Whale shark mapping: scientists uncover global distribution for the largest fish in the world
by Shreya Dasgupta
Polka-dotted and striped. Massive but docile. That’s the whale shark for you – the largest fish and shark in the world. But despite being major tourist attractions, the lives of these awe-inspiring creatures of the ocean remain far from being demystified.
However, a team of researchers from Australia may now have some answers to where these whale sharks (Rhinocodon typus) occur. They have, for the first time, predicted the current global distribution of these sharks across the Atlantic, Indian and Pacific oceans, and have also predicted where they could occur in the future.
“My interest for whale sharks started when I realized we knew so little about this species which is important both in fisheries (tuna purse seine) and tourism, providing high revenue in both activities,” says Ana Micaela Martins Sequeira, lead author of the study. Sequeira started studying whale sharks at the University of Adelaide, South Australia for her PhD in 2009.
Whale shark. Photo by Zac Wolf.
In the new study published in Global Change Biology, the team collected 4,336 records of shark sightings from fisheries records over several years – 31 years for the Atlantic Ocean, 17 years for the Indian Ocean and 11 years for western Pacific Ocean.
The team then put together environmental variables (such as distance to shore, mean depth, and sea surface temperature), which they believe affect the shark’s distribution. Using these variables and the shark sightings, they developed models to predict where the sharks could currently occur. They also used climate projections for 2070, when climate change is expected to raise water temperatures by an average 2°C, to see if climate change will affect their distributions in the future.
Their results show that currently areas with highest suitability occur in the Atlantic Ocean, followed by the Indian and western Pacific Oceans.
The populations of whale sharks might also be globally connected, the authors write. For example, their predictions show that the sharks might be able to cross over from the Indian Ocean to the Atlantic, south of South Africa. The models also indicate the presence of a ‘corridor’ of suitable habitat that links the eastern and western Pacific.
“This corridor suggests that the possible migration between whale shark sub-populations in the eastern and western Pacific might still be possible in a near future when temperatures are predicted to be higher,” says Sequeira.
Among the environmental variables, sea surface temperature appears to be an important predictor of whale occurrence in the Atlantic and Indian Oceans, while depth is important in the Pacific and Atlantic oceans, write the authors.
Their predictions for 2070 show, that climate change could have two important effects on the whale shark distribution. First, there could be a weak pole-ward shift of their suitable habitat, and second, these habitats could shrink and may even be lost in some areas.
Corey Bradshaw, Professor and Director of Ecological Modelling at the University of Adelaide says, “We wouldn’t put too much emphasis on how much range will shrink by 2070, because it depends not only on the accuracy of the global circulation model projections, but also how much whale sharks will be able to tolerate outside of their normal physiological limits. We also don’t know much about how food supply will be affected, so it’s best just to say that an unknown shrinkage will arise.”
But the researchers do point out that their dataset could have some limitations.
“We can only obtain data from where the (tuna) fishing boats go, so it’s not a complete picture,” says Bradshaw. “Because there are no pelagic surveys specifically designed for whale sharks, and because tracking technology hasn’t been that successful, we had to rely on fisheries logbook data.”
“But the strong point is the large coverage compared with other datasets, which are available mostly at the scale of whale shark aggregation areas, for example in Ningaloo, Western Australia,” adds Sequeira.
Whale sharks are currently listed as Vulnerable by the IUCN Red List. Some threats to the sharks include collisions with ships, illegal fishing and hunting, capture in fishing nets as bycatch, poorly-regulated tourism, as well as climate change. But with whale shark populations likely globally connected, and influenced strongly by climatic conditions, it would be wise to revise current whale shark management policies using a precautionary approach, suggest the authors.
“We need to police illegal harvest a lot better (it’s still happening in places like Indonesia, and probably elsewhere), and we certainly can’t isolate populations from the rest of the larger metapopulation by focusing only on certain regions,” says Bradshaw.
Reference: Sequeira AM, Mellin C, Fordham DA, Meekan MG, Bradshaw CJ (2013) Predicting current and future global distributions of whale sharks. Global Change Biology. doi: 10.1111/gcb.12343