A study conducted by the Madagascar Whale Shark Project has revealed that juvenile whale sharks swim to Madagascar, particularly the island of Nosy Be, to feed. This is the first major scientific survey of it’s kind in the area and shows it is a newly-identified hotspot for these fish.
The whale shark (Rhincodon typus) is the world’s largest fish and is classed as globally endangered on the RIUCN RedList.
The study, published in Endangered Species Research, identified 85 individual sharks (all juvenile – ranging from 3.5 to 8 m in total length) in a single season using photographs of their distinctive spot patterns. In addition to visual identification of the sharks, the team attached eight satellite tags to some of the sharks to track their movements in near real-time.
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.”
“No one thought there were that many sharks, they don’t seem to be there all year round – they come back for the food.”
The study showed that the sharks were travelling an average 21km per day and spent most of their time in shallow waters between 27.5-30 degrees Celsius around the tagging area in Nosy Be.
The team uploaded photographs of the sharks’ markings to a global database of sightings known as the Wildbook for Whale Shark. They found no overlap with data collected from other feeding areas in the Indian Ocean, suggesting the juvenile whale sharks had not migrated from Mozambique or other neighbouring areas.
“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.
Satellite tags attached to the whale sharks to track their movements revealed that 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 2 individuals swam down to the southern end of Madagascar. One of those sharks then swam back to Nosy Be, a total track of 4,275 km.
“It was exciting to see that there is a second hotspot for the sharks in the area,” said Diamant. “Madagascar clearly provides an important seasonal habitat for these young whale sharks, so we need to ensure they are effectively protected in the country.”
Over the last decade, populations have declined dramatically globally, due to overfishing, accidental catches and boat strikes. The vast majority of the fish are found in the Indo-Pacific, where numbers have declined by more than 50% over the last 75 years.
“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.
Diamant S, Rohner CA, Kiszka JJ, 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. Endang Species Res 36:49-58. https://doi.org/10.3354/esr00889