Bradford RW, Harasti D, Westlake EL, Thomson R, Feutry P, Baylis S, Mayne B, Anderson C, Hillary R, Gunasekera R, Butcher P, Louden B, Gallen C, Patterson, TA (2025). Eastern grey nurse shark, Carcharias taurus, population abundance and trend. Report to the National Environmental Science Program. CSIRO Environment, Hobart, Tasmania.
Overview
This report provides an estimate of abundance and trend in abundance for the adult component of the Critically Endangered eastern Australian grey nurse shark population using the close-kin mark-recapture (CKMR) method.
CKMR is a powerful tool that relies on genetically identifying related individuals. The basic premise is that the degree of relatedness within the sampled population will provide an estimate of adult abundance. Twenty-one Parent-Offspring (POP) pairs, six Full Sibling (FSP) pairs, and 148 Half Sibling (HSP) pairs were identified in the dataset. All POPs and 146 HSPs passed QC and were used in deriving an estimate of abundance. FSPs are not used in the CKMR model.
Adult GNS abundance in 2023 is estimated to be 1,423 (95% CI: 921 to 1925 adults; CV[1] = 0.18) adults. The model further estimates the annual rate of increase to be 5% (95% CI: 2.3 – 7.1%). We would expect this to be close to the maximum rate of increase for this species. The current estimate uses a refined genetic method and a revised dataset from newly collected samples, resulting in fewer data being discarded due to the use of an Australian-specific growth curve to identify related individuals. This has resulted in an update of the previously reported abundance estimate (N2017) to 1,096 adults (CV = 0.15).
The NSW DPIRD provided data from necropsied GNS that allowed for the derivation of a growth curve specific to the eastern Australian GNS population. The Australian growth curve has resulted in a CKMR model that performs better than when using a growth curve from USA GNS.
Acoustic tagging projects undertaken in NSW provided an opportunity to investigate juvenile survival rates. Survival is expected to be high for GNS based on their low reproductive output strategy. Initial results indicate juvenile survival for ages 0 to 4 to be approx. 85% per annum, with survival of older ages increasing to at least 90% per annum. This result, however, should be taken with caution due to the small sample size and possible male bias. However, the results indicate that there is information on survival from telemetry data from internally implanted long-life acoustic tags, but any further tagging should focus on females and the youngest available animals.
[1] The CV, or coefficient of variation, is the ratio of the standard deviation to the mean. The higher the coefficient of variation, the greater the level of dispersion around the mean.
