Eckert, S.A.

Abstract:

Hawksbill sea turtles (Eretmochelys imbricata) nesting in Barbados were outfitted with time-depth recorders (TDRs) with temperature sensors to investigate the form and patterns of diving behaviour during the inter-nesting interval (INI; average 14.7 days). All females, regardless of size, surfaced infrequently during dives of average 56 min duration, and the majority of dives (90%) were spent in the bottom phase at 15–25 m depths, which corresponded to the depth of benthic habitat at each location. Diving activity was highest while commuting to and from the nesting beach (about 1–2 days each way), with a level of quiescence during the intermediate period (i.e. the majority of the INI). Despite little thermal variation in seawater at this latitude (13.1°N), the length of the INI was influenced by ambient sea water temperature. Diving behaviour was consistent with females conserving energy reserves built up at foraging grounds prior to arrival at the nesting beach and minimising time spent in the water column away from safe refuge at night. The frequency of surfacing and the depths at which females spend most of their time varies between sites even within one species and may be crucial in managing the risks to animals temporarily residing offshore from important nesting beaches. 

Abstract:

Background: Leatherback turtles are renowned for their trans-oceanic migrations. However, despite numerous movement studies, the precise drivers of movement patterns in leatherbacks remain elusive. Many previous studies of leatherback turtles as well as other diving marine predators have analyzed surface movement patterns using only surface covariates. Since turtles and other marine predators spend the vast majority of their time diving under water, an analysis of movement patterns at depth should yield insight into what drives their movements.

Results: We analyzed the movement paths of 15 post-nesting adult female Pacific leatherback turtles, which were caught and tagged on three nesting beaches in Mexico. The temporal length of the tracks ranged from 32 to 436 days, and the spatial distance covered ranged from 1,532 km to 13,097 km. We analyzed these tracks using a movement model designed to yield inference on the parameters driving movement. Because the telemetry data included diving depths, we extended an earlier version of the model that examined surface only movements, and here analyze movements in 3-dimensions. We tested the effect of dynamic environmental covariates from a coupled biophysical oceanographic model on patch choice in diving leatherback turtles, and compared the effects of parameters measured at the surface and at depth. The covariates included distance to future patch, temperature, salinity, meridional current velocity (current in the north–south direction), zonal current velocity (current in the east–west direction), phytoplankton density, diatom density, micro-plankton density, and meso-zooplankton density. We found significant, i.e. non-zero, correlation between movement and the parameters for oceanic covariates in 8 of the tracks. Of particular note, for one turtle we observed a lack of correlation between movements and a modeled index of zooplankton at the surface, but a significant correlation between movements and zooplankton at depth. Two of the turtles express a preference for patches at depth with elevated diatoms, and 2 turtles prefer patches with higher mezozooplankton values at depth. In contrast, 4 turtles expressed a preference for elevated zooplankton patches at the surface, but not at depth. We suggest that our understanding of a marine predator’s response to the environment may change significantly depending upon the analytical frame of reference, i.e. whether relationships are examined at the surface, at depth, or at different temporal resolutions. Lastly, we tested the effects of accounting for ocean currents on the movement patterns and found that for 13 of the 15 turtles, the parameter governing distance to the next patch decreased.

Conclusions: Our results suggest that relationships derived from the analysis of surface tracks may not entirely explain movement patterns of this highly migratory species. Accounting for choices in the water column has shown that for certain individual turtles, what appears to be favourable habitat at depth is quantitatively different from that at the surface. This has implications for the analysis of the movements and diving behaviour of any top marine predator. The leatherback turtle is a deep diving reptile, and it is important to understand the subsurface variables that influence their movements if we are to precisely map the spatial dimensions of favorable leatherback habitat. These results present a new view into the drivers of diving patterns in turtles, and in particular represent a way of analyzing movements at depth that can be extended to other diving species. 

Abstract:

This study presents a comprehensive genetic analysis of stock structure for leatherback turtles (Dermochelys coriacea), combining 17 microsatellite loci and 763 bp of the mtDNA control region. Recently discovered eastern Atlantic nesting populations of this critically endangered species were absent in a previous survey that found little ocean-wide mtDNA variation. We added rookeries in West Africa and Brazil and generated longer sequences for previously analyzed samples. A total of 1,417 individuals were sampled from nine nesting sites in the Atlantic and SW Indian Ocean. We detected additional mtDNA variation with the longer sequences, identifying ten polymorphic sites that resolved a total of ten haplotypes, including three new variants of haplotypes previously described by shorter sequences. Population differentiation was substantial between all but two adjacent rookery pairs, and FST values ranged from 0.034 to 0.676 and 0.004 to 0.205 for mtDNA and microsatellite data respectively, suggesting that male-mediated gene flow is not as widespread as previously assumed. We detected weak (FST = 0.008 and 0.006) but significant differentiation with microsatellites between the two population pairs that were indistinguishable with mtDNA data. POWSIM analysis showed that our mtDNA marker had very low statistical power to detect weak structure (FST \ 0.005), while our microsatellite marker array had high power. We conclude that the weak differentiation detected with microsatellites reflects a fine scale level of demographic independence that warrants recognition, and that all nine of the nesting colonies should be considered as demographically independent populations for conservation. Our findings illustrate the importance of evaluating the power of specific genetic markers to detect structure in order to correctly identify the appropriate population units to conserve. 

Abstract:

Accidental entanglement of leatherback sea turtles (Dermochelys coriacea) in the gillnet fisheries of Trinidad is the most serious conservation problem faced by the species and threatens to undo several years of proactive conservation and innovative management by the government of Trinidad and Tobago and many local non-government organizations (NGOs). The entangle- ment problem also places a severe strain on the ability of Trinidad fishers to operate economically, and is so severe that many are unable to fish during the sea turtle nesting season.

Undisputed among stakeholders is that incidental capture is the largest single source of mortality to leatherbacks in the country, killing more leatherbacks than all other factors combined. Because it supports the second largest known nesting aggregation in the world, the Republic of Trinidad and Tobago plays a uniquely important role in the survival of this species on a global scale. With this in mind, incidental capture and mortality to reproductively active females in waters under the Republic’s jurisdiction constitute a major threat to this Critically Endangered (cf. IUCN) species on both Atlantic basin and global scales.

In an attempt to open a dialogue on these issues, and facilitate a stakeholder driven process of solution-making, a National Consultation was hosted by the Wider Caribbean Sea Turtle Conservation Network (WIDECAST) and the Fisheries Division (Ministry of Agriculture, Land and Marine Resources) in February 2005. Invited participants included fishers drawn from all affected communities, including representatives from Tobago, local and national NGOs, the government’s primary natural resource management agencies, the Ministry of Foreign Affairs, and a small number of international fishing and conservation experts.

The goal of the meeting was to review the problem of sea turtle bycatch in coastal gillnet fisheries, in particular along the north and east coasts of Trinidad where most leatherback nesting takes place, and to apply the shared expertise of the forum to devising a series of potential solutions suitable for field-testing and evaluation by fishers and natural resource management professionals. To this end, twin objectives were proposed: fishers must be better off economically as a result of any proposed solution to the bycatch crisis, and the incidental capture and mortality of leatherback sea turtles in coastal fisheries must cease.

The goal of the meeting was met through technical presentations in a conference setting, open- forum question and answer sessions, an all-day field excursion to coastal fishing communities and fishing depots, Working Group discussions, plenary consensus on recommendations, and publication of this Proceedings document.

Participants acknowledged that the problem is a difficult one, and that no single solution would likely suffice for all areas and all fisheries. Thus it was proposed that a series of investigations be designed to evaluate, under realistic field conditions, various bycatch reduction options including: new bait types (e.g. artificial, dead and non-traditional baits) to enhance hook-and- line fishing as a replacement for gillnets; new technologies, techniques, or gear modifications (e.g. power take-up reels, alternate net materials, FADs; net-fishing at different depths); and creative approaches to net avoidance (e.g. sonic ‘pingers’, shark silhouettes). It was agreed that each of these options should receive equal weight during the experimental phase, and that the results of each trial should determine subsequent experimental priorities.

New regulatory regimes, and in particular the implementation of time and area closures, were also discussed. The recommendation was made that gillnets be banned from 1 March to 31 May within a region extending from the south end of Fishing Pond Beach to the west end of Paria Beach, and extending 8km offshore. Other types of gear would be allowed. There was concern over the government’s capacity to enforce the closure, however, and the need for improved marine resource management capacity was noted. Also noted was the need to harmonize the Fisheries Act (specifically the 1975 Protection of Turtles and Turtle Eggs Regulations) and the Conservation of Wild Life Act, such that protection to the leatherback turtle at all times was unambiguous.

With regard to evaluating fishery alternatives, the meeting uniformly agreed that active fishermen must be involved in the testing and development of each new method, with oversight and assistance by relevant experts. It was proposed that the best mechanism for initiating the field-testing component would be to invite proposals from relevant national and international experts (see “Project Implementation Notes”), and that fund-raising, including the paid participation of fishers, would need to occur on a case-by-case basis.

Furthermore, there was consensus that the following criteria be taken into account when evaluating the various mitigation options:

• What - will the experiment measure (objectives and variables)?
• How - will the experiment be conducted (materials and methods)?
• Where - will the experiment be conducted?
• Who - will conduct and evaluate the results of the experiment?

There was also consensus that the following Evaluation Criteria be adopted:

•  Can the new technique catch fish?
•  Is it economically viable (i.e. producing equivalent or increased revenue)?
•  Does it reduce adverse impact to leatherback sea turtles?
•  Can it be managed/regulated?
•  Is it logistically feasible for local conditions?
•  Is it biologically and commercially sustainable?
•  Will it be supported/accepted by the stakeholders?

Finally, there was widespread interest among participants that a mechanism be created, perhaps by Government, to facilitate an ongoing dialogue between fishers and natural resource managers, and embracing the expertise of communities and NGOs, on subjects of fisher concern regarding the bycatch issue and the hardships endured during the seasonal struggle to fish in the presence of large numbers of leatherback turtles.