M.S. Marine Biology
Second Degree Name
M.S. Coastal Zone Management
The whale shark (Rhincodon typus) has a global distribution in warm to warm temperate oceans, and is a species of high conservation concern currently categorized as Endangered on the International Union for Conservation of Nature (IUCN) Red List. Despite its dire conservation status and concerns about the growing number of ecotourism interactions with this species worldwide, relatively little information is available on key aspects of whale shark biology such as growth rates, reproductive rates, survival rates and breeding habitats. In particular, critical information such as age and growth of whale sharks is needed to improve the management and conservation of this species. Robust knowledge of life history parameters is needed to improve demographic models for whale sharks and enable better evaluation of their vulnerability to fishing pressures and recovery from population declines.
Whale sharks are well known to form aggregations in specific locations, with one such site being the South Ari Atoll in the Maldives. My study aimed to expand knowledge of the population dynamics, including age and growth, of whale sharks at the South Ari Atoll by calculating growth parameters and rates from encounters with free-swimming sharks over a decade (April 2006 to May 2016). A total of 1545 encounters with 125 individual sharks were recorded during this time period. To obtain the most accurate information on the sizes of whale sharks, total lengths were estimated by three different measurement methods (visual, laser photogrammetry, and tape), and linear regression was utilized to investigate how these different methods compared to one another. The results showed that visual estimates tended to underestimate sizes of the larger sharks, and laser and tape measurements yielded similar results to one another (R2 = 0.824). New sharks observed at the South Ari Atoll during the study period were significantly smaller than returning sharks, suggesting that young sharks may be recruited to the South Ari Atoll, where they stay and grow until reaching maturity before leaving the area.
To the best of my knowledge, my study is the first to infer growth parameters and rates from measurements of free-swimming whale sharks. Estimates of von Bertalanffy growth parameters for combined sexes, calculated from 180 encounters with 44 individual sharks (Males (n=40), Females (n=4), TL=3.16 m – 8.00 m), yielded an L¥ of 19.56 and a k value of 0.021. Analyzing 177 encounters with 40 male sharks (TL=3.16 m - 8.00 m) exclusively provided an L¥ of 18.08 and a k value of 0.023. These values correspond to a male age at maturity of ~25 years and a longevity of ~140 years, exceeding those estimated for whale sharks captured off Taiwan based on analysis of biannual vertebral rings (male maturity =17 years; longevity (combined sexes) = 80.4 years). There have been few growth studies, mainly from vertebral analysis, that have produced wide ranges in L¥ (14 – 20.5) and k values (0.017 – 0.037). These differences underscore the need for additional regional studies to obtain population specific estimates of these key life history parameters.
Cameron T. Perry. 2017. Age and Growth of Whale Sharks (Rhincodon typus) near the South Ari Atoll, Maldives. Master's thesis. Nova Southeastern University. Retrieved from NSUWorks, . (437)
Available for download on Monday, April 15, 2019