As the current century progresses, the extinction risk for sharks, rays and chimaeras* is set to rise in southern Australia with warming waters exacerbating the impact of commercial fishing, new research has found.
Southern sharks and their relatives are also likely to compete with and be ‘pushed out’ by tropical species, such as bull and tiger sharks, as they migrate southwards into increasingly warmer waters.
The study, co-authored by Australian Marine Conservation Society (AMCS) shark scientist Dr Leonardo Guida, was published in the leading journal Fish and Fisheries. It found that if fishing persists at current levels, up to six currently threatened species, including the critically endangered school shark and endangered maugean skate, will have their recovery hampered because of competition from migrating tropical and sub-tropical species and changing habitats.
Although the prognosis is worrisome, the groundbreaking study’s risk assessment of the combined impacts of fishing and global warming provides direction as to where action is needed most to improve the outcomes for threatened species.
The co-authors say that they expect the number of threatened species to rise as global warming exacerbates the fishing risk.
Lead author Dr Terry Walker, research associate with Monash and Melbourne Universities said: “Since the early 2000s, extensive management reform greatly reduced risks from fishing to the populations of sharks, rays and chimaeras distributed off southern Australia. However, depending on the magnitude of climate change in the current century, risk will again rise.
“The species at highest risk from climate change inhabit the inner regions or reefs on the continental shelf or have inshore nursery areas. These species, such as school shark, bronze whalers and elephant fish (chimaeras), and their habitats will variously experience changing water temperature, freshwater runoff, dissolved oxygen concentration, ocean acidity, and damage from the increased storm and marine heatwave frequency and intensity.”
Co-author Professor Richard Reina from Monash University said: “Our study shows the urgent need for broad, integrated assessments of anthropogenic risks to sharks and rays over a long timeframe, in order to most effectively manage and conserve these ecologically and commercially important but vulnerable species.”
Dr Guida added: “The information provided is not only a snapshot of a possible future for sharks and rays in southern Australia, but is a critical tool to shape how Australia fishes into the future and best supports the recovery of its threatened sharks and rays.”
Using information across a variety of disciplines, including biology, oceanography and climate science, the paper examined shark and ray populations throughout the entire water column, spanning coastal and open ocean waters through to those living in the cold, dark depths beyond 700m.
Applying fishing levels similar to today, and those from the early 2000s when shark fishing peaked in Australia, the study assessed the future for sharks and rays in southern Australia under internationally agreed climate warming scenarios of low, medium and high greenhouse gas emissions.
The results of the study show that as the East Australian Current strengthens with global warming, and pushes warmer water further south, tropical species will likewise extend their range further south. Tropical species will then begin to compete with established, cooler water species, such as the critically endangered whitefin swellshark, and force them to head westwards away from Bass Strait and into cooler water.
Dr Guida added: “Global warming is literally going to push southern sharks and rays into a corner, because they can only go so far south and west. It’ll not only change which sharks and rays are present in southern Australia, but also how the ecosystem functions as a whole – it’s a change I’ll likely see unfold in my lifetime.
“We not only have to curb and commit to net zero emissions, but adjust our fishing practices while we’re still that little bit ahead of the curve. The framework produced by this study is a critical and much-needed tool to help us recover and protect our threatened sharks and rays.”
Notes for editors:
* Chimaeras are often included under the collective label ‘shark and rays’ for simplicity, but this often results in lower awareness of these species, and ultimately less research on them, compared to true sharks and rays. Chimaeras, sharks and rays are all members of the animal group “Chondrichthyes” – the cartilaginous fish. Chimaeras are also commonly referred to as ghost sharks, rat fish, spookfish, or rabbit fish.