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Individual whale
sharks can be recognised from the distinct spot patterns on their
dorsal fins and behind their gills. Scarring damage to the fins
can also be a good visual identification. However, it still takes
a discerning eye to distinguish between individual sharks as well
as knowledge of previously sighted sharks.
Scarring and
spot patterns are therefore not an easy or reliable means of
identification. Most of the people who report whale shark
sightings have no background in whale shark identification and
often forget to note scarring or spot patterns when encountering
the gentle giant, so overwhelmed are
they by the experience.
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| Prominent
scarring on a whale shark dorsal fin. Photo Udo
Engelhardt |
With the aim of facilitating whale shark identification, MCSS set up a Whale Shark Tagging Programme in Seychelles based on a pilot project in 1996 (see next issue for more details). Since then, the
programme has used various tag types to monitor the distribution and migration of whale sharks in the
western Indian Ocean.
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Identifying sharks …. Initially the tags used were thin 'spaghetti' tags which were effective but
difficult to read. In 2001, these were replaced by placard tags similar to those used in Belize by the
UK Darwin Initiative Whale Shark Project. The tag has a coloured bar indicating the year of tagging, the
letter 'S' representing the Seychelles programme and an identifying number. The tag is anchored to the
back of the shark, normally close to the dorsal fin, by a titanium anchor dart that is inserted into the
shark’s thick skin by a tagging applicator.
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Placard tag
inserted next to the dorsal fin. Photo Udo
Engelhardt |
Tracking the Sharks …. The big advantage of the basic visual tag is that you do not need any special
equipment to track the shark; you simply check each shark for its number and can build up a picture of
its movements by repeated sightings. This is good in theory but it is also the major weakness in that it
requires someone to not only find the shark again but get close enough to read the tag; unfortunately
experience has shown that these sharks are seldom seen and re-sighting data is not easily generated.
This has heralded the use of high technology or 'smart tags' which can transmit information over
distances so that it is not necessary to actual re-sight the shark to confirm where it is. The
Seychelles programme is using three types of these advanced tags each giving remote information in
different ways.
Archival Pop-Off Tags (image in the box to the right) are compact tags, about the size of a small hand light, that record and store
data on depth, temperature and light intensity (i.e. approximate geographical position). They are
designed to 'pop-off' and float to the surface at a set time and then transmit the data by ARGOS
satellite to the project team. The translated data provides an overall picture of the shark’s movements
as well as its daily diving and surfacing habits.
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Spot2 Positional Tags transmit whenever they are at the surface and by cross triangulation between the
receiving Argos satellites they can give accurate position fixes. The whole tag is encapsulated in a
buoyant casing to ensure that the aerial is above the water for transmissions when the shark is near the
surface. Spot2 tags are however relatively bulky, causing drag in the water, which in turn places more
stress on the anchor point and makes it more difficult to keep them attached to the shark.
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| Spot2 Tag before
painting with antifouling paint. |
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Acoustic Tags are designed for close quarters tracking. The tag has an acoustic transponder which
sends out a 'sonar-like' signal which is picked up by an in-water hydro-phone. This system gives
extremely accurate fine scale details of the sharks movements and diving/surfacing behaviour but is
extremely resource intensive, requiring a boat to follow the shark for 48 to 72 consecutive hours. This
can give excellent data but the logistical requirements and the shark's capability to travel up to 35
kilometres a day tend to restrict the usage of this technology.
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An acoustic tag
ready for deployment |
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