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

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 = 0.09-0.14) are consistent with those expected of long-lived sharks, which highlights their sensitivity to fishing pressure and conservation concerns.

Asymptotic Growth of Whale Sharks Suggests Sex-Specific Life-History Strategies

“…Using a back-calculation approach on vertebral increments, Wintner (2000) demonstrated that the earliest growth trajectories may be J-shaped, implying a slow initial start that results in a growth profile that conforms better to a logistic rather than von Bertalanffy model. Indeed, use of a logistic model in Ong et al (2020) produced estimates of asymptotic length (10.7 m) more similar to our estimates for males, and a slower initial growth rate in general might account for the differences in predicted age-specific growth rates between our study and others. Although there are very few published observations of growth rates for whale shark pups, a full-term embryo (sex not provided) raised for 120 days in Taiwan (taken from a commercially harvested mother; Chang et al, 1997) and a male embryo raised for 1157 days in the Okinawa Churaumi Aquarium (Nishida, 2001) displayed linear growth trajectories in excess of rates predicted even by the present study (Figure 4).…”

“…Wintner (2000) estimated a size (15 m) that was closer to our estimates for females when theoretical data points were added to the calculation of the VBGM, but a size of almost 20 m when the relationship was unconstrained by these points. Similarly, Ong et al (2020) predicted asymptotic sizes of 22 m for whale sharks off the coast of Taiwan using a von Bertalanffy model Taiwan (Hsu et al, 2014) Annual rings Size ranges of whale sharks used in each study are displayed in the Supplementary Material (Supplementary Figure 3).…”

“…We compared our data set of individual growth trajectories with published estimates of whale shark growth profiles (Wintner, 2000;Hsu et al, 2014;Perry et al, 2018;Ong et al, 2020). For those studies providing growth parameter estimates, we plotted the individual shark growth trajectories from Ningaloo as line segments overlaid on respective von Bertalanffy growth curves, whereby length and age combinations for initial observations were presumed to fall on the estimated age-at-length predicted by the growth model.…”

“…Interpretation of some of these data is complicated by problems in estimating ages from vertebrae (as is the case with other large sharks; Harry, 2018;Natanson et al, 2018). Furthermore, new research that has validated aging techniques using a bomb radiocarbon approach (Ong et al, 2020) shows that in analyses where biannual patterns in the deposition of bands growth has been assumed, growth rates were overestimated.…”

Ontogenetic growth pattern of the extinct megatooth shark Otodus megalodon—implications for its reproductive biology, development, and life expectancy

“…The situation may be particularly problematic for older individuals because not all growth bands may be deposited in vertebrae past a certain age (Francis et al 2007;Passerotti et al 2014;Harry 2017). Nevertheless, where well-demonstrated cases of annual band deposition are known (Goldman et al 2012, and references therein;Ong et al 2020), the relationship of growth bands to time or age can still be broadly correlated (Natanson and Deacy 2019), and remains a widely used life-history assessment technique for diverse modern elasmobranchs (e.g. Ong et al 2020;Rolim et al 2020).…”