More protections for tiger sharks needed after new study indicates possible population wipe out

A new study which shows a genetically separate population of tiger sharks on Australia’s east coast may have been wiped out by fishing pressures and shark control programs, illustrates better protections are urgently required for the iconic sharks, marine conservation groups say.

The new paper led by Dr Alice Manuzzi from the Technical University of Denmark, identified from current and historical DNA samples that a ‘southern’ population of tiger sharks from as far south as Victoria had existed but appeared to have disappeared from as recently as the 1930s. The study follows previous research (1,2) which reported a shocking 71% decline in tiger sharks along Australia’s east coast in just three decades.

Dr Leonardo Guida, shark scientist with the Australian Marine Conservation Society said the study was another red flag for the species and indicated urgent action to better protect them was required.

Tiger shark pups from a pregnant female killed on shark control equipment. Image acquired via right to information“Tiger sharks are one of the world’s most iconic species, and here on our east coast it looks like a population disappeared from under our noses. This study is the canary in the coalmine, if ever there was one, and we need to do more to prevent further declines in tiger shark numbers,” said Dr Guida.

“Tiger sharks are caught incidentally in the commercial fisheries of New South Wales and Queensland and are kept for their meat and fins. In Australia, their numbers are declining and fishing rules specific to this species are sorely needed (3).”

The study’s author, Dr Manuzzi said: “Fishing may have played a significant role in the changes we’ve identified in these two genetically different populations. The decline identified seems to coincide with the introduction of shark control programs and increases in commercial shark fishing. We think the southern population preferred to be more residential and coastal compared to the northern population which is hypothesised to be more widespread and able to migrate offshore.

“Localised depletions of tiger sharks could be more common than we anticipated. We should consider localised protection measures, such as local seasonal closures or marine reserves to properly match the geographic scale of the population.”

Dr Guida added: “We need a population count of tiger sharks on the east coast and independent monitoring on all fishing vessels so we know how many tiger sharks we have left, how many are dying, and what fishing limits must be put in place to ensure their population can persist in a healthy condition.”

The paper also flags shark control programs in Queensland and New South Wales as possible contributors to their decline because drumline gear (suspended fishing hooks) tends to capture large numbers of tiger sharks.

Although both states have made some progress in trialing and implementing non-lethal strategies to improve beach safety, 51 shark nets are still installed off beaches in New South Wales, and 27 nets and 383 drumlines off 86 beaches in Queensland. In Queensland, tiger sharks are one of the most frequently caught sharks on drumlines as well as one of 19 target species that are shot and killed if caught on a drumline.

Lawrence Chlebeck, marine biologist from Humane Society International (HSI), said: “These programs contribute to the ongoing decline in tiger sharks on Australia’s east coast. This species is vital to the health of delicate marine ecosystems, including the Great Barrier Reef. Not only do they keep food webs in check, but also influence the behaviour of fish and grazers to ensure the health of key habitats such as coral reefs and seagrass beds.

“New South Wales and Queensland need to modernise these uselessly destructive shark control programs and introduce new technologies that reduce the risk of shark bite without the terrible cost to marine wildlife.”

AMCS and HSI are investigating whether tiger sharks should be nominated as a threatened species to improve its protections under Australia’s environment laws.

References
1. Brown, Christopher J., and George Roff. “Life-History Traits Inform Population Trends When Assessing the Conservation Status of a Declining Tiger Shark Population.” Biological Conservation 239 (November 1, 2019): 108230. https://doi.org/10.1016/j.biocon.2019.108230.
2. Holmes, Bonnie J., Wayne D. Sumpton, David G. Mayer, Ian R. Tibbetts, David T. Neil, and Mike B. Bennett. “Declining Trends in Annual Catch Rates of the Tiger Shark (Galeocerdo Cuvier) in Queensland, Australia.” Fisheries Research 129-130 (October 1, 2012): 38-45. https://doi.org/10.1016/j.fishres.2012.06.005
3. Simpfendorfer, Colin A., Andrew Chin, C.L. Rigby, S Sherman, and W. T. White. Shark Futures: A Report Card for Australia’s Sharks and Rays. James Cook University: Centre for Sustainable Tropical Fisheries and Aquaculture, 2019.

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