- Whale sharks don’t need help being spectacular. The world’s biggest fish is impressive in nearly every aspect, growing as long as 12 meters (40 feet) and weighing up to 21 tons.
- A new study in the journal Endangered Species Research used photo-identification techniques based on the sharks’ distinctive spots to discover a new hotspot for juvenile whale sharks around the tiny island of Nosy Be, in northwest Madagascar.
- This is a rare bit of good news for a species that, like many other sharks, is struggling to survive in oceans increasingly subject to the negative impacts of human activity.
Whale sharks don’t need help being spectacular. The world’s biggest fish is impressive in nearly every aspect, growing as long as 12 meters (40 feet) and weighing up to 21 tons. Their enormous mouths contain thousands of teeth, and their backs feature constellations of white spots that make them look like gliding, underwater solar systems. As a result, whale sharks (Rhincodon typus) are known as marokintana in Madagascar, meaning “many stars.”
But in addition to their beauty, whale shark spots have also proven vital to researchers. A new study in the journal Endangered Species Research used photo-identification techniques based on the sharks’ distinctive spots to discover a new hotspot for juvenile whale sharks around the tiny island of Nosy Be, in northwest Madagascar.
“No one thought there were that many [whale] sharks,” Stella Diamant, lead author of the study and founder of the Madagascar Whale Shark Project, told the BBC.
This is a rare bit of good news for a species that, like many other sharks, is struggling to survive in oceans increasingly subject to the negative impacts of human activity. The IUCN Red List categorizes whale sharks as endangered, with experts reporting a 63 percent decline in the species’ population over the last 75 years in the Indian and Pacific oceans. The main threats to whale sharks include overfishing, boat collisions and death as bycatch.
Teenage stag party
The study identified 85 whale sharks in one season in 2016, from September to December, off Nosy Be. Diamant says that over three seasons, from 2015 to 2017, her team observed 240 of the animals.
These numbers are significantly higher than reported figures from other coastal areas surrounding the African continent known for whale sharks. These individuals also represent only those animals successfully observed. Despite being the size of school buses and weighing up to three times an African elephant, whale sharks can be surprisingly elusive.
“There may also have been up to another hundred more sharks not observed,” Diamant tells Mongabay.
Interestingly, all of the sharks were juveniles of no more than 10 meters (33 feet) in length. They were also nearly all male: only 16 of the identified sharks were female, about 18 percent of those observed. This disproportionality in sex and the predominance of juveniles is not uncommon in whale shark sightings and holds true for many other known whale shark areas.
Diamant says researchers have yet to discover a whale shark area where juvenile females outnumber males.
She adds that while “the reasons are still unknown” for the segregation between males and females, it is “probably due to different energetic needs for each sex.”
The researchers think that the popularity of Nosy Be as a seasonal habitat for the juveniles relates to high prey availability in the fertile, plankton-rich waters surrounding the island.
“Juveniles need higher energetic intake because they are growing and they come to the coast where there are these plankton blooms,” Diamant says.
As well as plankton, whale sharks feed on small fish. In fact, researchers found the sharks around Nosy Be were often in the presence of schools of mackerel tuna feeding in the area, pointing to a potential connection between the whale sharks and the tuna.
“Baitfish often hide behind the whale shark, which helps the tunas and also gives the tunas a place to hide if other predators are around,” says Simon Pierce, a co-author of the paper and principal scientist at the Marine Megafauna Foundation. “We also think that there might be sound detection involved on the part of the whale shark, so hearing the tunas jumping might help the whale shark locate the baitfish.”
Although the details of this potential relationship between the whale shark and the mackerel tuna are lacking, Pierce and his team aim to explore it further later this year. Such research may be important not only for understanding whale shark ecology and behavior, but also for future conservation efforts.
Tourism potential and problems
While the research published by Diamant and her colleagues represents the world’s first study of whale shark populations in Madagascar, hints of a growing whale shark presence existed previously. Beginning in the early 2000s, fishermen, tour operators and NGOs all reported whale sharks sightings. In 2011, an ecotourism industry focusing on the whale shark began in Nosy Be, offering tourists the chance to swim alongside the gentle giants.
“It’s incredibly humbling … your mind is completely empty, but in a good way,” Diamant says of swimming with whales sharks.
While an important source of revenue to the region, a rising tourist industry also presents risks. Tourist vessels will go very close to the sharks, operators allow tourists to touch the sharks, and some even offer an experience where clients can “ride” the whale sharks by grabbing onto the fin or body as they move through the water.
“It clearly bothers them and affects their behavior,” Diamant says of both the touching and the riding. The latter activity is also potentially life-threatening for the tourists.
“[The shark] might dive deep and [the tourist] won’t be able to hold [their] breath long enough,” Diamant says.
It is not only irresponsible tour operators or misinformed tourists who engage in this behavior, but also “experienced free divers/spear fishers who just do it for the fun,” Diamant adds.
In response to this and other forms of misconduct, the Madagascar Whale Shark Project has created a voluntary code of conduct outlining appropriate guidelines for tour operators. Some of their suggestions include limiting interactions to one boat per shark with a minimum distance of 25 meters (82 feet) and time limit of one hour, as well as maintaining a distance of 3 meters (10 feet) between swimmers and sharks.
While Diamant says the code has had positive results so far, its voluntary nature and the lack of legal regulations regarding whale shark tourism in Madagascar mean that not all operators comply.
A whole new world
Whale sharks are not the only magnificent creatures attracting tourists to the waters of Madagascar; the region is also home to manta rays, devil rays, bottlenose dolphins, sea turtles, and a diverse array of coral, many of which are also globally endangered.
It even boasts a new species of whale. In 2011, researchers made the first observation of Omura’s whale (Balaenoptera omurai), a species of baleen whale, in Madagascar. Described in 2003, the elusive Omura’s whale is unusual in that, unlike most whales, it doesn’t migrate, preferring to stay in one place. Even stranger is its preference for the tropics: tropical waters do not offer an abundance of food supplies like colder waters. The slender and streamlined Omura’s whale sings a low, repetitive hum for hours on end, occasionally swelling into a chorus of multiple whale voices, potentially in an effort to woo females.
Many of these charismatic animals, and the waters they live in, remain unprotected, and whale sharks are no exception. Although there are two marine protected areas close to Nosy Be, no protective measures are in place for the sharks in the majority of their range.
Last year, the Convention on the Conservation of Migratory Species of Wild Animals listed whale sharks under Appendix I. This compels Madagascar, and other signatories, to prohibit hunting of the species except under very limited circumstances, as well as to establish other conservation measures. Still, researchers in Madagascar report recent records of both whale sharks and dolphins killed as bycatch. The species are particularly vulnerable to gillnets, large vertical “walls” of net designed to entangle massive numbers of fish by the gills.
Diamant and her co-authors suggest a restriction in gillnet use in areas associated with the whale shark. They also recommend keeping a close eye on the schools of tuna as a result of the observed, but not yet understood, connection between the two species.
While Diamant says “a marine reserve would be amazing,” any conservation measures must “benefit and empower local people as well.” Madagascar is considered one of the poorest nations in the world, with a majority of citizens living in extreme poverty.
Even as scientists discover new populations, the whale sharks’ conservation is undercut by unanswered questions. One in particular haunts Diamant.
“The main thing is: where do they go to mate?” she says. If we knew the answer, she says, “we can protect these areas and at least safeguard another generation of whale shark.”
Banner image: A Kenyan myth tells of god being so enamored with the creation of the whale shark that he dropped thousands of silver coins from heaven, which landed on the sharks’ backs and have remained there ever since. Image by Stella Diamant.
Citations:
• Brenier, A., & Vogel, A. (2017). Integrating conservation and development in madagascar’s marine protected areas. Fao Fish Aquacult Tech Pap, 603: 85−98.
• Cerchio, A., Andrianantenaina, B., Lindsay, A., Rekdahl, M., Andrianarivelo, N., & Easoloarija, T. (2015). Omura’s whales (balaenoptera omurai) off northwest madagascar: ecology, behaviour and conservation needs. Royal Society Open Science, 2: 150−301.
• Cerchio, S., Andrianarivelo, N., & Andrianantenaina., B. (2015b). Ecology and conservation status of Indian Ocean humpback dolphins (Sousa plumbea) in Madagascar. Adv Mar Biol, 72: 163−199.
• Diamant, S., Rohner, C., Kiszka, J., Guillemain d’echon, A., Guillemain d’echon, T., Sourisseau, E., & Pierce, SJ., (2018). Movements and habitat use of satellite-tagged whale sharks off western madagascar. Endangered Species Research, 36: 49−58.
• Everett, BI., Jiddawi, N., Wambiji, N., Boinali, K., Andriamaharo, T., Oodally, Z., & Chauca, I. (2015) WIOFish database:a catalogue of small scale fisheries of the western Indian Ocean: biennial report for 2014 and 2015. www. wiofish. org/ portal/ wiofishdb/ User Files/ Sys Docs/ bb _ content/ 10000/ 2/ WIO Fish % 20 Biennial % 20 Report % 202015. pdf (accessed 8 May 2018)
