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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.
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Leatherback
sea turtles nest occasionally on the beach at Canaveral
National Seashore. The leatherback is an endangered
species of sea turtle and is one of the largest in the
world. It can grow to be over 6 feet long and weigh
over 1,000 pounds.
Credit: NOAA
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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.
For more information contact the European Space
Agency
ESA Media Relations Division
Tel: +33(0)1.53.69.7155
Fax: +33(0)1.53.69.7690
www.esa.int
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