The whale shark (Rhincodon typus) is the world’s largest fish, growing up to 65 feet and weighing around 75,000 pounds! Despite this huge size, they prey on tiny organisms. Whale sharks are known as filter feeders; they feed by ingesting huge amounts of water, and filtering out plankton, small crustaceans and fish, which get trapped in their pharynx.
Unfortunately, the whale shark is listed as vulnerable on the IUCN Red List. They are hunted as part of the shark fin trade and in many areas of the world populations have declined dramatically over the last century. Similarly to other Elasmobranchs, the whale shark is slow to grow and reproduce- meaning populations do not recover from over-exploitation easily.
Despite its high conservation status and a recognised need for protection, relatively little is know about the whale shark. We do know that whale sharks may live at least 100-150 years, however, where they go during their long life remains a mystery.
Recently, tagging and genetic studies have enabled scientists to track the movement of these beautiful creatures, with some interesting results. For example, three main populations of whale shark have been identified: the Pacific, Caribbean and Indian Ocean. A recent study found that there are high rates of gene flow between these locations- meaning whale sharks are travelling long distances and interbreed with animals from distant populations.
This finding has been supported by numerous tagging studies; recently, a whale-shark named Anne was tagged and observed to migrate a whopping 12,000 miles- the longest ever recorded trans-Pacific migration!
These studies have also revealed ‘whale shark hotspots’. For example, a recent study identified The Nosy Be area, an island off the northwest coast of Madagascar, as a globally significant whale shark hotspot. A whopping 85 individual sharks were identified during the 3 month whale shark season back in 2016. This is a huge amount, given that just 33 individual whale sharks were recorded in Mozambique throughout the whole of 2016 and on 70 in Tanzania!
This region has been a whale shark tourism hotspot since 2011, and plays a large role in supporting the local economy. However, this vulnerable species is not currently protected in Madagascar. Hence, studies such as these have important conservation implications; protecting these regions could help to mitigate the impacts on the sharks within these areas.
The team used a tagging system called smart position or temperature transmitting tags (SPOT5) to track the sharks’ movements. When sharks are at the surface, the tag sends signals to a satellite, which are then relayed to ground systems, where they are used to calculate the location of the tag.
It is technologies like this that are enabling the quantification of elasmobranch populations across the globe, particularly in remote areas that are difficult to access, such as the open ocean. Elasmobranchs are notoriously free-ranging and can migrate long distances; science is helping to unravel the mystery of where they go.
C. J. A. Bradshaw et al., 2008. Decline in whale shark size and abundance at Ningaloo Reed over the past decade: The world’s largest fish is getting smaller. Biological conservation. 141: 1894- 1905.
J. V. Schmidt et al., 2009. Low Genetic Differentiation across Three Major Ocean Populations of the Whale Shark, Rhincodon typus. PLoS ONE. 4: e4988.
S. Diamant et al., 2018. Movements and habitat use of satellite-tagged whale sharks off western Madagascar. Endangered Species Research. 36: 49- 58.