Tracking a whale shark to learn from her itinerary

Rio Lady was first tagged in 2006 and has been observed and monitored for nearly two decades in one of the longest continuous tracking operations of a wild creature ever attempted

Thu, 08 Aug, 2024 - 12:06

Richard Collins

The basking shark, an annual visitor to our coasts, is a silver medal winner — it’s the second largest fish in the world and the biggest in the Atlantic. The gold award goes to the whale-shark of southern oceans. Whereas an adult basking shark may be 8 metres long, a whale-shark reaches 12 metres. In 1868, Edward Perceval Wright, Professor of Botany at Trinity College Dublin, saw a 15 metre-long specimen in the Seychelles.

The two giant shark species, although not closely related, have similar lifestyles. Giant 'vacuum cleaners', they swim near the surface with mouths wide open, drawing water through huge sieves to filter out small fish and plankton. ‘Keep moving’ is the motto of these lumbering juggernauts — they need a steady flow of water through their gill rakers to get enough to eat. But ocean temperatures are rising; keeping abreast of changes in plankton distribution is challenging. And there’s a new problem; poo samples, analysed three years ago, contained micro-plastics.

The whale-shark, dark-coloured with white spots, resembles a giant starling. Numbers are declining. The species is now classified as ‘Endangered’ by the International Union for the Conservation of Nature. The basking shark is faring a bit better but is deemed to be ‘Vulnerable’.

Despite their great size and relatively trusting natures, the private lives of these elephants of the ocean remain shrouded in mystery. Whales dive to great depths... a descent of 1,928m was the deepest recorded for any fish species. Why do sharks do this?

Mating of whale-sharks was observed for the first time, off St Helena, in 2016. Little is known about their social, and breeding, protocols. Females retain sperm to produce pups intermittently; the youngsters remain on board until birth. We still don’t know where the mothers go to give birth.

New technology, hopefully, will help answer some whale-shark questions. In a paper just published, scientists from the Universities of Rhode Island and Nova Southeastern shed new light on shark movements in the West Indies.

The whale shark Rio Lady. Picture: University of Rhode Island

‘Rio Lady’ is an individual shark so well known to researchers that she has been given a nickname. Tagged in 2006 in an attempt to track her movements, Lady surprised researchers by venturing out to mid-Atlantic east of Brazil. In 2018, a state-of-the-art monitoring device, known as a SPOT (Smart Position and Temperature Transmitter), was fitted to her dorsal fin. Whenever the fin emerged from the water, the SPOT produced a short data transmission which was picked up by satellites. The data enabled her location to be determined to within 250 metres. The device continued to operate successfully until 2023, in one of the longest continuous tracking operations of a wild creature ever attempted.

Lady travelled more than 43,000km in 1,700 days. She visited the Gulf of Mexico, the Caribbean and the open Atlantic off Bermuda, surfacing at more than 1,354 locations during her journey. There were "annually consistent patterns of movement between three regions". Knowing these shark itineraries, and where the fish are likely to be at particular times of the year, will help conservationists to focus protection measures more effectively.