4. Dec, 2017

Citizen Science contributes to world wide face book of whale sharks.

The whale shark (Rhincodon typus) is one of the many shark species I have never encountered. Perhaps one day when I am done with the Bahamian sharks (will I ever?), I shall cross the Atlantic to visit these gentle plankton gulping giants at Islas Mujeres.  During the summer months hundreds of whale sharks gather just north of the island in a seven mile radius to take advantage of the plankton rich waters created by the joining of the Gulf of Mexico and the Caribbean Sea.

Left:  The region behind the gills of whale sharks (above) exhibits suitable variation in spot pattern to enable individual recognition using image-matching software (below: see Arzoumanian et al. 2005).

Whale sharks have recently been upgraded from vulnerable to endangered under the IUCN Red List of Threatened Species.  There are still numerous reports of individuals being killed for food, including having their fins removed for soup. Their characteristics – docility, being the largest of all extant sharks, and tendency to aggregate seasonally at several accessible locations worldwide – have encouraged ecotourism and citizen-science opportunities, but also mean they can easily be targeted by fisheries in jurisdictions where they are not afforded protection.

Adult’s whale sharks grow slowly and only reach sexual maturity when they are 30 years  old, with their maximal age varying between 70 and 100 years.   It seems  that nobody has yet seen  whale sharks mating or a female giving birth to her pups. A  female species  captured in 1996 appeared to have  around 300 pups  in her womb each about 40 cm long. There are several  whale shark aggregation sites  -or hotspots- spread over the world  such as the Philippines, Islas Mujeres in Mexico, Belize, South Africa, Mozambique,  Qatar and the Gulf States, Galapagos Islands and the Maldives.

Still little is known about the whale sharks migratory habits and habitats. A recent paper in  Biomagazine* however described a large scale project of Australian researchers called  EcoOcean. Central in the project is the  Wildbook for whale sharks:  a visual database of whale shark  encounters of individually catalogued whale sharks. Here people can assist with whale shark research by submitting photos and sighting data. Since 1992 the  library has collected around 30.000 pictures of whale sharks over 57 countries, many taken by  snorkelers  and scuba divers visiting one of their hotspots.  This included in particular Ningaloo Reef and other sites along the along the extended Western Australian coastline. The ever growing data-base caused the number of 14 known hotspots to increase to 20. Top four sightings of whale sharks were from the locations Mexico-Atlantic, Western Australia (Ningaloo Marine Park), Mozambique and the Philippines (e.g. Cebu)

The project has contributed to a  better understanding  of the  wale sharks mobility  as well as their regular habitats.  An unique aspect of the project is that creation of the  picture library was realized with the help of Citizen Science: pictures taken by snorkelers or  scuba divers while visiting popular sites of whale sharks.  The technique used to identify the sharks in the EcoOcean project  is  straightforward, and  based on pattern recognition software using the white dot patterns behind the  gills of the whale shark as  a standard (see picture above). This particular area on the shark’s skin needs to be photographed in the correct orientation. Participants also upload, when possible, other relevant sighting information for storage and future analysis, includ­ing sighting location, sex, and estimated total length

As noticed in my earlier Blog,  using 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. But it is  much cheaper than tagging of sharks with telemetric devices, a more invasive method that can only  be applied on  a relatively small number of species. An it also allows to search for  individual sharks by comparing its unique pattern with a larger data base (somewhat like a fingerprint of a foreign visitor collected by the US customs at the airport).  Another interesting side of this method is that UW photographers can contribute to collect such a data base. 

The first  whale shark that entered the library  was a wale shark called  Stumpy,  because of   the unusual shape of the upper lobe of its caudal fin. Stumpy’s unique patterning was later identified  in 69 photographs submitted to the library between 1995 and 2016. During these two decades, Stumpy’s  total length  remained relatively unchanged at 7.40 m which closely matches  the 7.44 m estimates calculated from lengths reported by citizen scientists between 2008 and 2015. Photographic evidence also showed that Stumpy’s claspers had become elongated for the first time in 1998 and 2001, respectively – an indication that they had attained maturity*.  Other whale sharks  from the data base  are   A-103, a faithful customer of  Ningaloo reef  in Australia  estimated to be about 21 years, and BZ-011  that has been spotted over 5 years along the coast of Belize, but nowhere else. A surprising exception was A-424  who  showed far more mobility. After  been identified  first in Australian waters,  she was spotted later  in  Indonesia about 2.700 km  further to the north-west.  Smallest species were found in Indonesia (4 meter)  largest in Galapagos (11 meter). In general, long distance migratory routes were  extremely rare. However this did not  rule out  the possibility that the date base  might cover only a small part of the world population, or that  the  identified whales sharks had  visited other  more remote and  unknown locations.

Taking these cinsiderations in account,  Citizen Science  seems to have demonstrated that:

-Whale  sharks are far less migratory than other shark species (like for example tiger sharks ak)

-Exhibit a high degree of site fidelity  with longevity conservatively estimated to be at least 80 years

-Males and  females are strongly segregated with a preference for their own distinct habitats (in accordance with other predator sharks ak). For example  at the Galapagos, 99% of sexed individuals were female while  in Maldives and South Africa, only 9.43% and 9.60%, respectively, of the sexed whale sharks that were submitted were females


Sources and links

*Norman B. et al. (2017) undersea constellations: the global biology of an endangered marine megavertebrate further informed through citizen science  Bioscience xx: 1–15.   Published by Oxford University Press on behalf of the American institute of Biological Sciences 

Hsu HH, Joung SJ, Hueter RE, and Liu KM. 2014. Age and growth of the whale shark (Rhincodon typus) in the north-western Pacific. Mar Freshwater Res 65: 1145–54.

Norman B and Stevens J. 2007. Size and maturity status of the whale shark (Rhincodon typus) at Ningaloo Reef in Western Australia. Fish Res 84: 81–86

Arzoumanian Z, Holmberg J, and Norman B. 2005. An astronomical pattern-matching algorithm for computer-aided identification of whale sharks Rhincodon typus. J Appl Ecol 42: 999–1011.

Davies TK, Stevens G, Meekan MG, et al. 2013. Can citizen science monitor whale-shark aggregations? Investigating bias in mark–recapture modeling using identification photographs sourced from the public. Wildlife Res 39: 696–704