The largest living fish on Earth roams less than previously thought, a new study finds. Whale sharks are solitary creatures which have been previously difficult to monitor due to high individual mobility. However, research in the Indo-Pacific region, carried out by Dr Clare Prebble, Whale Shark Project researcher for the Marine Megafauna Foundation (MMF) has discovered that whale shark aggregations segregated by life stage and gender, experience minimal interactions between one another and limited movement across specific aggregation sites.
Prebble found that despite their high mobility many whale sharks, particularly juvenile males, are predominantly localised in specific coastal areas, as well as around islands in the tropics and subtropics. They have been observed targeting predictable feeding opportunities such as fish spawning events, but have shown low levels of movement between three major feeding sites in the Western Indian Ocean (Mozambique and Tanzania) and the Arabian Gulf (Qatar), where groups rarely swam more than a few hundred kilometres north or south from their established individual areas.
The whale sharks were monitored by the MMF using various innovative methods of data collection, from ‘biological passports’ to photo identification. With photo IDs, the MMF used the natural markings on each animal to identify individuals. Each shark has a unique spot pattern, much like a human fingerprint, which is thought to remain unchanged throughout their life. A shark’s pattern is photographed and run through software on the Wild Book for Whale Sharks platform (www.whaleshark.org), a system that has been adapted from NASA software used to identify star constellations. This database compares each photograph to all the identified sharks in the world and informs the MMF which shark is which.
However, the MMF’s newest data collection technique is the biochemical analysis of tissue biopsies, known as ‘stable isotope analysis’. This involves the production of ‘isoscapes’ – the mapping of isotopic gradients over a range of geographic scales. Isotopes vary spatially within ecosystems, with heavier variants found more in near-shore environments than offshore, and more at higher latitudes in the poles than at the equator. These isotopes are passed up through the food web to the tissues of consumers (such as the whale sharks).
‘Every time a whale shark eats something it is accumulating evidence of where and what it was eating, like putting a stamp in a passport,’ explains Dr Prebble. ‘When we look at the isotopic composition of their tissues, it’s like we’re leafing through their passport looking at what they’ve been doing the last few weeks or months.’
The whale shark is considered an endangered species by the International Union for the Conservation of Nature (IUCN) Red List. The Indo-Pacific whale shark population is thought to have shrunk by 63 per cent over the past 75 years, with more than half of the world’s population having been killed since the 1980s alone. Today, the world’s largest feeding areas only host a few hundred sharks at most.
As they are long-lived, slow-growing, and late to mature, whale sharks are particularly vulnerable to human threats. The WWF claims that ‘demand for their meat, fins and oil remains a threat to the species, particularly by unregulated fisheries. They are victims of by-catch, the accidental capture of non-target species in fishing gear. Whale shark tourism [worth over $100million each year worldwide] presents a threat to the species as it can interrupt their feeding and sharks can be injured by boat propellers.’
The incidental catching of whale sharks in gillnets results in an estimated 50 to 60 per cent of accidentally captured sharks being released alive and five to ten per cent dying. The fate of the remaining 30 to 40 per cent captured unfortunately remains unknown due to inaccurate reporting. The IUCN claims that ‘restrictions on mesh size, net length and fishing locations can help to avoid whale shark catch. Training in safe release, or provision of by-catch reduction technologies (deliberate weak points in nets or other ways to avoid entanglement) could help to reduce the likelihood of shark injury or mortality when interactions occur.’
With the MMF now understanding that many whale sharks are highly localised, with their worlds much more intertwined with our own than previously thought, it is urging conservation management techniques be established. ‘Our results show that we need to treat each site separately, and ensure good conservation management is in place, as the sharks may not re-populate if they’re impacted by people’s activities,’ Prebble claims. As a range of threats including potential future concerns such as climate change and increased marine pollution events occur, a lack of conservation could potentially lead to higher mortality, displacement from preferred habitats and the rising concern of extinction.
Get the best of Geographical delivered straight to your inbox by signing up to our weekly newsletter and get a free collection of eBooks!