Australia’s Critically Endangered eastern grey nurse shark population is increasing slowly, a project that combined rigorous biological sampling and genetic sleuthing has found.
Scientists collected biopsy samples from more than 300 grey nurse sharks off the coast of New South Wales. The DNA of these sharks was profiled to reveal family relationships that can help to estimate population size.
Based on this work, the number of adults in the eastern grey nurse shark population is estimated to have increased by about 5% annually between 2017 and 2023, from about 1096 to 1420 adults.
While the breeding population remains extremely small, this modest increase is an encouraging sign for the recovery for this threatened species.
It indicates that conservation actions such as the removal of fishing threats from key aggregation sites – as outlined in The National Recovery Plan for the Grey Nurse Shark – are helping to slowly recover the species.
Many of the places where grey nurse sharks aggregate for breeding, pupping and feeding in NSW and Queensland are protected.
CSIRO and the NSW Department of Primary Industries and Regional Development (NSW DPIRD) led the research, with funding support from the Australian Government’s National Environmental Science Program Marine and Coastal Hub.
“Grey nurse sharks have one of the lowest reproductive rates of any shark,” Dr David Harasti of NSW DPIRD says.
“They breed once every two years and produce only two pups after a year-long gestation period. They are also long-lived and slow growing.
“This makes their recovery fragile, especially given the low levels of genetic diversity in the eastern population, which makes it vulnerable to even small environmental changes or disturbance from human activities.”
The adult population size and trend for the eastern grey nurse shark population were estimated using ‘close-kin mark-recapture’. This technique developed by CSIRO combines advanced genetics and statistical modelling.
“Close-kin mark-recapture works on the premise that the number of close-kin pairs (in this study, two animals that share a parent) in a population relates directly to the size of the adult population,” Dr Toby Patterson of CSIRO says.
“For example, samples from a small adult population typically would display a high rate of close-kin pairs, while samples from a large adult population would display a low rate of close-kin pairs.”
CSIRO researchers analysed the DNA of 376 tissue samples collected from grey nurse sharks to identify the close-kin pairs.
NSW DPIRD sourced new tissue samples from live animals released via the NSW SMART drumline program, in addition to conducting targeted scuba diving surveys.
“NSW DPIRD scuba divers that are trained in safe biopsy techniques used a hollow stainless-steel biopsy tip attached to a hand spear with a purpose-built quick release adaptor,” Dr Harasti says.
“In a procedure involving minimal interaction, a diver would approach a shark underwater and obtain a small tissue sample at close range, while a second diver operated a stereo-video camera system to record an accurate length measurement.
“The sampling took place in NSW from the Solitary Islands off Coffs Harbour down to Cabbage Tree Island off Port Stephens, in depths ranging from eight metres at shallow sites such as Hayden's Reef off Forster, to 30 m at sites such as Fish Rock off South-West Rocks.
“While collecting tissue from the sharks can be tricky, after a few days’ sampling we had become experts at sneaking up on the sharks to obtain samples!”
Importantly, this new, accurate length information was used to estimate the age of sharks using a new model developed specifically for Australian grey nurse sharks that links the shark’s age to its length.
“Accurate estimation of the sampled shark’s age is an important input into the close-kin mark recapture population estimate,” Dr Patterson says.
“Due to a lack of Australian data and models, previous estimates were based on data from other countries in the species’ range.”
Dr Harasti says the new research methods and breadth of data provided by this project have enabled a strong evidence base for the Australian and NSW governments to assess conservation actions for the grey nurse shark.
“It seems we’re heading in the right direction to ensuring a more positive future for the grey nurse shark along our east coast,” he says.
The grey nurse shark is one of 110 priority species in the Australian Government’s Threatened Species Action Plan 2022–2032.
“This early sign of recovery is great news for this long-lived species,” Threatened Species Commissioner, Dr Fiona Fraser, says.
“However, we need to stay mindful that recovery takes time and ongoing effort.
“Grey nurse sharks are still listed as Critically Endangered on the east coast of Australia.”
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