Annual Bands in Vertebrae Validated by Bomb Radiocarbon Assays Provide Estimates of Age and Growth of Whale Sharks

Joyce J L Ong,Steven E Campana,L Paul Fanning,Mark G Meekan,Hua Hsun Hsu

doi:10.3389/fmars.2020.00188

Abstract

Conservation and management strategies for endangered and threatened species require accurate estimates of demographic parameters such as age and growth. The whale shark, Rhincodon typus, is the largest fish in the world and is highly valued in the eco-tourism sector. Despite conservation concerns and advances in our understanding of their life history, basic demographic parameters for growth, longevity and mortality are of questionable accuracy; previous growth studies could not agree whether the vertebral growth bands were formed annually or biannually. Here, we provide the first validation of the annual formation of growth bands within the vertebrae of the whale shark using bomb radiocarbon assays. Ages of up to 50 years were estimated from sectioned vertebrae of sharks collected in Taiwan and Pakistan. There was no cessation of the formation of growth bands in the vertebrae of older sharks and our study provides the oldest observed longevity for this species. Initial estimates of growth (k = 0.01–0.12) and natural mortality rates (M = 0.09) are consistent with those expected of long-lived sharks, which highlights their sensitivity to fishing pressure and conservation concerns.

Highlights

Accurate and reliable estimates of the age and growth of individuals in a population are central to effective strategies for the management and conservation of any species
We showed that growth bands in sectioned vertebrae can provide accurate estimates of sharks aged up to 50 years old
We found no evidence that vertebral counts underestimated the age of older individuals, as can be the case for porbeagle and white sharks (Francis et al, 2007; Hamady et al, 2014), presumably because the much larger asymptotic body size means that there is no cessation of vertebral growth in the older sharks we sampled in our study

Summary

Introduction

Accurate and reliable estimates of the age and growth of individuals in a population are central to effective strategies for the management and conservation of any species. Estimates of age are usually obtained from counts of the annual growth bands formed within otoliths, which are calcified structures within the skull case (Campana, 2001). Annual Bands in Whale Sharks of exploited species (e.g., orange roughy Hoplostethus atlanticus; Smith et al, 1995), or compromise the effectiveness of recovery programs for species that are threatened or endangered For this reason, many studies have sought to validate the timing of the production of growth bands (Campana, 2001). The timing of the deposition of bands can be validated by comparing carbon isotope values within vertebrae, with an isotope baseline chronology of known age (Campana, 2001; Campana et al, 2002; Goldman et al, 2012)

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Conclusion