Endangered turtles' trek along ocean currents revealed by satellite

Scientists have been using tracking sensors to follow the long treks of individual leatherbacks, then overlaying their routes with sea state data, including near-real time maps of ocean currents gathered by satellites including ESA's ERS-2 and now Envisat.

They are working to uncover connections between the apparently meandering routes followed by turtles and the local ocean conditions, and so develop strategies to minimise the unintended but deadly threat posed to leatherbacks by deep-sea fishing.

These giant reptiles - known to reach 2.1 metres in length and weigh in at 365 kg - briefly come ashore to lay their eggs on beaches across French Guiana and neighbouring Suriname, the turtles' last remaining major nesting sites in the Atlantic Ocean. Around nine weeks later the hatchlings emerge en masse and head into the sea, one day to return when they reach maturity and lay eggs themselves.

However each turtle's return is by no means certain. While in open water the turtles have been known to dive as deep as 1,230 metres in search of food, most of the time they do not venture deeper than 250 metres down, leaving them vulnerable to the hooks of longline fishermen – hundreds of thousands of such hooks are deployed daily across the Atlantic.

Ongoing 'bycatching' of leatherbacks by fishermen has left the 100-million-year-old species on the brink of extinction in the Pacific and Indian oceans. In the Atlantic their numbers are higher – partly due to a ban on longline US fishermen operating in the Ocean's northern section - but the turtles are still being lost at an unsustainable rate.

A paper was recently published in Nature summarising the work done so far in tracking leatherbacks through the Atlantic, submitted by a team of researchers from France's National Centre for Scientific Research in Strasbourg, neighbouring Louis Pasteur University, the French Guiana Regional Department of the Environment and the company Collecte Localisation Satellites (CLS) in Ramonville, specialising in satellite-based systems for location-finding, data collection and Earth Observation.

Pacific leatherbacks follow narrow migration corridors. Researchers hoped that if their Atlantic counterparts acted in the same way then fishing could be restricted across these zones.

Starting in 1999 individual turtles were tracked using the CLS-run Argos system, based on radio-emitting tags whose position can be tracked worldwide to a maximum accuracy of 150 metres. Six American NOAA spacecraft currently carry Argos receivers, with ESA's MetOp series due to join the system following their initial satellite launch next year.

The turtles' tracks were then overlaid with maps of sea level anomalies obtained by merging data with the radar altimeter aboard ESA's ERS-2 with another aboard the NASA-CNES satellite TOPEX-Poseidon.

ERS-2, like its successor Envisat, is part of the select group of satellites equipped with a Radar Altimeter (RA) instrument. By firing thousands of radar pulses off the surface of the sea every second extremely precise ocean height measurement is made possible. Height anomalies detected by this type of sensor are often indicators of the presence of ocean currents and eddies: warm currents can stand up to a metre above colder waters.

By merging multiple radar altimeter results together, the result is a more frequent and higher resolution measurement of sea level anomalies than any one spacecraft could achieve. For example, now that ERS-2's global mission is over, results from Envisat's RA-2 instrument are being combined with similar data from the joint French-US Jason spacecraft and the US Navy's GFO.

"The altimetry data has been very useful to our work because we have been able to check the turtles' trajectory against ocean currents," said Philippe Gaspar, co-author of the Nature paper and Head of the Satellite Oceanography Division of CLS. "What we have found is that their relationship with currents alters considerably over the course of their journeys.

Unlike their Pacific relations, the Atlantic leatherbacks do not follow narrow migration corridors but disperse widely - to begin with, the leatherbacks carry out long nearly straight migrations either to the north or to the Equator, swimming across currents as they encounter them. One made it to within 500 km of West Africa before turning back, another came close to Nova Scotia.

"Then having either made it to the Gulf Stream area or to the equatorial belt, the turtles tend to slow down and follow the frontal areas associated with local ocean current systems, which are generally rich in marine life."

Unfortunately fishing fleets target these frontal systems for exactly the same reason, so these turtles are placing themselves in danger. This finding means limited closures of Atlantic fishing areas is unlikely to have much impact in turtle bycatch reduction, and other solutions will have to be considered, such as turtle-friendly fishing gear and hooks recently developed by NOAA and endorsed by the World Wildlife Fund.

Meanwhile leatherback tracking continues on an ongoing basis, Gaspar added: "We are now looking at estimating the swimming speed of turtles during their trips by obtaining their total velocity from the Argos receivers, then subtracting the current velocity made available to us by altimetry. This has never been done before and should provide us with useful information on the energy they expend throughout their migration."

French schools have been given the chance to take part as part of an educational oceanographic scheme called Argonautica, with classes participating in the Argo-luth project, analysing turtle movements against outputs from MERCATOR, a model that presently covers the North and Equatorial Atlantic Ocean and assimilates radar altimeter data on an operational basis.