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

Abstract

Age and growth data are central to management or conservation strategies for any species. Circumstantial evidence suggests that male whale sharks (Rhincodon typus) grow to asymptotic sizes much smaller than those predicted by age and growth studies and consequently, there may be sex-specific size and growth patterns in the species. We tested this hypothesis by using stereo-video and photo-identification studies to estimate the growth rates of 54 whale sharks that were resighted over a period of up to a decade at Ningaloo Reef. We found that male growth patterns were consistent with an average asymptotic total length (TL) of approximately 8-9 m, a size similar to direct observations of size at maturity at aggregation sites worldwide and much smaller than the sizes predicted by earlier modeling studies. Females were predicted to grow to an average asymptotic length of around 14.5 m. Males had growth coefficients of K = 0.088 year −1 , whereas limited resighting data suggested a growth coefficient of K = 0.035 year −1 for females. Other data including re-sightings of an individual male over two decades, records of sex-specific maximum sizes of individuals captured in fisheries and data from juveniles growing in aquaria were also consistent with the suggestion of sex-specific growth profiles for the species. We argue that selection for sex-specific growth patterns could explain many of the otherwise enigmatic patterns in the ecology of this species including the tendency of the species to form aggregations of juvenile males in coastal waters.

Individual haplotyping of whale sharks from seawater environmental DNA

“…Managing these elusive animals can be additionally challenging because the majority of their life cycle is spent in the open ocean, and they only form coastal aggregations seasonally in response to peaks in planktonic prey (Meekan et al, 2006; Rowat & Brooks, 2012). For these reasons, we lack estimates of many of the most basic demographic parameters for the species (Meekan et al, 2020).…”

Population structure, residency, and abundance of whale sharks in the coastal waters off Nosy Be, north‐western Madagascar

“…The drivers of this pattern are unclear. Male whale sharks are likely incentivized to seek out predictable and high-value prey sources to speed their growth to maturity, whereas females may seek out less risky areas elsewhere (Meekan et al, 2020). The apparent absence of adult sharks at Nosy Be lends support to the hypothesis that larger whale sharks are primarily oceanic (Ketchum, Galván-Magaña & Klimley, 2013;Ramírez-Macías et al, 2017;Perry et al, 2020).…”

First Insights Into the Horizontal Movements of Whale Sharks (Rhincodon typus) in the Northern Arabian Sea

“…The migratory behavior of whale sharks, in combination with their slow growth rates (Meekan et al, 2020) and K-selected life history, make populations highly vulnerable to anthropogenic pressures, such as ship strikes (Speed et al, 2008;Lester et al, 2020), bycatch and targeted fishing (Capietto et al, 2014) and pollution (Boldrocchi et al, 2020). These threats are of particular concern as the species is classified as "Endangered" on the IUCN Red List (Pearce and Norman, 2016).…”