Jaws hold clue to tiger shark ecology

Using DNA retrieved from historical tiger shark jaws collected over the past century, an international group of scientists has revealed that population decline is changing the genetic diversity of one of the ocean’s apex predators.

This includes the discovery of a previously unidentified tiger shark population in eastern Australian waters, which now appears to have disappeared entirely from the south-eastern range of the species’ distribution, say researchers including Flinders University’s Professor Charlie Huveneers in a new journal article.

The new study published in Scientfic Reports, led by Dr Alice Manuzzi from Denmark Technical University, found significant change in the genetics of tiger sharks (Galeocerdo cuvier) from eastern Australia across the past six decades.

“This updated research using historical DNA from tiger shark trophy jaws dating back to the early 1900s suggests one newly identified local population has reduced to a point where it has very likely has been lost,” says co-author University of the Sunshine Coast marine ecologist Dr Bonnie Holmes.

“The population’s genetic signature starts to reduce from the 1970s – coinciding with increases in commercial shark fishing and the introduction of shark control programs.”

Professor Charlie Huveneers, from the Southern Shark Ecology Group.

The DNA samples were gathered from tiger shark jaws sourced from the Gulf of Carpentaria, Coral Sea and Tasman Sea that have been found in museums, national fishery institutes and personal collections.

Until now, it was believed that tiger sharks in Australia were always part of a large Indo-Pacific population, whose movements extended several thousand kilometres across the Pacific and Indian Oceans, with enough gene flow to make them less susceptible to local impacts.

Flinders University co-author Professor Huveneers says declining shark populations can affect marine ecosystems by influencing the abundance and behaviour of prey species and through changes in predator-prey interactions.

“Tiger sharks are top predators in marine ecosystems, feeding on a wide range of organisms including seabirds, turtles, dolphins, and various fish species,” he says.

“Seasonal changes in tiger shark abundances have been shown to affect the behaviour and preferred habitat of their prey as tiger sharks are top predators in marine ecosystems.”

Contemporary tissue samples from 2000-2015 came from fin-clips from sharks caught in the Queensland Shark Control Program, the New South Wales Shark Meshing Program, commercial and recreational landings, and sharks caught for tagging and tracking research purposes.

Genetic diversity is the fuel that drives future evolutions.

As one of the world’s largest shark species, tiger sharks are top-level predators in marine ecosystems, feeding on a wide range of organisms including seabirds, turtles, dolphins and various fish species. This shark species is listed globally as near-threatened.

The scientists say eradication of semi-independent evolutionary lineages will not only have a local effect on tiger shark abundance, but also the evolutionary potential of the species as a whole and the ecosystem services they provide.

Professor Professor Einar Eg Nielsen from the Technical University of Denmark says the documented Australian decline in tiger sharks numbers is likely caused by a combination of factors.

“Our study shows that tiger sharks can have local and genetically isolated populations at a restricted geographical scale – such as the southeast Australian coast – and that these local populations are vulnerable to direct exploitation and shark control programs,” says Professor Nielsen.

“When we, through genetic analysis, better understand the distribution and migration of shark populations and their responses to human activities over historical time, we are better able to design proper management plans and actions at the appropriate geographical scale.”

The research highlights the need for more genomics work to be undertaken on vulnerable and endangered large shark species across Australia, especially those still harvested in commercial fisheries, like hammerhead sharks.

The tiger shark project is part of a larger project umbrella ‘GENOJAWS’ where the scientists have studied retrospective genetics/genomics of four big sharks: sand tiger shark, mako shark, white shark and tiger shark.

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