Stars are starting to pop out in the Arabian night skies as the sun dips behind the horizon, tinging everything an inky blue colour. Soon the stars will be mirrored on the Arabian sea, but not because the ocean is mirror-like… these stars are moving. In fact, these stars adorn a beautiful whale shark (Rhincodon typus), the largest fish that roams our oceans. Found in shallow and open waters throughout the world’s tropical and subtropical environments, they are known to have predictable coastal occurrences of these sharks such as off Australia and Mexico. Traveling thousands of miles to get there, they are often seen on the surface but have recently been discovered to also dive down to depths of 984-1,640 feet (300-500 meters).

But why would the stars need to dive down into the twilight zone? Thanks to advances in satellite telemetry techniques, scientists have expanded our knowledge of the migratory movements of these species and allows us a sneak peek at what drives these complex movements. A team of scientists, led by Lucy Arrowsmith of the Oceans Graduate School at the University of Western Australia, have described their horizontal patterns of movement to identify key drivers of this species’ movement patterns in the northern Arabian Sea along the coast of Gujarat, India.

“Before 2001, whale sharks in the northern Arabian Sea and along the Indian state of Gujarat were subject to targeted fisheries that hunted these animals for their fins, skin and meat. However, these fisheries largely ceased operations after May 2001 when the whale shark was included in the Schedule I of the Indian Wildlife (Protection) Act, 1972. In 2004, the Wildlife Trust of India (WTI) launched the ‘Whale Shark Conservation Project’ to spread awareness of the species and its protective status, to aid in changing the mindset of fishers towards whale sharks and reduce this species being killed in fishing nets. This initiative was very successful, with fishers also being reimbursed for their damaged nets if whale sharks were accidentally caught and released,” explained Arrowsmith about how this particular project came about. “In 2011, scientists working with the WTI and from the Australia Institute of Marine Science (AIMS) began tagging whale sharks that were accidentally caught in these nets before releasing them. This was the first-time whale sharks in this region were tagged, and a huge step towards understanding their movements and aiding conservation!”

For Arrowsmith, this research sees the fruition of a life-long passion. “I’ve always had a fascination with the ocean and everything to do with it! Growing up in the UK and away from the beach, meant I got my first shark experience at The Deep Aquarium, but I’ve always been captivated by [them]. It wasn’t until […] I got the chance to undertake a placement in The Bahamas at the Cape Eleuthera Institute, that this dream of working with sharks and stingrays became a reality,” she says. “After this incredible experience, I knew I wanted to work with sharks! They are by far my favourite shark species, and every time I’m in the water with one it blows my mind (I may also have a little cry).”

It boggles the mind of the PhD candidate that there is still so much to discover about whale sharks, and that lack of knowledge combined with their slow growth and maturity rates makes their populations highly vulnerable to anthropogenic pressures (such as ship strikes, bycatch and targeted fishing, and pollution). These threats are of particular concern as the species is classified as “Endangered” on the IUCN Red List. Ultimately, this has motivated satellite tagging programs to identify their movement patterns and assist in identifying areas of potential threats to the species in order to target effective conservation planning. The team tagged eight sharks that were accidentally caught in purse seine fishing nets in the northern Arabian Sea from 2011 to 2017 using Smart Positioning Tags (SPOT tags). For studies of shark movements, SPOT tags are a highly versatile and cost-effective satellite transmitting tag that is commonly attached to the dorsal fin. With whale sharks reaching lengths of 40 feet (12.1 meters) or more, according to the CITES Identification Manual the ‘first dorsal and tail (caudal) fins reach to over 4.9 ft (1.5 m) in height in mature adults.’ Plenty of room to attach a tag there!

“It was great to have seven of the eight tags successfully transmit locations! We extracted daily sea surface temperature (SST), salinity, surface elevation and water velocity data from [the data from the tags]. The daily SSTs were averaged to produce SST plots overlaid with each whale shark’s location, [which] show extensive ranges of water temperature, and by visually examining these plots we can identify links between movements and features of physical (such as fronts) or biological (such as phytoplankton) oceanography,” says Arrowsmith. “We began analyzing the longest tracks first, one of which went for 137 days, traveling a distance of 1,386 miles (2,230 kilometers). We found that this shark (a 22.3 ft/6.8 m female) stayed in an SST range of 71.6 to 84.2° F (22 to 29° C; a range that is already known to be favourable for this species) and followed frontal systems between water masses and cold-water eddies. With the other tracks, we found that the sharks also remained within a very narrow temperature range, and when the SSTs reached temperatures of 95° F (35° C), such as in 2015 when the strongest El Nino Southern Oscillation (ENSO) phenomenon for the past four decades occurred, the sharks remained close to the coast where cooler water temperatures were observed. This was fantastic to observe, as it gave further evidence for the habitat preferences of this species and highlighted how, potentially, whale sharks may avoid very warm surface waters which may be necessary to maintain optimal metabolic rates.”

The team are ecstatic to have contributed to baseline information for these sharks in this area regarding their movements. “Sharks in the Indian Ocean are known to be under considerable threats from anthropogenic impacts and based on AIS fishing data, there is intense fishing activity off the coast of Maharashtra, where our sharks were tagged,” lamented the researcher. “Therefore, it seems likely that there is an overlap between fishing vessels and the whale sharks in this region. Highlighting these threats and demonstrating the complexity of whale sharks movements is vital for the conservation of this species, something we hope our study can assist in doing!”

Arrowsmith hopes to continue studying these charismatic animals, underscoring the need for management and conservation strategies for these animals on a global scale. “I feel incredibly lucky to be able to work so closely with fantastic scientists investigating these behaviours and movements,” Arrowsmith said, looking out towards the horizon. There, just as the last of the sun’s rays are extinguished, does a constellation-studded dorsal fin appear.