Science

Endangered hammerhead sharks migrating to waters as far south as Perth, study finds

An endangered species of hammerhead shark, typically found in tropical waters, is moving into waters as far south as Perth, a new study has found.

The scalloped hammerhead shark is rarely seen south of the Abrolhos Islands, according to the Department of Biodiversity, Conservation and Attractions.

But a new peer-reviewed study, published in Austral Ecology, reported recurring sightings of the rare species in Perth metropolitan waters in both the summer of 2019-20 and 2020-21.

Lead author Andrea Lopez said it was the southernmost recurrent group of scalloped hammerheads reported in Australia, pointing to a potential extension of their distribution in WA.

But she said the southerly shift could spell danger for the species.

“At the moment, because the north-western shark fishery is closed, [this species] doesn’t interact with many fisheries up north.

“However, if [fishers] are in the presence of them [in Perth]these animals might be interacting with the local temperate shark fisheries.”

Conservation status under review

Scalloped hammerheads are currently listed as “conservation dependent” in Australia, but their conservation status is under review.

In 2019, the International Union on the Conservation of Nature elevated the scalloped hammerhead’s status to critically endangered.

Scalloped hammerhead sharks are allowed to be caught by recreational fishermen.(Wikimedia Commons: Kris Mikael Krister)

The species is commercially caught in very small numbers of less than a ton annually, according to the Department of Primary Industries and Regional Development (DPIRD).

But they are allowed to be caught by recreational fishers, who can collect up to three scalloped hammerheads as a part of their large pelagic mixed-species bag limit.

Director of aquatic resource management Nathan Harrison said DPIRD was currently working the Northern Territory, Queensland, New South Wales and Australian government agencies on a national scalloped hammerhead management strategy.

Warmer waters potentially influencing sea change

The study made no conclusions as to why the species was appearing to migrate further south.

But Ms Lopez said it was consistent with other studies that showed a relationship between warming water temperatures and species moving further south.

“So similar results from a study on tiger sharks were published last year.”

According to the CSIRO, the overall ocean surface around Australia has warmed by around 1 degree Celsius since 1910, with eight of the ten warmest years on record occurring since 2010.

map of oceans around Aus, oceans warming by over 0.16 degrees Celsius in the south east.  0.08 in the eastern tropics.  0.12 rest
A map showing trends in sea surface temperatures from 1950 to 2017.(Supplied: Bureau of Meteorology )

The frequency of marine heat waves has also increased.

University of Western Australia oceanographer Charitha Pattiaratchi said it was possible the upward trend in ocean temperatures played a part in the southward migration of the species.

But he said ocean temperatures varied on a much larger scale throughout a season or year.

“So there is a change in the temperatures, but 1 degree in let’s say 20 years is quite small when we can have 2 to 3 degrees changes every month or year,” he said.

Under high emission scenarios, WA’s ocean temperatures are projected to increase by between 1.5 and 4.1 degrees by 2090, according to data from the CSIRO and Bureau of Meteorology.

Different regions of WA had different projections of temperature increase, with the monsoonal north projected to see the highest increase in the state.

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