promoted that is not limited to addressing only the turtle issue. Copyright © 2007 John Wiley & Sons, Ltd
Age and growth determination by skeletochronology in loggerhead sea turtles (Caretta caretta) from the Mediterranean Sea
No full textSkeletochronology was applied to humerus bones to assess the age and growth rates of loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea. Fifty-five dead turtles with curved carapace lengths (CCL) ranging from 24 to 86.5 cm were collected from the central Mediterranean. Sections of humeri were histologically processed to analyze annual growth marks. Two approaches were used to estimate the somatic growth in the form of a von Bertalanffy growth function. The first approach was based on calculating the total number of growth marks, which corresponds to the age of turtles at death. The second approach estimates the carapace length at old growth marks in order to provide the growth rate of each turtle. The observed individual growth rates ranged from 1.4 to 6.2 cm yr(-1), and showed both elevated inter- and intra-individual variability possibly related to the environmental variability experienced by turtles during their lifetime. Both approaches gave similar results and suggest that Mediterranean loggerhead turtles take 14.9 to 28.5 years to reach a CCL of 66.5 to 84.7 cm. This size corresponds to the average size of nesting females found in the most important Mediterranean nesting sites and can be considered the approximate size at maturity
Spatio-temporal distribution and migration of adult male loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea: further evidence of the importance of neritic habitats off North Africa
No full textTen adult male loggerhead sea turtles, captured by trawlers or dip nets, were satellite-tracked from a neritic foraging ground in the Mediterranean in order to investigate adult spatio-temporal distribution and breeding migration. Five individuals migrated to potential breeding sites in Libya and one to Greece. The results complement previous studies and show that: (1) the Tunisian shelf may be more important for turtles from Libyan rookeries than previously thought; (2) male tracks corroborate a conservation hotspot previously identified for juveniles; (3) the north African coast represents a preferred migratory corridor, unless open sea routes are more direct; (4) adult males may exhibit high fidelity to relatively small areas, without evident seasonal differences; (5) adults home ranges were smaller and more neritic than juveniles frequenting the same area; (6) males may frequent multiple courtship areas; (7) the average remigration interval of males frequenting this region is longer than 1 year. © 2012 Springer-Verlag Berlin Heidelberg
Estimation of age at maturity of loggerhead sea turtles Caretta caretta in the Mediterranean using length-frequency data
No full textIt is widely accepted that the age at sexual maturity of sea turtles is a critical parameter for studying population dynamics and persistence. Estimates of the age at maturity for such longlived species are derived using somatic growth models, which are still lacking for several regions of the world. In the present study, the growth rate of the loggerhead sea turtle Caretta caretta in the Mediterranean was investigated using a length-frequency analysis of a dataset collected over a 19 yr period (1990 to 2008). A total of 2255 individuals were measured in the central Mediterranean, with turtle size ranging from 16.8 to 97.5 cm curved carapace length (CCL). Monthly length-frequency histograms were constructed, and strong size modes were identified, assumed to represent individual cohorts. Growth rates were calculated by tracking the progression of the modes, by means of a modal progression analysis. Annual growth rates ranged from 0.37 to 6.5 cm yr -1. A von Bertalanffy growth function was used to estimate the time required by turtles to grow within the observed size range. The results indicate that turtles would take from 23.5 to 29.3 yr to reach 80 cm CCL, considered an approximation of the size at maturity. This estimation integrates and confirms a previous estimate obtained using a different method. It provides information vital to understanding the population dynamics of loggerhead turtles in the Mediterranean, and highlights the value of datasets of longterm series when investigating critical demographic parameters. © Inter-Research 2011
Geometric morphometrics, scute patterns and biometrics of loggerhead turtles (Caretta caretta) in the central Mediterranean
No full textWe investigate for the first time allometric vs. non-allometric shape variation in sea turtles through a geometric morphometrics approach. Five body parts (carapace, plastron, top and lateral sides of the head, dorsal side of front flippers) were considered in a sample of 58 loggerhead turtles (Caretta caretta) collected in the waters around Lampedusa island, Italy, the central Mediterranean. The allometric component was moderate but significant, except for the plastron, and may represent an ontogenetic optimization in the case of the head and flippers. The predominant non-allometric component encourages further investigation with sex and origin as potential explanatory variables. We also reported the variation of marginal and prefrontal scutes of 1497 turtles, showing that: variation of marginals is mostly limited to the two anteriormost scutes, symmetry is favored, asymmetry is biased to one pattern, and the variation of marginal and prefrontal scutes are linked. Comparisons with other datasets from the Mediterranean show a high variability, more likely caused by epigenetic factors. Finally, conversion equations between the most commonly used biometrics (curved and straight carapace length, carapace width, and weight) are often needed in sea turtle research but are lacking for the Mediterranean and are here estimated from a sample of 2624 turtles.</jats:p
Histological validation of gonad gross morphology to sex juvenile loggerhead sea turtles (Caretta caretta)
No full textSea turtles exhibit sexual dimorphism only as adults, hence diagnosing the sex of hatchlings and juveniles requires the employment of different techniques that vary in their level of accuracy and costs. In order to validate the observation of external gross morphology of gonads as a sexing method for juveniles, we compared results obtained in this way with those obtained through histology in 99 loggerhead turtles with curved carapace length (CCL) ranging from 24.0 to 69.0 cm, found in the Adriatic Sea and in the central Mediterranean. Sex was correctly diagnosed in 92.9% of the 99 cases. The highest error rate due to wrong or uncertain sexing was found in turtles with a CCL less than 30.0 cm (33.3%). In turtles with a CCL of 30.0-40.0 cm and 40.0-50.0 cm, the error rates were low (5.3% and 6.7%, respectively), while no errors occurred in larger individuals (CCL greater than 50.0 cm). The results show that gonadal morphology is a reliable sexing method for large juveniles, but for those of less than 30 cm CCL we recommend verification by histology
Adult sex ratios of loggerhead sea turtles (Caretta caretta) in two Mediterranean foraging grounds
No full textSea turtles show temperature-dependent sex determination (TSD) and information on sex ratios at different life stages is necessary both for population dynamics models for conservation and to shed light on the possible adaptive value of TSD. Adults represent the less abundant class of sea turtle populations and adult sex ratios at foraging grounds are very difficult to obtain. We first analysed biometric data of 460 juvenile and adult loggerhead sea turtles ranging from 60 to 97.5 cm curved carapace length (CCL), in which a clear bimodal distribution of tail length (the main secondary sexual character of adult males) was observed in the size class >75 cm CCL. We then sexed 142 adult turtles in this size class collected from the Tunisian shelf and from the southeastern Tyrrhenian Sea, observing a proportion of females of 51.5% (95% CI: 41.2-61.8%; n=97) and 40.0% (95% CI: 25.7-55.7%; n=45) respectively. Our results complement previous studies and support their findings of similar and more balanced sex ratios in adult and juvenile loggerhead turtles in the Mediterranean, in contrast with highly female-biased sex ratios of hatchlings. © 2014 CSIC
Biases and best approaches for assessing debris ingestion in sea turtles, with a case study in the Mediterranean
No full textIn a sample of 567 loggerhead turtles (Caretta caretta) from the central Mediterranean, debris occurrence varied
according to methods and turtle source, and was up to 80% in pelagic turtles. Frequencies of plastic types, size and
color are also reported. These results and a critical review of 49 studies worldwide indicate that: (i) the detected
occurrence of plastic (% turtles) is affected by several factors (e.g., necropsy/feces, ecological zone, type and date
of finding, captivity period for feces collection), (ii) mixed dataset and opportunistic approaches provide results
which are biased , not comparable, and ultimately of questionable value, (iii) only turtles assumed to have had a
normal feeding behaviour at the time of capture or death should be considered, (iv) turtle foraging ecology and
possible selectivity may undermine the use of turtles as indicator species for monitoring marine litter, as recently
proposed for the Mediterranean
