Proyecto del Tiburón Ballena del Atlático Mexicano-CONANP

Global distribution of Domino

The whale shark is in all warm tropical seas of the world, except the Mediterranean. Occasionally they are sighted in oceanic waters, although they are generally in groups feeding near coastal areas. Although widely distributed, they are infrequently seen, with the exception of some presumably preferred coastal areas, where they are regularly observed in relatively great numbers (of some tens to few hundreds) during some months of the year (CITES, 2002). The whale sharks location when not sighted near shore is unknown. When they come near the coast, it is generally related to planktonic production blooms.

Experiments have been made that analyze air temperature, sea temperature, salinity profiles, as well as current and wind patterns in areas where whale sharks have been sighted (Iwasaki, 1970 in Colman, 1997). Sightings were registered in 18° - 30°C temperatures, but they seemed to prefer 21° - 25°C, where waters rich in nutrients arise to the surface. The author suggests these conditions of temperature can be optimal for the production of the plankton and nekton on which the whale shark feeds. In other sources, the optimal temperature rank for the species has been published as 28° - 32°C, and in other parts with a wider range of 20°C a 32°C, although literature reports that the whale shark can remain during prolonged periods at depths of 240m where the temperatures are 10°C (Eckert & Stewart, 2000). There are also sightings reported at temperatures of 29°C of the coast of Texas (Hoffman et. al., 1981 in Colman 1997) and between 23.5° and 26°C in the Galapagos Islands (Both & Papastavrou, 1988 in Colman, 1997).

R.typus is generally a solitary animal; nevertheless, it can exhibit massive groupings of more than 100 individuals or show small associations of 5 to 20 individuals (Beckley et al., 1997; Colman, 1997a).

Whale shark distribution is characterized as being highly seasonal, with the appearance of groups during certain months in places where plankton thrives as result of periodic breeding of fish or invertebrates (Fowler 2000, Norman in press, Heyman et al. 2001, all in CITES 2002). This species is highly migratory, and the tracking of some individuals by means of satellites demonstrates some very long distance migrations, including a trip of more than 2,000 km towards Asia from the northwestern coast of Australia in 2002 (Norman pers. com. in CITES 2002), a trip of 550 km completed in a few weeks (Graham and Roberts, in prep in CITES 2002), a migration of 2,000 km from the sea of Mindanao in the Philippines to 280 km to the south of Vietnam made in two months (Eckert et al. in press in CITES 2002), and a migration of 13,000 km from the Gulf of California (Mexico) to the area of Tonga during 37 months (Eckert and Stewart, 2000).

There seems to be a seasonal and spatial segregation amongst populations, frequently sighting similarly sized and same sex individuals in a certain area (Norman 1999 in CITES 2002), while other groups of different ages and predominantly opposite sex are in other areas(Eckert and Stewart 2001, Graham and Roberts in prep in CITES 2002). By similarity with other migratory sharks, different sexes and different age groups can make different migrations. Therefore, the young animals can have different migrations than mature ones, and mature males and females can also undergo migrations of different durations and different lengths. For example, the migrations of mature females in some species are tied with reproduction cycles, which last two years (Hueter 1998 in CITES 2002). Tracking of tagged individuals and DNA studies have demonstrated that the male white sharks migrates from one ocean to another, whereas the females tend to remain in coastal waters of the continent in which they were born (Boustany et al. 2002, Pardini et al. 2001 in CITES 2002). In spite of undertaking such long migrations, studies involving tagging and photographic identification have demonstrated that sharks return to the same feeding areas in later years (Taylor 1994 in CITES 2002, Norman 1999 in CITES 2002, Graham et al. in prep. in CITES 2002). This behavior has been described in many shark species (for example, Walker 1996 in CITES 2002) and anadromous fish pecies.

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Role of the species in its ecosystem (CITES, 2002)

The role of the whale shark in its ecosystem is not known, but being a great plankton consumer, it can be similar to small whales. Although the species feed occasionally on eggs released by congregations of reef fish (Heyman et al. 2001), is not considered probable that this located predatory activity has an important effect on those species’ populations (only a very small proportion of teleosts fish’s fertilized eggs become adults). Traditional tuna fishermen know that whale sharks are associated with schools of tuna fish (Anderson and Amhed 1993, Silas 1986, Au 1991, Waller 1996) and have been used like "natural device for the congregation of fish" by the tuna fishermen of the Pacific and the Caribbean (for example, Stretta et al. 1996). Orcas, Orcinus orca, (O’Sullivan and Mitchell 2000) and, in the case of young individuals the blue marlin and blue shark ( Norman in press), are considered as natural predators.

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Oceanographic characteristics of the area

In the marine portion of the northeast part of the Yucatan Peninsula , a series of natural phenomena take place, like the Yucatan upwelling and Cabo Catoche gyre. There are few places in the world that present upwellings on western ocean margins (Coast of Somalia, East coast of Florida, Cold Cape in Brazil) and among these, the Yucatan upwelling displays the greatest speed, comparable to speeds of the main upwellings on eastern ocean margins. The continental platform on the east coast of Quintana Roo presents a width of 1 to 2km, allowing the circulation of the Yucatan Current at depths of 225 to 250km. In contrast, the platform that extends out to sea off the northern coast of the peninsula measures more of 260 km , being the widest platform of the Caribbean .

When the deep water mass arrives to this corner of the Yucatan Peninsula, it encounters shallower depths that cause its ascent towards the surface of the sea (Francisco Ruiz pers and Gyory ). According to Marino, the high temperature, density and nutrient gradient that separates upwelling water from surface water extends towards the west approximately until a diagonal between Scorpions Reef and Lagartos River . During the spring and summer there is little mixture between these two water layer which "induces significant fertilization" in the photic layer (where light penetrates) of the area, increasing the phytoplankton biomass.

The upwelled deep water stays below surface in most of the area. Nevertheless, a gyre in front of Catoche Cape creates a dome where deep water can arrive to the surface. When this gyre is completed, the surface water full of phytoplankton makes contact once again with the upwelling, providing microorganisms with new nutrients. This re-supply explains why the greatest amount of phytoplankton, measured in average chlorophyll concentration, and photosynthetic efficiency, represented in new gross production, "appears precisely to the northeast of Catoche Cape , immediately after contact between the gyre water and the recently ascended Yucatan upwelling water" (Merino, 1992).

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