Sharks and rays
One hundred and thirty seven species of sharks and rays occur in the WIO, of which 15 are endemic to the region. The highest elasmobranch diversity in the region has been recorded from Mozambique waters, with 108 species (73 sharks and 35 rays). Ten species are endemic just to South African waters.
Sharks are at the top of the food chain especially in the coral reef environment. Sharks have slow reproductive rates with many species only producing a handful of offspring when the adults are 10-15 years old. The more primitive species lay eggs, while the most advanced species are viviparous, meaning the fetal sharks are connected by placenta in utero, and born live.
Published references on sharks and rays in the WIO are very rare, and the available data are based primarily on gray literature and testimony, often inaccurate. In the last decade, however, this situation is changing as interest in sharks from an ecological standpoint, and for conservation, has increased.
Sharks are widely dispersed in tropical waters, including the WIO, but heavy mortality from fishing impacts has dramatically reduced their numbers in many locations. At present, the most important locations for sharks are the islands of the Mozambique channel, and the southern islands of the Seychelles, likely due to their isolation.
The Iles Éparses (Scattered Islands) are among the few locations with good shark populations: aggregations of juvenile Galapagos sharks (Carcharhinus galapagensis) in Bassas da India, nursery areas for grey reef sharks (C. amblyrhynchos) in Juan de Nova, blacktip reef sharks (Carcharhinus melanopterus) and lemon sharks (Negaprion acutidens) in Europa and nurse sharks (Nebrius ferrugineus) on Geyser. Other notable sites include the Zelee bank especially for grey reef shark, and Europa for the presence of hammerhead shark schools (Sphyrna mokarran).
Mayotte and associated banks (Zélée and Geyser) are regularly frequented by sharks. In Mayotte, scalloped hammerheads (S. lewini) are observed at the beginning of the austral winter (July-September), in which schools can exceed 20 individuals.
Reef Manta Ray (Manta alfredi)
Grey reef shark (Carcharhinus amblyrhynchos)
© David Obura
Porcupine ray (Urogymnus asperrimus)
© Julien Wickel
Hammerhead (Sphyrna zygaena)
© Alessandra Maccari
Whale shark (Rhincodon typus)
© David Obura
Oceanic whitetip shark (Carcharhinus longimanus)
© Julien Wickel
Threats - Sharks are increasingly becoming a threatened fish group in the WIO, as they are highly vulnerable to fishing, and their misplaced reputation as man-eaters promotes an attitude of eradication or control rather than one of conservation, among the public and managers. Based on voluntary declared FAO records there is evidence that shark catches in the WIO have more than halved after reaching a peak of 180,000 Mt in 1996. About 100 million sharks are fished annually in the world, either as accidental catch in seine and/or gill nets, or direct exploitation for their fins and other products. Of this total, 30% is from the Indian Ocean, and mainly from the south-western part of the Indian Ocean. Catching of sharks for their fins, the most valuable part as they can be dried easily and sold at prices over $100 per kg is the greatest threat to shark populations in the WIO, and is banned in many countries.
Key oceanographic features of the WIOAreas of aggregation of whale shark in the WIO (hatched areas). The three sectors A, B and C correspond to the presence of pelagic whale sharks during different seasons from data collected by purse seiners: (A) January, (B) from April to May; (C) from August to September (Rowat, 2007 in Kiska et al., 2009; unpublished).
Observations of great white sharks in the WIO. The stars correspond to sparse observations or catch (data from Cliff et al., 2000 and MAYSHARK, unpublished data, in Kiska et al., 2009; unpublished).
The Great White shark (Carcharodon carcharias) - Great White sharks are naturally low in abundance, and range from cold temperate waters to tropical waters though in the latter they tend to prefer cooler deeper waters. In the WIO, Great Whites are most common in South Africa, and across a broad range of sizes. Elsewhere in the WIO they are rare, though individuals have been caught or seen throughout the region, and mostly as large pregnant females, suggesting they may pup in warmer waters. They have low reproductive potential, probably have a low natural mortality, and presumably possess a low capacity for density- dependent compensation to rapid declines in population size. It is therefore reasonable to conclude that populations are vulnerable to recruitment overfishing and all forms of non-natural mortality. However, their population status is poorly known over the species range owing to a lack of robust abundance indicators, and quantitative stock assessments are not currently possible.
Whale sharks (Rhincodon typus) -
From the first whale shark described in 1828 from the Indian Ocean, the region continues to be one of the most important areas for whale sharks. Whale sharks are a broad ranging species with seasonal migration patterns over 1000s of km, though may be resident year-round in equatorial zones. Globally they are found in many areas with surface sea water temperatures of 18–30°C, and range across the entire Indian Ocean. Unusually for sharks, females give birth to large numbers, even thousands, of young.
The species has however been the subject of several targeted fisheries and thus sustained massive, rapid declines in population numbers. A number of fisheries targeting whale shark have developed within the Indian Ocean, some from traditional roots, such as in India, Pakistan and the Maldives that originated to supply the oil from the shark’s liver for waterproofing boats This escalated especially in India during the 1990s to supply the demand in Taiwan for ‘Tofu shark’. Reported figures indicate a peak in this Indian fishery of 279 sharks in 1999 but that despite increased effort only 160 were taken in 2000. This fishery was closed in 2001 when the species received protected status. The fishery in the Maldives previously took 20–30 whale sharks per year but proved unsustainable with declining catches and the fishery was stopped in 1995.
Whale shark tourism has rapidly grown in importance in the WIO, with predictable seasonal sightings known in Kenya (e.g. Diani), Tanzania (e.g. Mafia) and Mozambique (e.g. Tofo). In the islands, two main aggregation areas (feeding and nursery) have been identified: the granite islands of Seychelles (Mahe in particular) and the northwest coast of Madagascar, especially near the island of Nosy Be. The dynamics and inter-relatedness of these populations are unknown, but represent a significant opportunity for blending conservation, research and economic development.
Other sharks and rays - Reef and oceanic sharks are widely dispersed but their populations greatly reduced through fishing mortality. In the WIO the bull shark or Zambezi shark (Carcharhinus leucas) is most strongly implicated in attacks on people, in both mainland and island sites, fuelling the general fear of sharks and low commitment to their conservation in most regional countries. In austral winter in Mayotte, manta rays (Manta alfredi) and scalloped hammerhead sharks (Sphyrna lewini) are particularly abundant near steep reef slopes. Geyser Bank may constitute a nursery area for tawny nurse sharks (Nebrius ferrugineus) and Zélée Bank could be a nursery ground for grey reef sharks (C. amblyrhynchos). At Tofo in southern Mozambique, there is a major manta ray (M. alfredi) aggregation that has been investigated for several years. Annual population size estimates range from 150 to 450 individuals and the super-population estimate was 800 individuals.
Guitarfish are known from the region, though exploited and highly depleted.
Management and conservation - Reducing excess mortality of sharks is a fisheries issue requiring action on gear types and their operation, and preventing the most destructive markets, especially for shark fins. These actions need to be taken at regional and global levels. An additional solution is the creation of marine protected areas focused on key locations and habitats of importance to sharks, and some of the remaining high-density populations of sharks.
Because of their importance in fisheries and as charismatic species, sharks have been mentioned in multiple global legal instruments, which can support actions at multiple levels. The examples below illustrate these with reference to the Whale Shark:
IUCN Red List - the whale shark is listed as Vulnerable in the IUCN Red List
- CITES - Appendix II, 2002. This status should allow for the closer monitoring of and restriction in international trade in whale shark products, and by so doing assist in the conservation of the species on a global scale.
- Convention on Migratory Species (CMS) - Appendix II, 1999. On listing, a call was made for co-operative actions by 2001–2002, however, it was not until November 2005 that the CMS approved a ‘Recommendation for the conservation of migratory sharks’ proposed jointly by Australia, New Zealand and Seychelles.
- UNCLOS - Agreement on Straddling Fish Stocks and Highly Migratory Fish Stocks. This agreement recognizes that as the whale shark is a highly migratory species, coordinated management and assessment of shared migratory populations would promote an understanding of the cumulative impacts of fishing effort on the status of shared populations. To date, no such measures have been proposed.
- FAO - International Plan of Action for Sharks - there is a potential framework for whale shark conservation. Unfortunately implementation of even National Plans of Action by FAO members has been extremely limited thus hampering plans for an international instrument.
Key References - Bruce (2009); Chabanet & Durville (2005); Colman (1997); Eckert & Stewart (2001); FAO (2000); Fowler S (2000); Hammerschlag & Fallows (2005); Jamon et al. (2010); Kiszka et al. (2009); Marshall & Barnett (1996); Marshall et al. (2011); Romanov et al. (2010); Rowat (2007); Rowat et al. (2009); Wickel et al. (2009); Wickel et al. (2010). --> References