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Satellite PAT tag as deployed in the MCSS programme. |
In
last issue of Sagren we discussed the basic principles of
satellite tags and the
Argos
system giving the user information about the position and
movements of their animals. This is only part of the
systems capability, tags attached to sharks and other
animals can also provide information on the behavior of
the study animal and monitor the environmental conditions
encountered by the animal.
These
tags are called data recording tags and one of the most
popular forms is the Pop-Off Archival tag that was
designed to deal with the problems associated with marine
animals that spend much of their time underwater. The tag
comprises of an Argos Platform Terminal Transmitter (PTT)
coupled to a data logger with a variety of sensors. These
tags monitor, store and summarise data during the period
of deployment and then at a user set date the tag releases
or pops-off from the animal. Once at the surface it
transmits the stored data to the
Argos
system and a whole range of factors will then influence
data transmission (see issue 2.3).
MCSS uses
PAT tags made by Wildlife Computers for the
Seychelles
whale shark programme. These tags have been set to monitor
depth, temperature and light intensity, the latter
allowing for estimates of geographical position to be
calculated for each day.
One of the
major shortcomings or constraints of the
Argos
satellite system is the amount of information that can be
sent. The satellites work on high frequency radio
transmissions, with all tags transmitting on one
frequency. To increase the chance of receiving the
transmissions from a number of tags, transmission times
have to be very short (max. 960milliseconds). Similarly,
an individual tag is not allowed to transmit continually -
the shortest repetition allowed is once every 40 seconds.
In terms of
data transmission this presents very real constraints as
this effectively allows only 256 bits (32 bytes) per
transmission. As such tag manufacturers have designed some
very clever operating systems to condense data for
transmission.
MCSS PAT
tags use 12 continuous depth and temperature ranges as a
means of compressing the data from each study cycle period
(3 hours in our case). As such only 24 records are needed
to describe the full 3 hour period. Even so if a tag has
been deployed for even a short period (e.g 3 months), it
would have accumulated nearly 18000 depth and temperature
records, not including the light intensity readings. Here
the software programming is key to trying to maximize how
the data is transmitted. Some tags allow the user to
prioritise transmission routines for the different data,
others have complex cyclic repeating systems but all work
inside the maximum of 256 bits per transmission.
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Temperature at Depth data from a single PAT tag
showing the oceanographic environment encountered
by the tag |
The
information from this type of data is very revealing and
has shown us how whale sharks spend most of their time in
a very narrow depth and temperature range. These tags are
also able to provide basic oceanographic information as
the depth and temperature data can be combined into
temperature-at-depth profiles showing the stratification
of the water columns the sharks pass through.
More
sophisticated tags are being developed and tested; the
SPLASH tag made by Wildlife Computers combines the data
logging features of a PAT type tag with the regular
transmissions of a location tag. Essentially every time
the animal comes to the surface it transmits not only its
identity code but as much of the stored data as it is
able. These tags also record diving profile information
which could give an insight into the sharks foraging
habits.
Another
development in tagging technology is the combination of
true research-grade oceanographic sensors into tags such
as is being tested by the Sea Mammal Research Unit at
St. Andrews University
,
Scotland
. Their Satellite Relayed Data Logger (SRDL) has been in
use on marine mammals for over 10 years and a recent
evolution of the tag being used on Beluga whales, has
enabled researchers to get real oceanographic data from
under the ice flows in the Storfjorden, Svalbard.
You can download a copy of an article about
this in pdf format from the Argos site by clicking
here (1.2mb).
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| Oceanographic sensors incorporated in
the SRDL tag have allowed data to be gathered from previously inaccessible areas;
outputs shown from MAMVIS visualisation system. Courtesy
of the Sea Mammal Research Unit, St. Andrews. |
Bearing in mind the constraints of data
transmission on the
Argos
system the output from these tags is quite remarkable and
may herald a new dimension satellite tagging and
tracking….
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