Good news for whale sharks
Juvenile males at least seem to stick close to home, rather than wandering far and wide to fall foul of the fishing industry or ship-strikes.
That’s the conclusion of a team of researchers who have been processing the rapidly expanding photo-identification database of Indian Ocean whale sharks.
The new study centred on Ningaloo Reef in Western Australia, where young male whale sharks turn up every March to be joined by large numbers of snorkellers. Where they go after July was previously unknown.
The sharks tend to aggregate where food is seasonally abundant in the Indian Ocean – at Ningaloo, the Maldives, Mozambique and the Seychelles – but what was previously unclear was whether it was the same population that migrated between these sites.
The new research, published in Royal Society Open Science, suggests that the sharks don’t go very far at all, and return to the same sites year after year.
Whale sharks have distinctive spot patterns, and by running a database of more than 6000 images through a semi-automated matching program the team of seven scientists, led by Australia-based Samantha Andrzejaczek and Mark Meekan, identified about 1000 individuals. Of these, 35% were seen at the same Indian Ocean site in more than one year, and none were found to move across the ocean.
One shark was tracked between Mozambique and the Seychelles, but generally the populations appeared to be isolated, with juvenile males returning regularly to the same sites. Juveniles photographed at Ningaloo in 1992 were seen up to 19 years later, with many sightings in between and some returning in up to six consecutive years.
Females and adult males were rarely spotted at these sites, leading to speculation that they don’t necessarily behave in the same way as juveniles. It can take whale sharks up to 30 years to reach maturity.
With the IUCN Red List status recently upgraded to Endangered, reliable information about whale-shark movements is regarded as vital to efforts to protect them. The study concludes that staying close to home is good news for the sharks, because it allows conservation and management to be concentrated on smaller, single-jurisdiction areas.
The scientists now want to increase the number of study sites and photographs taken to boost their knowledge of migration patterns further.
The report, The Ecological Connectivity of Whale Shark Aggregations in the Indian Ocean: a Photo-Identification Approach, can be read here
