New research has found about one-quarter of oceanic shark habitat is actively fished, leaving sharks with few places to hide from the longlines they frequently get caught in.

Published in Nature, the study highlights the urgent need to protect pelagic sharks, which are reported to be in decline, via increased conservation efforts.

Pelagic (or oceanic) sharks are highly migratory, covering vast areas of the ocean – including areas targeted by fisheries. Large pelagic sharks account for around half of all identified shark catches from fisheries.

The extent of habitat overlap with industrial fisheries has been so far difficult to determine, as data from fisheries about catch numbers can be incomplete.

Johan Gustafson.

Griffith Centre for Coastal Management PhD candidate Johan Gustafson was part of the international team of researchers who tracked the movement of 1600 sharks across 23 species using satellite tags.

The team also monitored the movements of global fishing fleets to see where their paths crossed with the tagged sharks.

This process found the areas frequented by protected species had much higher overlap with longline fisheries, suggesting that more protected areas were needed to sustain these populations of sharks.

The results revealed that 24% of the space used by sharks in an average month falls under the footprint of pelagic longline fisheries, which are responsible for catching most sharks from the open ocean.

Areas of ocean that are frequented by protected species, such as great white sharks and porbeagle sharks, had higher overlap with longline fleets (around 64%).

“This study shows the high overlap and mortality rates of unregulated longline AIS-equipped fishing fleets throughout all oceans, and the impact it can have on an already stressed shark population,” Gustafson said.

“Commercial drift longlines can be many kilometers long and contain thousands of hooks and are responsible for the majority of shark catches, including the White, Oceanic whitetip, tiger, bull, Mako and hammerhead sharks.

“Almost two-thirds of these species are listed as either endangered or vulnerable and near threatened. Coupled with the low growth rates or sharks, recovering populations (ie. White and porbeagle sharks) are being further hindered, and further decreasing other shark species populations.

“Areas of low hotspot-fishing overlap occur in areas of low productivity, and sharks seems to be in transit through them, though more low overlap areas may exist and we need to determine their value to shark populations and how to incorporate them into marine protected areas.

“Management is a complex issue – how do we police large areas of the high seas? How do we monitor the fishing fleets from multiple nations? How do we make sure the catch data is accurate and unbiased?

“These are the future issues we are facing, and it may take some time and technological advancements to undertake unbiased and fair monitoring of the high seas fishing fleets.”

The study was led by Professor David Sims, who is part of the Global Shark Movement Project based at the Marine Biological Association Laboratory in Plymouth, UK.

“Our results show major high seas fishing activities are currently centred on ecologically important shark hot spots worldwide,” Professor Sims says.

The shark hot spots identified in Australian waters were found to be the South Australian Basin, northwest Australia, the southern Great Barrier Reef, New Zealand shelf waters and around the Kermadec Islands northeast of New Zealand.