85 juvenile sharks visited various spots of the island nation’s coast in a single season, says study
Lead author Stella Diamant with a tagged shark Credit: Simon J Pierce, www simonjpierce.com
A new study published in the journal Endangered Species Research has revealed that juvenile whale sharks swim to Madagascar, a newly-identified hotspot for these huge fish, to feed. Eighty-five individual sharks were identified in a single season using photographs of their distinctive spot patterns.
An isolated “island continent”, famed for animals and plants that exist nowhere else in the world, Madagascar’s nutrient-rich waters are also home to an incredible array of marine life attracting increasing numbers of tourists.
Whale sharks are primarily seen around the small island of Nosy Be, in northwest Madagascar. This area is a globally important hotspot for large marine species, including manta rays, sea turtles, humpback whales and even rare Omura’s whales.
The study, "Movements and habitat use of satellite-tagged whale sharks off western Madagascar" is part of the Madagascar Whale Shark Project, a collaboration initiated in 2016 by researchers from the Marine Megafauna Foundation, Florida International University, and Mada Megafauna.
Lead author and project leader Stella Diamant said: “We’ve found that whale sharks regularly visit Nosy Be between September and December. That has led to a growing ecotourism industry, as people travel to see and swim with these gigantic, harmless sharks. We’re still learning about their population structure and movement patterns, but it’s clear the area is an important hotspot for the species.”
Whale sharks are the largest fish in the world, growing up to 20 metres long. However, all of the sharks seen in Madagascar have been juveniles of less than nine metres.
“We identified 85 individual whale sharks over our first season in 2016. Some of the sharks were present across several months. They spend a lot of time in the area and seem to come here to feed,” Diamant said.
The marine biologists uploaded photographs of the sharks’ unique spot patterns to Wildbook for Whale Sharks (a global database of sightings) and compared them with data collected from known feeding areas in the Indian Ocean, including Djibouti, the Maldives, Mozambique, Seychelles and Tanzania, but found no overlap.
“Whale sharks are a globally endangered species due to overfishing, accidental catches and boat strikes. Major declines in sightings have been seen in Mozambique, where we’ve documented a 79 per cent decline in sightings since 2005, and the Seychelles. I was hoping that some of those sharks might have shifted over to Madagascar”, said co-author Simon Pierce, co-founder and principal scientist at the Marine Megafauna Foundation.
“Unfortunately, that doesn’t seem to be the case. It’s great news for Madagascar though. These sharks can be a major asset for the country. There’s already a good marine ecotourism industry developing”, he added.
As part of this study, the team attached eight satellite tags to immature whale sharks to track their movements in near real-time. They found that the sharks spent most of their time in shallow waters between 27.5-30 degrees Celsius around the tagging area in Nosy Be.
Half of the tagged sharks also visited a second hotspot near Pointe d’Analalava, 180 km south of Nosy Be. Five of the sharks swam over to Mayotte and the Comoros islands, and two swam right down to the southern end of Madagascar. One of those sharks then swam back to Nosy Be, a total track of 4,275 km.
The sharks are slow-swimmers, travelling an average 21 km per day. Three sharks were re-sighted in the Nosy Be area the following season after having lost their tags.
“It was exciting to see that there is a second hotspot for the sharks in the area. We will be exploring the area later this year. Madagascar clearly provides an important seasonal habitat for these young whale sharks, so we need to ensure they are effectively protected in the country”, concluded Diamant.
Madagascar is a known location for shark fishing and finning. Whale sharks are currently afforded no formal protection except in two Marine Protected Areas located to the southwest and northeast of Nosy Be.
“Over the last decade, shark populations have declined dramatically in Madagascar due to overfishing. However, the most significant threat to this species is the incidental catch in coastal gillnets and industrial purse seiners operating offshore”, said Jeremy J Kiszka, a marine biologist at Florida International University and co-scientific lead of the Madagascar Whale Shark Project.
Whale sharks are listed as “endangered” on the IUCN Red List of Threatened Species since 2016 and received an Appendix I listing the UN Convention on Migratory Species in 2017. As a signatory to the Convention, Madagascar is obligated to protect the sharks and their migratory habitat in national waters.
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