Last updated July 31, 2019 at 5:08 pm
Sharks, including protected species, are at risk from commercial fishing, with even remote hotspots affected by longline fishing fleets.
Even the most remote parts of the ocean appear to offer highly migratory sharks little refuge from industrialised longline fishing fleets, according to a major new international study published in the journal Nature.
Large sharks inhabiting the open ocean account for more than half of all identified shark catch globally in target fisheries or as bycatch.
Regional declines in abundance of some populations such as shortfin mako shark – the fastest shark in the sea – have led to widespread calls for catch limits in the High Seas where there is currently little or no management for sharks.
But precisely where in the vast expanse of the oceans sharks aggregate and how much fishing takes place in those chosen habitats remains poorly known globally, even though it will be crucial to selecting sites to conserve sharks.
A quarter of all shark habitats are within active longline fishing zones
More than 150 scientists from 26 countries have now addressed this knowledge gap by collating movement data from nearly 2000 sharks tracked with satellite transmitter tags and cross-referencing with data from global fishing fleets.
They found multi-species pelagic (those that live in the ocean) shark hotspots were mostly located in boundaries in the sea between different water masses that are highly productive and food-rich.
They then calculated how much the hotspots were overlapped by global fleets of large, longline fishing vessels – the type of fishing gear that catches most pelagic sharks – with each vessel capable of deploying 100 km long lines bearing 1200 baited hooks on a daily basis.
“We found 24% of the mean monthly space used by sharks globally falls under the footprint of pelagic longline fisheries,” says Associate Professor Huveneers.
“It means about one-quarter of shark habitats fall within active fishing zones, presenting a threat to these iconic ocean predators and demonstrating an urgent need for conservation efforts to protect declining pelagic shark populations,” he says.
For commercially exploited sharks such as North Atlantic blue and shortfin makos sharks overlap was much higher, with on average 76% and 62% of their space use, respectively, overlapping with longlines each month.
“Even internationally protected species such as white and porbeagle sharks had overlap values exceeding 50%,” Associate Professor Huveneers says.
“The South Australian Basin was found to be an important hotspot for pelagic sharks, providing some refuge for these species as fishing vessel activity for pelagic sharks is relatively sparse in this area.”
Related: See Charlie Huveneers swim with sharks in the film 700 Sharks
“Our results show major high seas fishing activities are currently centred on ecologically important shark hotspots worldwide” he says.
Equally alarming was that shark hotspots showing high overlap with longline fishing were often also subjected to high fishing effort, a potential ‘double whammy’ for sharks that will result in higher catch rates and potentially accelerate declines in abundance.
“Some shark hotspots were exposed to higher than average fishing effort for as much as half the year,” says Dr Nuno Queiroz, a lead researcher in the study from the University of Porto in Portugal.
New maps of shark hotspots need to influence marine protected areas
Given the high fishing effort in hotspots of many species for significant portions of the year, and the very few tracked hotspots free from exploitation, the study reveals exposure risk of sharks to fisheries in the high seas is spatially extensive – stretching across entire ocean-scale population ranges for some species.
The team’s findings indicate large sharks – some of which are already endangered globally – face a future with limited spatial refuge from industrial longline fishing effort.
The researchers propose that the detailed maps of shark hotspots and exposure to longline fishing effort they have produced can provide a ‘blueprint’ for use in deciding where to place large-scale marine protected areas (MPAs) aimed at conserving sharks, in addition to the need for strict quotas to reduce catches elsewhere.
“Some of the shark hotspots we studied may not be there in as little as a few years’ time if management measures are not put in place now to conserve the sharks and the habitats on which they depend,” Professor Sims says.
Charlie Huveneers will be participating in a special Q&A after a free screening of 700 Sharks at SCINEMA at The Science Exchange, Adelaide, on Saturday 24 August.