Juvenile Male Whale Sharks Prefer to Return Home Each Year

A team of scientists analyzed photographs that were taken over the past several years in order to track the movement of whale sharks. Whale sharks were previously believed to travel great distances during migrations but the researchers found that most didn’t travel too far from home. The findings are in a paper that was just published in the journal Royal Society Open Science.

Whale sharks (Rhincodon typus) are the largest fish on the planet. They can weight up to 47,000 pounds and some individuals reach over 40 feet at maturity. Although huge, whale sharks don’t pose any threat to humans—they’re actually filter feeders, consuming over 45 pounds of plankton each day. Their size and docile personalities make them popular with ecotourists and divers. Many companies profit when they aggregate in areas such as Australia’s Ningaloo Reef. The animals have become endangered, however, and scientists still don’t know their migration patterns. Whale sharks are difficult to track and study, especially when they move away from reefs and back to the open ocean. Previously, it was believed that all whale sharks were part of the same population, migrating from site to site.

A research team collected photographs taken by scientists, tourism operators, and tourists as part of a citizen science project. Photographs could be used to identify individual animals since every whale shark has a unique set of markings. The team collected location information and used a computer program to help process all of the photos. This allowed the researchers to develop a map of the sharks’ movements over the past six years.

The researchers found that none of the tracked whale sharks travelled very far. The same sharks tended to return to tourism sites each year and none crossed the Indian Ocean, as previous studies had suggested. Interestingly, juvenile males were overrepresented in samples—scientists still aren’t sure where females and adult males go.

The team’s findings provide new insights into whale shark movements. The researchers found that the sharks returned to the same sites each year. Whale sharks were recently added to the endangered species list and the study may aid conservation efforts. The authors believe that a new tracking method needs to be developed in order to solve mysteries such as the location of females and adult males.

REFERENCE

Andrzejaczek et al. The ecological connectivity of whale shark aggregations in the Indian Ocean: a photo-identification approach. Royal Society Open Science (2016).

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