Why bull shark numbers in Australia could begin to drop

·Environment Editor
·3-min read

Habitat nursing juvenile bull sharks in NSW is under extreme threat from climate change, scientists warn.

Mangroves were previously thought to be the species’ primary feeding ground, but a two-year study, led my Macquarie University, turned this assumption on its head.

Focusing on the Clarence River, in the Northern Rivers district, analysis of tissue samples revealed prey living in salt marshes is more important to young bull sharks.

Urbanisation of coastal areas and climate change are leading to the decline of bull shark habitat. Source: Getty (File)
Urbanisation of coastal areas and climate change are leading to the decline of bull shark habitat. Source: Getty (File)

This environment is key to their survival and growth before they become large enough to venture into the open ocean.

Unfortunately for these young sharks, global warming and urbanisation of coastal areas are causing this habitat to decline, study co-author Dr Vincent Raoult from the University of Newcastle warns.

"It’s estimated we’ve already lost 50 per cent of our salt marshes across the world,” he said.

Scientists examined shark samples taken from the Clarence River. Source: Yuri Niella / Macquaire University
Scientists examined shark samples taken from the Clarence River. Source: Yuri Niella / Macquaire University

Speaking with Yahoo News Australia, Dr Raoult added protecting salt marshes had not previously been associated with large shark conservation.

“Salt marshes are very productive environments,” he said.

“We think that salt marshes are a really important habitat for larvae of various species of crustaceans.

“These form a huge source of food for a lot of fishes which are then fed on by the bull sharks.”

What is a salt marsh?

  • Salt marshes are a type of coastal wetland that becomes flooded by salt water tides.

  • They typically contain decomposing plant material which can smell like rotten eggs.

  • Large numbers of fish and invertebrate species shelter in them.

  • By buffering the coast from waves, they protect shorelines and stop erosion.

Around 50 per cent of the world's salt marshes have already been destroyed. Source: Getty
Around 50 per cent of the world's salt marshes have already been destroyed. Source: Getty

Why study could be 'good news' for sharks

A 2018 Queensland study found catch numbers of large apex sharks have declined by between 74 and 92 per cent over the last 50 years.

Juvenile sharks feed in salt marshes before leaving the safety of the estuary for the ocean. Source: Estuarine, Coastal and Shelf Science
Juvenile sharks feed in salt marshes before leaving the safety of the estuary for the ocean. Source: Estuarine, Coastal and Shelf Science

Commercial fishing is the primary driver of their demise, and regulating the industry can be “complicated” Dr Raoult said.

“What really was surprising from the study is there are other things we can do,” he added.

“Typically, we don't think of ways to preserve habitat, and the good news is rehabilitating salt marshes is actually relatively easy.

“All you have to do is remove the weirs and the dams which have dried them out and they will recover progressively.

“Really, this is a word of warning that if we do lose more salt marshes, then it's going to be an added threat to shark abundance on our coastlines.”

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