Genes reveal grey nurse sharks at risk
A genetic study has shown the endangered population of grey nurse sharks off the east coast of Australia cannot rely on its cousins for help.
The grey nurse, Carcharias Taurus, is listed as 'vulnerable' worldwide and 'critically endangered' off the east coast of Australia by the International Union for the Conservation of Nature.
They also found the grey nurse population off the east coast of Australia has little genetic variation between individuals, increasing the threat of extinction to this group.
Stow and team collected 193 fin and muscle samples of grey nurse sharks from five regions - Japan, South Africa, Brazil, north-west Atlantic and eastern and western Australia.
The team used two genetic markers - mitochondrial DNA and microsatellites within the nuclear DNA - to measure genetic diversity between regions and within groups.
They found strong genetic differences between regions and across ocean basins.
"What that implies is there is low levels of migration among populations," says Stow. "And in Australia the eastern population is less likely to be replenished by migration."
He says the genetic analysis shows the grey nurse does not even migrate within Australia, with the western Australia grey nurse a distinct group.
Long-term survival at risk
Stow says the low genetic variability between individuals in the east coast population will make it harder for the grey nurse to survive long term in the area.
"Genetic variation is necessary for the population to adapt to new situations, it [genetic variation] is the raw material for evolution," he says.
Stow says the team is now looking at the causes behind this low level of genetic variation.
However initial work suggests it is not related to the decline in population numbers off the east coast.
"We believe the low genetic variation … is a historic consequence of a small number of individual [sharks] founding the original population," says Stow.
He says the findings have "upped the ante" in terms of the risk that the species will disappear from eastern Australia's waters.
"There is a pretty good chance we could lose the species off the east coast of Australia," says Stow.
He says it is unknown how this will affect the ecosystem within its range.
"When you knock out a top-order predator there could be ecological ramifications that we can't predict."
Stow says the findings that each population is a genetically discrete identity supports the previous belief that ocean temperature acts as a barrier to migration for the species.
The grey nurse is found in warm-temperate inshore waters and cannot tolerate waters with an average temperature below 15°C. They are also uncomfortable in tropical waters.
The findings have implications for how the species should be managed.
"In contrast to pelagic species, such as whale sharks and white sharks that may cross ocean basins mandating cooperative international efforts for conservation, grey nurse shark needs to be managed regionally," says Stow.