A short post today to tell you about the long residency of an individual whale shark at the Ningaloo Reef feeding aggregation in Western Australia. These huge animals require an enormous amount of their planktonic food, and so they travel the seas between sites where rich food sources are transiently found. One of these sites is at Ningaloo Reef off the coast of Western Australia, where whale sharks come each April through July to feed on coral spawn. Since 1995, whale sharks visiting Ningaloo have been individually documented by photo identification of their spot patterns. These patterns are stored in the Wildbook for Whale Sharks (http://www.whaleshark.org/), which now holds more than 7000 individual animal profiles.
Whale sharks are believed to live as long as 80-100 years, growing to as much as 18 meters over their lifetime. Despite this large size range, most feeding aggregations are populated by sharks of roughly similar size, and it is hypothesized that animals may move from one aggregation circuit to another as they age. To determine long term movements of a highly migratory animal is difficult however, as most tagging methods have durations of a few months to a few years.
Photo-identification, however, is believed to last for the life of an animal, with spots enlarging as the animal grows, but their pattern remaining consistent. Norman & Morgan now report on the recurrence of one individual whale shark at Ningaloo Reef for at least 20 seasons. “Stumpy”, so named for his deformed caudal fin, was the first shark photographed at Ningaloo in 1995, and was most recently seen in 2016. He has been photographed at Ningaloo in 19 of these 22 seasons. Based on his size in 1995, Stumpy was believed to have been about 15 years old, making him now 36. Since 2001 he has displayed the elongated clasper morphology of mature male sharks. Another whale shark, Zorro, has also been photographed at Ningaloo Reef over this period of time
Interestingly, Stumpy and Zorro do not appear to have grown measurably over the past 20 years. Both animals were measured at roughly 7.5m in 1995, and are the same size now. Whale shark growth is poorly understood. It was believed to slow dramatically in adult animals, but 10-12 m sharks are not infrequently seen. As whale sharks can clearly grow larger than 7.5 m, the near zero growth rate of Stumpy and Zorro was unexpected.
These results show that whale sharks can maintain a presence at repeated feeding sites over decades-long periods of time. They also raise interesting questions about what was believed to be a low but regular growth rate for adult whale sharks. Why have Stumpy and Zorro not grown over the last 20 years? Will their growth begin again at a later point in their life? What does this mean for our size-based estimations of whale shark age? Will Stumpy and Zorro leave Ningaloo when they do grow larger, perhaps joining an aggregation with overall larger animals?
The manuscript is: Norman, BM & Morgan, DL. The return of “Stumpy” the whale shark: two decades and counting, Front Ecol Environ 2016; 14(8):449–450, doi:10.1002/fee.1418.