Estimating the rate of quasi-extinction for the Australian grey nurse shark (Carcharias taurus) population using deterministic age- and stage-classified models
Otway, N.M., Bradshaw, C.J.A. and Harcourt, R.G., 2004. Estimating the rate of quasi-extinction for the Australian grey nurse shark (Carcharias taurus) population using deterministic age- and stage-classified models. Biological Conservation, 119: 341–350.
Grey nurse sharks off the eastern coast of Australia are listed as “endangered” and may number no more than 300 in New South Wales and southern Queensland waters. They are an inshore, coastal dwelling species and its population was severely reduced by commercial fishing in the 1920s and spearfishing in the 1960s. The population has continued to decline despite the protection afforded the species in NSW waters in 1984. The shark is long-lived with a maximum age between 25 – 35 years, has late sexual maturity of 6 – 8 years and only two pups are born every 2 years. The shark’s distribution, specific habitat requirements, reproductive biology, and small population size render them particularly vulnerable to extinction in the waters off eastern Australia. The time to reach quasi-extinction (a population comprising 50 or fewer females) was estimated for the grey nurse shark population in these waters using current estimates of abundance and human-induced rates of mortality. The estimated minimum population size in 2002 was 300 individuals. Human-induced mortality assessed from recovered carcasses was estimated at 12 per year of which 75% were females. The time to quasi-extinction was modelled for worst-case, probable and best-case scenarios. The models used population sizes of 300 (worst-case), 1000 (probable) and 3000 (best-case). Human-induced mortality was added to the model assuming either all carcasses were being recovered (best-case), or more conservatively, that only 50% are reported (which is considered realistic). Depending on the models applied, if all carcasses are being reported, quasi-extinction times for worst- possible and best-case scenarios range from 14 – 20 years, 96 – 129 years, and 380 – 488 years, respectively. If under-reporting is occurring, time to quasi-extinction ranges from 8-11 years, 49 – 68 years and 206 – 433 years, respectively. In all scenarios modelled, the grey nurse shark population will decline if no further steps are taken to remove human-induced sources of mortality. More precise estimates of abundance and annual survival rate are now needed to refine the modelled predictions.