18. Mar, 2016

Using natural body markings to identify a shark

Identification of an individual shark can  be helpful to learn more about  its  behavior, age, distribution, habitat use, population  and  migration patterns. Not only about  that individual shark but also the group to which it belongs.  For example,  if a certain GHH (great hammerhead) shark has Bimini as its winter habitat but then shows up along the Florida coast in spring, its migratory pattern could be representative for  the entire group of GHHs that left their winter location. The most  frequently used method  for shark identification is tagging. There are several methods of tagging that serve different purposes.  Plastic numbered  ‘cattle’ tags, ‘dart tags’ containing a capsule or a microchip under the skin, are ways to learn more about a sharks migratory paths (see also http://sharkangels.org/issues-facing-sharks/tagging-sharks  But they require recapture of the  shark  by a  marine expert or fishermen that catch a shark  with a long line or net and then return the information to the taggers.  Same holds for data-loggers, small devices  that collect  enormous amounts of data but also require that the shark is recaptured for downloading the collected data. Acoustic and satellite  tags are more advanced  telemetric  devices  that allow  to track a  moving shark with special apparatus for receiving the emitted signals.

Matching natural  body markings  is  an alternative and  shark-friendly way to  identify individual sharks, although it does not allow real time tracking of the shark.  It also requires a picture of the shark to be compared with a larger data base (somewhat like a fingerprint of a foreign visitor collected by the US customs at the aiport).  An interesting part of this approach is that UW photographers can contribute to collect such a data base. Given that some some guidelines are provided on how to capture  the right  image of the markings of a specific shark. In some sharks like the  great white and  tiger shark  the meandering  line  separating the white lower  and the pigmented upper part of  the lateral  body seems  to follow a typical  ‘idiosyncratic’  pattern (see the picture above of tiger shark Emma and a crop of her  head markings).  Other species (like C. Taurus, the sand tiger shark) show typical individual patterns of pigmentation spots.  

Compared with  tagging,  identification via natural markings is inexpensive, non-invasive and reliable over a much longer period.  But, as said earlier, they depend on pictures of  pigment spots or markings on selected areas on one side of the body.  It becomes easier when a  shark has distinctive marks like a scar, a mutilated  dorsal or pectoral  fin.  I remember a male white shark that showed up  at Isla  Guadalupe in Mexico nicknamed  ‘Shredder’, because of its heavily damaged  first  dorsal fin, probably inflicted by a sea lions sharp teeth (see this picture taken by Dr Steve. https://www.flickr.com/photos/drsteve/2925698193/).  Of course these cases are mainly of interest for those that return to the same spot every year, and want to check if the 'regular guys' are still around.  

A new and promising scientifically based matching technique  is computer-aided pattern recognition of natural pigmentation spots. It has been successfully  applied on various species of sharks like the whale, leopard, nurse, the great white and the sand tiger shark**

Sources

**Castro, A.L.F. & Rosa, R.S. (2005) Use of natural marks on population estimates of the nurse shark Ginglymostoma cirratum, at Atol das Rocas Biological Reserve, Brazil. Environmental Biology of Fishes,

**Van Tienhoven, A.M. A computer-aided program for pattern-matching of naturalmarks on the spotted raggedtooth shark Carcharias Taurus Journal of Applied Ecology (2007), 4, 273–280

 **Arzoumanian, Z., Holmberg, J. & Norman, B. (2005) Anastronomical pattern-matching algorithm for computer-aided identification of whale sharks Rhincodon typus. Journal ofApplied Ecology, 42, 999–1011.