Extremely low sample size allows age and growth estimation in a rare and threatened shark

Abstract

Understanding life history parameters is key to assessing biological productivity, extinction risk, and informing the management of exploited fish populations. Age-and-growth analyses in chondrichthyan fishes (sharks, rays, and ghost sharks) is primarily undertaken through counting band pairs laid down in vertebrae. For rare, threatened, and protected species such as river sharks (family Carcharhinidae; genus Glyphis) of northern Australia, obtaining sufficient samples of vertebrae may not be possible. Here we use a very sample size, selective size-class sampling, and back-calculation techniques to provide age and growth data on the Speartooth Shark Glyphis glyphis from which comprehensive sampling is not possible. Ten individuals were sampled from the Adelaide River, Northern Territory, Australia. Length-at-age models were applied to the observed and back-calculated data with the sexes combined due to the small sample size and growth estimated using a multi-model framework. Band pair counts produced age estimates of 0-11 years. Most model parameter estimates for length-at-birth (L0) and asymptotic length (L{infty}) were biologically plausible. The model averaged parameters for the observed data were [Formula] total length (TL) and [Formula] TL, and for back-calculated data were [Formula] TL and [Formula] TL. Overall, the parameter standard errors and model residual standard errors were lower for the back-calculated data due to the addition of interpolated data. Analysed samples were restricted to juveniles and sub-adults as adult G. glyphis have not been encountered in the Northern Territory. The ageing results suggest an age-at-maturity of >12 years for this species. The lack of mature individuals in the sample means that this analysis should be considered as a partial growth curve with length-at-age estimates that are valid over the available age range. The results presented here provide the first age and growth estimation for river sharks.