Information is key – but lacking for sharks and rays in the Western Indian Ocean (commentary)

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  • Due to overexploitation, at least 27 percent of the 222 different shark and ray species found in the Western Indian Ocean (WIO) are considered threatened, meaning that they are classified as either Vulnerable, Endangered or Critically Endangered by the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species. These species face a high risk of extinction and need urgent conservation intervention.
  • Although the majority of sharks and rays pose no threat to humans, we pose a major threat to them, primarily through fisheries. Shark fisheries have existed for many decades, although historically they were primarily caught as unwanted bycatch. However, they are now increasingly being targeted due to the high demand for meat for local consumption and export, and for their fins for the global shark (and ray) fin trade.
  • Ensuring that sharks and rays are sustainably managed is important not only because they provide an important source of food and income for many coastal communities, but also because they serve an important function in maintaining balanced and healthy ecosystems through their roles as apex and meso predators, and as food for other, larger marine species. However, information needed to sustainably manage shark and ray populations is sorely lacking in the WIO.
  • This post is a commentary. The views expressed are those of the author, not necessarily Mongabay.

Great white sharks, bull sharks, tiger sharks: All are household names, synonymous with media hype, and made infamous through films and documentaries. These “deadly” names represent just three species of sharks and, aside from the real shark nerds, few people are able to name many others.

Yet, globally, there are over 1,250 species of sharks and their relatives the rays and chimaeras. Some of these are known from just a few specimens, some have gone extinct before they were formally named, and the majority we know very little about. Most choose not to bite humans even if they could, and many would not be capable, even if they wanted to.

In the Western Indian Ocean (WIO), there are over 220 different species of sharks and rays. Again, the majority of these pose little or no threat to humans. The whale shark, for example, is the biggest fish in the sea, but eats some of the smallest creatures in the ocean: phytoplankton and zooplankton, tiny plants and animals cast adrift in the water column. Many people have never heard of wedgefish, or “shark-rays” (they look like sharks, but are actually rays), which primarily feed on crustaceans. Wedgefish pose no threat to humans but, because their fins fetch enormous sums of money in the global fin trade, they are silently becoming one of the most threatened groups among the sharks and rays. There are also very timid sharks, such as the dark shyshark, which grows to just 60 centimeters (about 24 inches) in length. The name “shyshark” comes from its characteristic placing of its tail over its eyes when it feels threatened – hardly a man-eating monster!

A filter feeding whale shark off Nosy Be, Madagascar. Photo Credit: Stella Diamant/Madagascar Whale Shark Project.

Although the majority of sharks and rays pose no threat to humans, we pose a major threat to them, primarily through fisheries. Shark fisheries have existed for many decades, although historically they were primarily caught as unwanted bycatch. However, they are now increasingly being targeted due to the high demand for meat for local consumption and export, and for their fins for the global shark (and ray) fin trade. In developing nations, more and more people are migrating to the coast, and sharks and rays are increasingly becoming an important source of food and income. This increased threat is unsustainable, as few shark and ray species can tolerate high levels of fishing mortality. In a sense, sharks and rays are more akin to mammals than other fish species, as they generally take a long time to reach sexual maturity and have few offspring with long gestation periods. As such, many are not able to produce enough offspring to maintain stable populations while they are concurrently being diminished due to fishing.

Due to overexploitation, at least 27 percent of the shark and ray species found in the WIO are considered threatened, meaning that they are classified as either Vulnerable, Endangered or Critically Endangered by the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species. These species face a high risk of extinction and need urgent conservation intervention. Ensuring that sharks and rays are sustainably managed is important not only because they provide an important source of food and income for many coastal communities, but also because they serve an important function in maintaining balanced and healthy ecosystems through their roles as apex and meso predators (meso predators are mid-level predators that both prey and are preyed upon), and as food for other, larger marine species. However, information needed to sustainably manage shark and ray populations is sorely lacking in the WIO.

The Wildlife Conservation Society’s WIO shark and ray program recently assimilated all available information on threatened shark and ray species in the WIO, highlighting the major gaps in our knowledge of shark and ray biology, ecology, and fisheries. The database identifies research priorities for shark and ray species in the region so that conservation and research activities can focus on these key aspects.

One of the biggest information gaps for threatened shark and ray species in the WIO is movement behavior. Understanding a species’ movement is critical when considering potential protection measures. Movement behavior will dictate the required size and placement of a marine protected area (MPA) and determine whether an MPA could offer any protection at all for the species. Spatially and temporally predictable aggregations, such as the whale shark aggregations off Tofo in southern Mozambique, and off Nosy Be in north-west Madagascar, increase a population’s vulnerability to fisheries, as many individuals can be caught at the same time, and during ecologically important events, such as mating. Effective protection of such species, therefore, also requires an understanding of their aggregation dynamics.

A wedgefish (Rhynchobatus djiddensis) swimming along the sea floor in the Ponta Partial Marine Reserve in southern Mozambique. Photo Credit: WCS.

Another major information gap is reproductive ecology, which includes aspects such as gestation period, frequency of birth, number of offspring, and nursery localities. Nursery localities and gestation period can help to inform the timing and location of seasonal closures (another form of MPA, whereby a specific area is closed to fishing for a specified period of time). Frequency of birth and litter size determine how quickly a species can rebound from threats such as fishing. For example, a blue shark gives birth to an average of 35 pups every year, whereas a reef manta ray primarily gives birth to one pup every two years, after a 12-month gestation period. Therefore, blue sharks can tolerate a higher level of fishing mortality than reef mantas.

Understanding the threats that different shark and ray species face is also important. Threats differ depending on numerous aspects, such as quality of (and associated demand for) their fins, animal size, and where the species lives. Coastal sharks and rays (which live close to shore) are impacted by habitat destruction from coastal development and pollution, but face their greatest threat from traditional and artisanal fishers. These fisheries use a variety of gear types, from spears and tidal traps to handlines and indiscriminate gillnets. In comparison, pelagic shark and ray species (which live out in the open ocean, predominantly away from the coast) face their greatest threat from industrial fisheries. These fisheries include massive purse seine vessels which have nets up to 1.5 kilometers (about 1 mile, or more than 4,920 feet) long and up to 150 meters (492 feet) deep, or longlines that can be more than 100 kilometers (more than 328,000 feet) long!

Many different shark and ray species are caught in traditional and artisanal fisheries, but some of the notable species that are caught in high numbers throughout their range include hammerhead sharks and wedgefish, which are primarily targeted for their fins. These fisheries also target mobula rays and manta rays such as the shortfin devil ray for their meat, but primarily for their gill rakers, which fetch high prices in the Asian medicinal trade. In the pelagic fisheries, common species that are caught include blue sharks and mako sharks, which are primarily targeted for their meat. The lesser known kitefin shark, a deepwater species that lives close to the bottom, is also caught in large numbers for its meat but also for its squalene, a compound primarily obtained from shark liver oil that is used in a variety of products such as cosmetics and health supplements.

So, although sharks are often vilified, they are unfairly being persecuted in extremely high numbers on a daily basis. Fortunately, there are many organizations around the world fighting to protect them. Last Saturday was Shark Awareness Day, which should be a time for us to reflect upon the impact we are having on shark and ray populations globally, and to realize that we are privileged to share the ocean with these magnificent creatures that evolved over millions of years to be perfectly adapted to their environment. So, in the spirit of Shark Awareness Day, educate yourself and others, and together we can ensure that these creatures will swim safely for another million years.

A curious dark shyshark off the coast of South Africa. Photo Credit: Mike Markovina.

Dave van Beuningen is a shark and ray conservation assistant for the Wildlife Conservation Society’s Madagascar & Western Indian Ocean shark and ray program. Rhett Bennett is a shark and ray conservation officer for the Wildlife Conservation Society’s Madagascar & Western Indian Ocean shark and ray program.

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This story first appeared on Mongabay

South Africa Today – Environment



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