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Sea turtles, along with other turtles and tortoises, are part of the order Testudines. The seven living species of sea turtles are: Flatback, Green Sea Turtle, Hawksbill, Kemp's Ridley, Leatherback, Loggerhead and Olive Ridley. All species of sea turtles are listed as threatened or endangered. The leatherback, Kemp's Ridley, and hawksbill turtles are critically endangered. The Olive Ridley and green turtles are endangered, and the loggerhead is threatened. The flatback's conservation status is unclear due to lack of data.
Although greatly differentiated in their general life-style and feeding habits, the seven species of sea turtle still share a common lifecycle that may encompass prolonged periods in the open sea at all developmental stages. During these periods, turtles are known to move widely between different habitats or specific locations, ranging large oceanic areas away from the continental shelf. In recent years, many features of the open-ocean movements of sea turtles, such as their general extent, pattern and course, have become known, mostly thanks to the findings obtained through satellite telemetry. However, the effects of environmental factors on these movements are still poorly known, and only in a few cases has the integration of satellite tracking data with oceanographic techniques such as mathematical modelling of current fields and analysis of remote sensing data, provided useful clues on how turtle movements are affected by the current circulation pattern. This report attempts to integrate all the previous work done on the oceanic movements and nesting sites of marine turtle with a conclusion of its application in India.
Remote sensing is defined as the acquisition of information about an object without being in physical contact with it. Information is acquired by detecting and measuring changes that the object imposes on the surrounding field, be it an electromagnetic, acoustic, or potential. In remote sensing, information transfer is accomplished by use of electromagnetic radiation (EMR). A geographic information system (GIS) is a computer-based tool for mapping and analyzing things that exist and events that happen on Earth. Geographic Information System (GIS) is a computer based information system used to digitally represent and analyse the geographic features present on the Earth' surface and the events (non-spatial attributes linked to the geography under study) that taking place on it. The meaning to represent digitally is to convert analog (smooth line) into a digital form.
Various forms of remote sensing and GIS applications have been used to study the oceanic movements of turtles, which has been briefly described below:
The Global Positioning System (GPS) is a burgeoning technology, which provides unequalled accuracy and flexibility of positioning for navigation, surveying and GIS data capture. The GPS uses satellites and computers to compute positions anywhere on earth. The GPS is based on satellite ranging. That means the position on the earth is determined by measuring the distance from a group of satellites in space.
TrackTagâ„¢ GPS loggers have been deployed on adult female loggerhead sea turtles during the pre-nesting period in May 2006 and removed during the inter-nesting period in June 2006. In addition, six time-depth recorders (TDR) were also used. On retrieval, all GPS locations were plotted to examine the turtles' movements. Data when the turtles were ashore were removed from all analysis and were confirmed by direct field observation. Using the Geographic Information Systems (GIS) package ArcView 3.1 key area was identified by overlaying the turtle GPS fixes on existing features, including sea depth parameters, Natura 2000 marine habitats and maritime zones. The location of turtle GPS fixes was analysed (using the "query" and "summarise" tools) with respect to the chosen features, to indicate areas that are in need of increased protection.
Argos is a global satellite-based location and data collection system. It allows any mobile object equipped with a compatible transmitter to be located across the world. It also offers the possibility of collecting data from measurement sensors connected to this transmitter. Argos platforms automatically transmit messages that are received by satellites in low polar orbit. The satellites then transfer the messages to ground receiving stations. The ground stations automatically transfer the messages to Argos Processing Centres. The Processing Centres calculate the position of the transmitters and process the data measured by the sensors. Finally, the Processing Centres automatically deliver the results to the users
Argos system users have the advantage of two independent positioning modes:
Argos location: Argos centres calculate a transmitter's location using the Doppler Effect on transmission frequency.
GPS positioning: On request from the user, a specific processing module extracts the GPS positions included in the messages, validates them and distributes them in the same format as the Argos locations.
Data on the position of nine loggerhead turtles were provided from Argos-linked, 180 g, Telonics ST-10 transmitters attached to the turtles with glass-fibre cloth and resin. The physical and biological environmental data that were used to describe the environment at and around the migrating loggerheads were sea surface temperature (SST), surface chlorophyll a density, and geostrophic current The SST data was multichannel SST (MCSST) from the University of Miami, with weekly temporal resolution and one-tenth of one degree of longitude and latitude spatial resolution. The chlorophyll density was estimated from two satellite sensors: Ocean Colour and Temperature Scanner (OCTS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Geostrophic currents were computed from satellite altimetry data from TOPEX/Poseidon
Comparisons between current speed determined from an acoustic Doppler current profiler along TOPEX/Poseidon track. The environmental data was linked to the turtle's position by spatial interpolation of the environmental data between grid points. Specifically, a cubic polynomial was used to estimate the environmental data at and around all turtles' locations from the observed environmental grid. A computer routine, `grdtrack', within Generic Mapping Tools (GMT) was used for the interpolation (Polovina, Kobayashi, & Parker, 2000)
The development of satellite telemetry in the late 1980s allowed the first in-depth studies of animal movement patterns. Since then, Service Argos, a joint venture between the Centre National d'Etudes Spatiales (CNES, the French space agency),
the National Aeronautics and Space Administration (NASA, USA) and the National Oceanic and Atmospheric Administration (NOAA, USA), has emerged as the predominant satellite-based system for tracking wildlife.
Despite the obvious power and advantages of the Argos system to track animals by satellite, the data generated are difficult for many biologists to exploit. A broad range of skills is required to efficiently download, collate, filter and interpret Argos data. Integration of animal movements with other physical (e.g. remote sensing imagery) and anthropogenic (e.g. fishery distributions) datasets presents additional technical and computing challenges.
The Satellite Tracking and Analysis Tool (STAT) is a freely available system designed for biologists who work on animal tracking; it includes a set of standardized tools and techniques for data management, analysis, and integration with environmental data. A variety of environmental data layers are available in STAT for integration with animal tracking. These data are sampled for all locations in the STAT database and can be downloaded and incorporated into tracking maps and animations.
Bathymetry - General Bathymetric Chart of the Oceans (GEBCO) is a global topographic dataset with one-minute (1') spatial resolution.
Sea surface temperature - Sea surface temperature (SST) data are gathered daily from the NOAA Geostationary Operational Environmental Satellites (GOES), and weekly and monthly from Advanced Very High Resolution Radiometer (AVHRR) satellite-based sensors.
Sea surface height and currents - Sea surface height (SSH), sea surface height anomaly (SSHa) and sea surface current (geostrophic velocity vector) data layers are obtained from the CNES/CLS Aviso/Altimetry project.
Chlorophyll - Global weekly and monthly chlorophyll data layers are obtained from NASA's Ocean Colour project SeaWiFS and MODIS satellite-based Sensor.
STAT uses GMT to generate a suite of map products that are automatically updated each day as new data become available for each tracked animal. Users can set a variety of preferences that determine the appearance of the map for each tracked animal and the method by which tracking data are filtered, and that include any of the environmental data layers.
Important inferences from the studies:
Measurements of sea-surface height anomalies (SSHA) made by the TOPEX/Poseidon satellite showed that in particular, most circuitous or curved routes taken by the turtles were super imposable onto SSHA, and the sense of the turtle circling within the anomaly was always in accordance with the rotation of the water masses. The tracked turtles were involved in many oceanic features of different extent and nature, such as eddies, and their routes closely matched the mesoscale features encountered for the 69-84% of their length. The linear parts of the routes, conversely, occurred within the current mainstream and showed a striking similarity in their course with those of surface buoys tracked in the same region.
Surface circulation accurately portrays the conditions the turtles experienced during the tracking period, as turtles mostly remained in the upper part of the water column. Satellite-relayed depth data showed that turtles generally dived to depths shallower than 70 m, reaching depths below 500 m only a few times in the tracking period.
Thus, for marine turtles and moving in the open sea, ocean currents and related features certainly represent an important environmental factor affecting their life and behaviour. Hatchlings and early juveniles live within major current systems, where they find suitable feeding areas, like fronts or convergence regions, and where they are safer than in coastal areas. Adults of pelagic species similarly exploit the biologically rich environments linked to current systems. As a counterbalance of such beneficial effects, currents typically carry pelagic sea turtles on macro-geographic journeys, often ranging over entire ocean basins. These (mostly passive) movements have no specific role in turtle development or inter-reproductive biology, and can rather be thought to have detrimental consequences, leading turtles away from their subsequent foraging or nesting areas where they move to sooner or later. The effects of currents are also mostly harmful when turtles cross open-sea stretches heading towards a specific target, like during the adults' shuttling migrations .These lateral currents can deflect turtles from the optimal route towards their target, displacing them sideways.
Detailed and meaningful post-hoc analysis can be done by integrating the findings obtained by satellite tracking sea turtles (and other marine migrants as well) with the available remote sensing data. This approach has recently allowed most useful insights into the determinants of long distance turtle movements, and can be expected to provide further exciting results when applied to previously tracked journeys showing unexplained phenomena such as the occurrence of migratory corridors. In addition, attempts could be made to monitor current action while tracking the animals. A new generation of satellite linked instruments has recently been developed, allowing the collection of useful environmental, physiological and behavioural data during animal tracking. Among these, the compass directions held by the moving animal is an important parameter to record in order to estimate the drift induced by currents (as well as to reconstruct fine-scale movements between successive satellite fixes).
Importance of the studies in India: Understanding Olive Ridley better
The olive ridley is mainly a "pelagic" sea turtle, but has been known to inhabit coastal areas, including bays and estuaries. Olive ridleys mostly breed annually and have an annual migration from pelagic foraging, to coastal breeding and nesting grounds, back to pelagic foraging. Trans-Pacific ships have observed olive ridleys over 2,400Â miles (4,000Â km) from shore. Olive ridleys are globally distributed in the tropical regions of the South Atlantic, Pacific, and Indian Oceans. In the South Atlantic Ocean, they are found along the Atlantic coasts of West Africa and South America. In the Eastern Pacific, they occur from Southern California to Northern Chile.
Olive Ridleys nest sporadically along the coastline of India. In West Coast, nesting beaches in Gujarat are Gulf of Kutch (Jamnagar with the maximum number of nest) and Gulf of Khambhat (Bhavnagar being a major nesting ground). In Maharashtra Olive Ridleys have been reported nesting near Gorai, Kihim, Manowrie, Versova and the beach between Ambolgad and Vetye in the Ratnagiri district. There are two beaches in Goa namely Morjim and Galgibag; and Kozhikode district (Calicut) in Kerala which are frequented by these turtles for nesting. Sporadic nesting has been recorded in Tamil Nadu, Andhra Pradesh, and in Sundarbans in West Bengal.
The most fascinating feature of Olive Ridley is their mass nesting called 'arribada' meaning mass arrival in Spanish. They choose narrow beaches near estuaries and bays to lay eggs. Each adult female lays approximately 100-140 eggs at a time. It is believed that they nest in an interval of one-four years. The nesting season is between November to March.
In India, arribada takes place in the east coast state of Orissa at three nesting grounds Gahirimatha, Devi river mouth and Rushikulya river mouth. The later being the site of mass nesting for the season 2002-03. Gahirmatha, Orissa, India used to be one of the largest arribada nesting sites in the world. Over the past five years at Gahirmatha, there has been an arribada nesting event in only two of those five years. Additionally, between 1996 and 2002, the average size of nesting females declined at that site, indicative of a declining population (Plotkin ed. 2007). Declines in solitary nesting of olive ridleys have been recorded in Bangladesh, Myanmar, Malaysia, and Pakistan. In particular, the number of nests in Terengganu, Malaysia has declined from thousands of nests to just a few dozen per year (Limpus 1995).
One of the major challenges faced by conservationist is the sheer lack of data and knowledge about the life cycle of the species. Virtually nothing is known about the location of nursery habitats of the various turtle species and populations, although this would represent a most important piece of information for conservation purposes. Until some years ago, so little was known about the fate of turtle hatchlings that the term 'lost year(s)' was used to refer to the first period of their life after they left the nesting beach.
Though initial surveys have been done on the species, a detailed study is required for an enhanced conservation effort. New technologies of satellite tracking and other remote sensing techniques facilitate detailed study of the habitat and behaviour of the animal, which were not possible before. Following studies can be possible done to understand the species better:
Detailed analysis of morphodynamics of the rookeries - it involves study of the interaction and adjustment of the seafloor topography and fluid hydrodynamic processes, seafloor morphologies and sequences of change dynamics involving the motion of sediment. This can be done using multiple sampling IRS PAN data sets. ERDAS software can be used for satellite image processing coupled with field observations. Major nesting sites that need to be focussed on are Devi, Gahirmatha and Rushikulya beaches in Orissa, Gulf of Kutch and Gulf of Kambhat in Gujarat, Gorai, Kihim, Manowrie, Versova beaches in Maharashtra, Morjim and Galgibag beach in Goa and Calicut district in Kerela.
Bathymetry of surrounding waters: is the study of underwater depth of lake or ocean floors using LIDAR systems.
Study of geo-environmental parameters of coast to identify alternate rookery sites - Turtles have been found to prefer landforms having a surrounding water depth of 5m or more. The depositional features of the deltaic environment are ideal sites for mass nesting. Besides possessing suitable sediment and morphological environments; the nesting sites have to be away from human activities and their seaward coastal profile should have a surf zone near the shore.
Study of long distance movements of Olive Ridleys nesting at Orissa coast: Review of literature revealed that though movements of marine turtles have been done in detail in other parts of the world, no significant work has been done on Olive ridleys in India. This again emphasizes the need of a detailed and organized work with the help of remote sensing techniques described before.