Whale Sharks

Introduction

Whale sharks are the largest living fish in the ocean and it is their sheer size and filter-feeding behavior that gives them the name ‘whale’ shark, they are in fact sharks not whales. They breathe underwater using gills and do not need to surface to breathe like mammals do. The species whale shark (Rhincodon typus) was first described and named by Andrew Smith in 1828, based on a specimen harpooned in Table Bay, South Africa (Smith 1828 in Colman, 1997).

Etymology

In Utila the whale shark is known as ‘Old Tom’ due to a story told by fishermen of a huge resident barnacle-encrusted fish which cruised the waters surrounding the island.

In Vietnamese culture the whale shark is known as a deity, and so called “Ca Ong”, which literally translates as “Sir Fish”.

In Mexico, and throughout much of Latin America, the whale shark is known as “pez dama” or “domino” for its distinctive patterns of spots. However, they go by “Sapodilla Tom” in Belize due to the regularity of sightings near the Sapodilla Cayes on the Belize Barrier Reef.

In Africa, the name associated with whale sharks is very evocative: “papa shillingi” in Kenya came from the myth that God threw shillings upon the shark which are now its spots. In Madagascar the name is “marokintana” meaning “many stars”.

Japanese also reference the stars by calling it “geger lintang,” meaning “stars in the back”. In the Philippines, whale sharks are referred to as “butanding”

Distribution

Whale sharks are a widely distributed and highly migratory species which can be found in global tropical and warm temperate seas, both coastal and oceanic (Compagno, 2001). While some waters they only pass through on migrations, they also form seasonal aggregations at eleven different known locations; Ningaloo (Australia), the Philippines, Mozambique, Seychelles, Maldives, Djibouti, Belize, Holbox (Mexico), North and South Gulfs of California and the North Gulf of Mexico (Rowat & Brookes, 2012). Whale shark migrations can be several thousands of kilometers long but whether these are linked only to feeding events or whether other aspects are involved is still unknown (Hueter et al, 2013).

Whale Shark Distribution Map Rowat Brookes

Figure from Rowat & Brookes, 2012

Whale sharks are found around Utila, Honduras year round although the peak number of sightings is estimated to be between March and April (Fox et al, 2013). This can vary dependent on productivity and food availability. They are usually found on the Northern side of the island, likely due to this being close to the edge of the continental shelf (Fox et al, 2013)

Diet & Feeding Behavior

Whale sharks (Rhincodon typus), Basking sharks (Cetorhinus maximus) and Megamouth sharks (Megachasma pelagios) are the only known filter-feeding sharks. While the other two species use bristle-like gill rakers and are only able to target low-density prey by ‘passive feeding’ (filtering huge amounts of water by swimming through it slowly) whale sharks use pad-like filters for active suction feeding as well as passive ram-feeding (Rowat & Brookes 2013). This is thought to enable them to feed on various prey aggregations of different densities due to using more than one feeding method as well as feeding both vertically and horizontally in the water column. They have teeth but these are very small (about 2mm) and are not thought to be used in feeding (Taylor, 2007).

During suction feeding the shark draws in a mouthful of water and then closes the pharynx and pushes this water through dense filters on the internal gills (Nelson & Eckbert, 2007; Heyman et al, 2001). This fine sieve-like apparatus, which is a unique modification of the gill rakers, prevents the passage of anything but fluid out through the gills, trapping anything above 2 to 3 millimetres (0.079 to 0.12 in) in diameter (Taylor, 2007). Material caught in the filter between the gill bars is then swallowed. Whale sharks have been observed closing their gills and “coughing” food to the front of their mouth presumably as a method of clearing a buildup of food particles in the gill rakers (Nelson & Eckbert, 2007).

Prey is dependent on season and location but whale sharks have been observed feeding on both planktonic species such as copepods, krill, and crab larvae, as well as nektonic species such as sardines, anchovies, small tuna, and squid (Nelson & Eckbert, 2007). They have also been found feeding on fish eggs released in mass spawning events (Heyman et al, 2001).

Anatomy

The whale shark is the largest living fish in the ocean, with the biggest individual caught by the Taiwanese fishery in 1987 being recorded as 20 meters long and weighing 34 tons (Chen et al, 1997). However specimens this size are rarely encountered and the largest seen around Utila, Honduras between 1999 and 2011 was 10.6 meters long (Fox et al, 2013).

Whale sharks are characterized by a pigmentation pattern of contrasting light spots and stripes on a dark background that is unique to each individual, a depressed head featuring a wide, forward facing mouth set between the eyes, and large fins, in particular the dorsal and caudal (tail) fins. They possess a lightweight cartilage skeleton which does not include a ribcage (Martin, 2005), reducing total body weight. However, like all sharks, whale sharks are negatively buoyant and are thought to use the hydrodynamic shape of their pelvic fins to generate upwards thrust while travelling horizontally. This allows them to glide great distances while still minimizing energy expenditure (Gleiss et al, 2011).

Reproduction & Life History

Whale sharks are thought to exhibit similar life history characteristics to many other shark species, notably a slow growth rate, a late maturation age and the production of numerous live young. In fact, after a pregnant female was caught in Taiwan in 1995, it was discovered to contain 304 embryos in various stages of development, making it one of the most fecund shark species in the world (Joung et al, 1996).The embryos within this female ranged from those still contained within egg-cases to already hatched, free-swimming and near-term embryos so the conclusion was made that the embryos develop associated with a yolk sac but then given birth to as live young (Schmidt et al, 2010). The range in developmental stages suggests the ability to store sperm for ongoing fertilization (Schmidt et al, 2010). However, it is important to note that there are huge gaps in scientific knowledge regarding whale shark reproduction as mating or pupping behaviors have never been observed, meaning that the pregnant female from Taiwan in 1995 remains the only source of information.

Sexually active males can be identified by a pair of calcified claspers located just behind the pelvic fins that are utilized during reproduction, whereas females instead have a reproductive slit. 95% of males are mature once they reach a size of 9 meters, although some have found to be mature at 8 meters (Norman and Stevens, 2007). Females are thought to mature either at the same size or larger, although this information comes from a limited number of stranding incidents (Beckley et al, 1997) and more study needs to be done in this area.

Threats & Conservation

The IUCN (International Union of Conservation of Nature) Red List of threatened species assesses population trends and data of species and assigns them a group ranging from Least Concern to Critically Endangered. Unfortunately a recent assessment of this list came to the conclusion that a quarter of the world’s sharks and rays are threatened with extinction (Dulvy et al, 2014). Whale Sharks are currently listed as vulnerable and their population trend is decreasing. This is due to a number of factors most prominently overfishing, both bycatch (incidental catch) and targeted fishing of Whale sharks.

Whale sharks have been targeted for a long time as other sharks have, for products such as meat, oils and other extracts used in pharmaceuticals. Although many countries have banned shark fishing there are some parts of Asia which still hunt them as the demand for shark-fin soup remains whilst it is regarded a high priced delicacy. As well as the demand for shark products, they can be incidentally caught as a bycatch of other fishing practices (Rowat & Brookes, 2013). This is often welcomed due to the perceived threat of sharks to humans and also their possible impact on depleting other fish species they feed on as well as possible damage to nets. Sharks are very susceptible to overfishing due to being slow to mature and to reproduce, as well as their migratory behavior (Fowler, 2000; Dulvy et al, 2014).

Other possible threats are pollution, boat traffic, habitat destruction for farming, commercial development and global climate change (Dulvy et al, 2014). Surface swimming and feeding makes Whale sharks vulnerable to ship collisions which can result in either mortality or injury and these boat or propeller strikes are commonly recorded (Rowat & Brookes, 2013).  The other threats may have effects on migratory routes or food availability and this can detrimentally affect populations.

The waters of Honduras were declared a shark sanctuary by Honduran Law in June 2011 after an initial ban on shark fishing in 2010. However the prevention of fishing is difficult to control due to the illegal trade of products and the need for patrol boats to prevent illegal fishing or unrecorded bycatch. CITES (Convention on International Trade of Endangered Species) included Whale sharks in Appendix II protection status in 2003 which means that any trade of products from these species is controlled and only permitted by license holders and under strict regulations.

However as sharks are becoming more popular as a tourist attraction their value alive is much greater than dead. It is necessary to maintain strict regulations of interactions with these animals so as not to disrupt their normal feeding behavior or distress them, as this may also have detrimental effects both long and short term (Norman, 2002; Rowat & Brookes 2013). Ecotourism interactions can also be used to collect data about the individuals sighted including photographs for identification purposes.

 

Thank you to WSORC volunteers Emma Novak and Rory Sedgwick from the University of Newcastle for compiling this up to date report on whale shark biology and life history.

 

 

References

 

  1. Beckley, L. E., Cliff, G., Smale, M. J. & Compagno, L. J. V. (1997). Recent strandings and sightings of whale sharks in South Africa. Environmental Biology of Fishes 50, 343-348. doi: 10.1023/A:1007355709632
  2. Dulvy, N. K., Fowler S. L., Musick J. A., Cavanagh R. D., Kyne P. M., Harrison L. R., Carlson J. K.,. Davisdson L. N., Fordham S., Francis M. P., Pollock C. M., Simpfendorfer C. A., Burgess G. H., Carpenter K. E., Compagno L. V. J., Ebert D. A., Gibson C., Heupel M. R., Livingstone S. R., Sanciangco J. C., Stevens J. D., Valenti S., White W. T. (2014) Extinction risk and conservation of the world’s sharks and rays. eLIFE. doi:10.7554/eLife.00590.001
  3. Fox S., Foisy, I., De La Parra, R. V., Galvan B. E., Graham, R.T., Hoffmayer, E.R., Holmberg, J., Pierce, S. J. et al. (2013) Population Structure and Residency of Whale Sharks Rhincodon typus at Utila, Bay Islands, Honduras. Journal of Fish Biology 83: 574-587. doi: 10.1111/jfb.12195
  4. Heyman, W. D., Graham, R. T., Kjerfve, B., Johannes, R. E. (2001) Whale sharks Rhincodon typus aggregate to feed on fish spawn in Belize. The Nature Conservancy, 62 Front Street, Punta Gorda, Belize
  5. Hueter, R. E., Tyminski, J. P., de la Parra, R. (2013) Horizontal Movements, Migration Patterns, and Population Structure of Whale Sharks in the Gulf of Mexico and Northwestern Caribbean Sea. DOI:10.1371/journal.pone.0071883
  6. Joung, S. J., Chen, C.-T., Clark, E., Uchida, S & Huang, W. Y. P. (1996). The whale shark, Rhincodon typus, is a live-bearer: 300 embryos found in one ‘megamamma’ supreme. Environmental Biology of Fishes 46, 219-223. doi: 10.1007/BF00004997
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