Age and growth of whale sharks (rhincodon typus) near the south ari atoll, Maldives

Abstract

Despite the growing number of interactions with whale sharks through ecotourism, little or no information is available on important aspects of whale shark biology, such as growth rates, reproductive rates, survival rates and breeding habitats (Holmberg et al., 2009). Critical information, such as age and growth of whale sharks, is needed to improve the management and conservation of the species (Hsu et al., 2014; Rohner et al., 2015). Accurate measurement of life-history parameters can improve demographic models for whale sharks and enable better evaluation of their vulnerability to fishing pressures and recovery from population declines (Rohner et al., 2015). For the Maldives, knowledge on the ages and sizes of whale sharks may also provide crucial information into migration routes and potential links between other Indian Ocean aggregation sites. This study aimed to expand on the knowledge of age and growth of whale sharks in the Maldives by calculating growth parameters and rates from encounters during the time period of April 2006 to May 2016. A total of 1545 encounters with 125 individual sharks were recorded during this time period. Total length estimates were taken via three different measurement methods (visual, tape, and laser) and linear regression was utilized to investigate how the different methods were related to one another. This study showed that visual estimates tended to underestimate large sharks and tape and laser measurements yielded similar results to one another (R2 = 0.824). New sharks to the South Ari Atoll were significantly smaller than returning sharks. This provides evidence that small sharks may be recruited to the South Ari Atoll, where they stay and grow until they reach maturity and then they leave the area. This study was the first of its kind to produce growth parameters and rates from measurements of free-swimming whale sharks. Growth parameters for combined sex, calculated from 180 encounters with 44 individual sharks (3.16m – 8.00m), yielded an L of 19.556m and a k value of 0.0211yr-1. Analyzing 177 encounters with 40 male sharks (3.16m-8.00m) changed these parameters to an L of 18.081m and a k value of 0.0234yr1. This corresponds to a male age of maturity of ~25 years and a longevity of ~140 years. Keywords: von Bertalanffy, laser photogrammetry, growth rate, total length