Envisat tracks leatherback turtles

Envisat tracks leatherback turtles

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The site where Europe’s spacecraft are launched into orbit, the Atlantic shoreline of French Guiana, is also the starting point for another hardly less remarkable journey: the epic migration of the critically endangered leatherback turtle.

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.

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.

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.