Whale sharks in Madagascar
Whale sharks are living in all the warm oceans of our planet. Population repartition and behavior still need to be understood, as well as their movements. Example of a new feeding area for juvenile whale sharks in Madagascar was described and their movements monitored using Argos PTTs.
Whale sharks (Rhincodon typus) have been classified as “endangered”, with a decline of more than fifty percent of the population globally. As around Philippines (see Whale sharks: big friendly giants of the ocean), the question in the Western Indian Ocean is whether the population is decreasing, or only moving elsewhere.
Individual sharks normally swim around 10, 000 to 15, 000 km each year, so satellite telemetry technologies such as Argos are the only practical way to know where they are spending most of their time, where they go, etc. Argos tags are tethered using a ~1.5 m line to be out of water as often as possible, since the signal does not transmit through water (only sound would, and then it won’t go through the atmosphere and up to the satellite).
The small island of Nosy Be, in northwest Madagascar is a globally important hotspot for large marine species, including manta rays, sea turtles, humpback whales and even rare Omura’s whales. Whale sharks are routinely sighted off this island. Eighty-five individual sharks, all juveniles, were identified in a single season using photographs of their distinctive spot patterns, and 261 in three years in this area. Some of the sharks were present across several months.
Eight sharks were tagged in October 2016. They spent most of their time in shallow waters between 27.5-30°C around the tagging area in Nosy Be and seem to come here to feed. 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. Three were resighted in Nosy Be in 2017 (having lost their tag).
Dedicated whale shark tourism has been developing in the area since 2011, so a code of conduct and advices on how to behave for operators proposing with shark seeing is important to ensure sustainable and safe interactions for both whale sharks and humans.
- Photo: whale shark with Argos tag (Simon J. Pierce)
- Stella Diamant, Christoph A. Rohner, Jeremy J. Kiszka, Arthur Guillemain d’Echon, Tanguy Guillemain d’Echon, Elina Sourisseau, Simon J. Pierce: Movements and habitat use of satellite-tagged whale sharks off western Madagascar, Endang Species Res, Vol. 36: 49–58, 2018, https://doi.org/10.3354/esr00889
- The study 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. www.marinemegafauna.org & http://www.simonjpierce.com
- Madagascar Whale Shark Project https://www.madagascarwhalesharks.org/