Marine researchers: Mafia island is a favoured home for whale sharks

26Jan 2017
The Guardian
Marine researchers: Mafia island is a favoured home for whale sharks

IN some places, whale sharks routinely swim between countries, even on a daily basis. But on Tanzania’s Mafia Island just off the Indian Ocean coast, the world’s largest fish are such regulars that scientists started to name them.

“There aren't too many places where you repeatedly see the same whale sharks,” said Simon Pierce, principal scientist and researcher for the Marine Megafauna Foundation. “These days, it's almost like visiting old friends.”

Yet the gentle giant sharks are dangerously close to extinction.
In July last year, Pierce advised the International Union for the Conservation of Nature (IUCN) to reclassify them as an endangered species on the group’s Red List.

There is no good estimation of the number of whale sharks internationally, but according to the global online photo-identification database of sightings, there are at least 8,000 identified whale sharks roaming waters worldwide.

Because whale sharks can be found year-round in Mafia, the island offers a rare opportunity for scientists to study them. Scientists can analyse their diet, movements, and growth, with one goal being to better develop strategies to protect them.

“The fact that Mafia whale sharks are so resident shows that both international and local legislation are needed to effectively protect them,” Pierce said.

COUNTING WHALE SHARKS

In the same way people have fingerprints, whale sharks have distinctive patterns and scarring. Researchers use these markings—located on the left side between the pectoral fin and the gills—to identify each individual fish.

In 2007, marine biologist Matthew Potenski from the Shark Research Institute began photo-identifying whale sharks off Mafia island. When Pierce and his colleagues did their own research in November and December last year, they were amazed that the same sharks documented back then were still swimming around the island.

“Once we established that these sharks were living here, not just visiting, it became clear that Mafia is an amazing site to learn more about whale shark ecology,” said Pierce.

His team’s research took place during the five-month stretch from October to February when it is easiest to see the sharks.

Thanks to their eating behaviours, the sharks move inland to forage on shrimp at the surface. Come February, when the sharks begin to retreat, electronic tagging shows they remain close by and swim deeper in the water.

“Presumably they're switching to another source of prey, but we haven't got that figured out yet,” Pierce said.

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