Comparing length-measurement methods and estimating growth parameters of free-swimming whale sharks (Rhincodon typus) near the South Ari Atoll, Maldives

Abstract

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Whale sharks (Rhincodon typus) are an endangered species whose growth and reproductive biology are poorly understood. Given their conservation concern, estimating growth parameters, as traditionally derived from vertebral samples of dead animals, is challenging. We used a non-invasive approach to investigate growth parameters of whale sharks frequenting the South Ari Atoll, Maldives, by analysing repeat measurements of free-swimming sharks over a 10-year period. Total lengths of the sharks were estimated by three measurement methods. Visual estimates underestimated the sizes of large sharks, whereas laser and tape measurements yielded results that were similar to one another. The Maldives aggregation consisted of primarily male (91%) juvenile (total length = 3.16–8.00 m) sharks and sharks new to the area were significantly smaller than were returning sharks, which suggests that this site may constitute a secondary nursery ground. Estimates of von Bertalanffy (VBG) growth parameters for combined sexes (L∞ = 19.6 m, k = 0.021 year–1) were calculated from 186 encounters with 44 sharks. For males, VBG parameters (L∞ = 18.1 m, k = 0.023 year–1) were calculated from 177 encounters with 40 sharks and correspond to a male age at maturity of ~25 years and longevity of ~130 years. Differences between these estimates and those from other studies underscore the need for regional studies.