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Sharkbites Saturday

It’s a whale, it’s a shark… No, it’s a whale shark!!

Whale sharks are pelagic gentle giants that attract many tourists to their aggregation sites, however, there is not much information on their mobile nature. Read on to learn about their aggregations in the Philippines.

Article: Araujo, G., Snow, S., So, C. L., Labaja, J., Murray, R., Colucci, A., & Ponzo, A. (2017). Population structure, residency patterns and movements of whale sharks in Southern Leyte, Philippines: results from dedicated photo‐ID and citizen science. Aquatic Conservation: Marine and Freshwater Ecosystems27(1), 237-252.

Background

Out in the open ocean, within tropical latitudes and warm temperate waters lives the world’s largest fish, Rhincodon typus, or as we commonly know it, the whale shark (Fig. 1). This allusive species was solely known as a filter feeder, feeding on surface zooplankton. Recently, scientists discovered, through fatty acid analysis, that they also feed on demersal (close to the bottom) macro plankton and small fish. Whale sharks can measure between 18-33ft, weigh up to 20.6 tons, and on average live up to 70 years which is only a couple years below our own lifespan.

Fig. 1 Whale shark swimming with his mouth open to filter feed. Photo Credit: Brian J. Skerry, National Geographic

Recent information

These highly mobile giants were thought to aggregate seasonally for feeding events but the latest acoustic telemetry studies, demonstrated strong site fidelity and long residency periods. In other words, they like to visit the same sites and stay for longer.

Because of their charismatic appeal, whale sharks are of great value to tourism (Personally, as a diver, it is on my bucket list to see one on a dive). Coastal communities such as those in the Philippines archipelago, benefit economically making the whale shark’s movement patterns of paramount interest. Only three major aggregations of whale sharks have been described in this archipelago, at Donsol, Cebu, and Panaon Island, Southern Leyte. The last one was first described in this study.

Guidelines for diver distance from whale shark. Resource: James Morgan, WWF

The Study

The researchers used photographic identification, taken by scuba divers, to describe the whale shark aggregations between 2013 and 2014. Dedicated visual surveys were done in 2013 and 2014 aboard tourist boats and motorized outriggers boats to monitor the population. Lastly, citizen science data (pictures and documentations tourists took during boat tours) was collected to estimate residency and explain their mobility.

 

Results

93 individuals, between 2013 and 2014, were identified and measured; 54 were males, 13 females, and 26 undetermined (Fig. 2 and Fig. 3). Overall, the average estimated total length was 5.72 ± 1.02m which indicated that individuals in the aggregation were mainly juveniles. The visual surveys estimated that 27% of the individuals suffered from partial or complete fin amputations resulting from fishing lines, boat propellers, and/or net entanglement. Another 45% showed multiple parallel scars caused by propeller impact.

2013 yielded a greater number of whale shark encounters when compared to 2014 (366 and 12). However, the data supports that individuals return over time and residency estimates from the citizen science data suggests that whale sharks stay an approximate 27.04 days in Panaon Island.

Fig. 2 Size and sex distribution of identified whale sharks at the study sites. Females= dark gray, males= black, and undetermined= light gray. Resource: Araujo et al, 2016

Number of whale sharks encountered at the study site between February and June for 2013 and 2014. Resource: Araujo et al, 2016

 

Importance

This study revealed the mobile patterns and behavior of whale sharks in the Philippines archipelago. Individuals from Panaon Island were spotted and Id in Taiwan, which is 1600 km away! The researchers concluded that given the distance travelled by these giants, management should be carried out as a single unit regionally in Southeast Asia. But, additional research is needed to single out and focus on the whale shark encounter variation at aggregation sites. Because whale sharks are listed as vulnerable species under the IUCN (International Union for Conservation of Nature) red list, this research contributes by informing management where they should concentrate their efforts and how to mitigate whale shark and boat encounters.

 

Happy Sharkbites Saturday!!

Additional Resources:

https://www.nationalgeographic.com/animals/fish/w/whale-shark/

https://www.worldwildlife.org/species/whale-shark

https://www.worldwildlife.org/magazine/issues/winter-2017/articles/all-swim

 

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