We measured the fork length (FL 2009–2019) and TL (2009–2011) of individual whale sharks using the EventMeasure software (see “Footnote 1”). Calibrations were conducted using the CAL software (see “Footnote 1”) and calibration accuracy was verified by measuring known lengths on a scale bar post-calibration in the swimming pool. If male sharks do asymptote in size at 5 m to reflect the likely ranges of targets in the field. Further support for asymptotic rather than indeterminate growth in whale sharks is provided by repeated observations of a 7.4 m TL mature male shark at Ningaloo Reef that has not changed in size in over two decades ( Norman and Morgan, 2016). This implies that male size is more likely to asymptote at a much smaller size range, given well-established trade-offs between growth and sexual maturation in fishes ( Roff, 1991 He and Stewart, 2001). This idea is difficult to reconcile with the direct observations of size at maturity of male sharks reported by several studies, as these show that (based on clasper morphology) males generally attain sexual maturity at around 8 m ( Norman and Stevens, 2007). Data from these animals has been used to extrapolate growth patterns of adults, with predicted asymptotic sizes sometimes exceeding the documented maximum size of 18 m for the species. (2018), have mostly sampled juvenile males (3–7 m TL). 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.Īll growth rate studies of whale sharks, including a recent photo-identification approach by Perry et al. 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). It is mostly derived from age analysis of the vertebrae of a few individuals ( n = 15) that have been stranded on shorelines in the western Indian Ocean ( Wintner, 2000) and from sharks ( n = 92) that were captured during a fishery (now closed) off the coast of Taiwan ( Hsu et al., 2014). Indeed, declines in abundance have led to whale sharks recently being classified as Endangered by the IUCN Red List ( Pierce and Norman, 2016).ĭata on the growth patterns of whale sharks is very limited. The immense size of these animals implies slow growth rates and great longevity ( Speakman, 2005), factors that are likely to promote low resilience to anthropogenic threats such as overfishing, warming oceans and ship strike. Whale sharks are the world’s largest fish, growing to maximum known sizes of 18 m total length (TL) ( McClain et al., 2015). 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. 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. 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. Females were predicted to grow to an average asymptotic length of around 14.5 m. 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 world-wide and much smaller than the sizes predicted by earlier modeling studies. 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. 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.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |