COMMERCIAL FISHING OVERLAPS THREATENED SHARK SPECIES HOTSPOTS

Commercial fishing overlaps threatened shark hotspots worldwide. A ground-breaking study led by Marine Biological Association (MBA) scientists and published in the journal Nature reports that large sharks – some of which are already endangered globally – face a future with limited spatial refuge from industrial longline fishing effort, even in the remotest parts of the ocean.

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 (areas beyond national jurisdiction; ABNJ) where there is currently little or no management for sharks.

Where in the vast expanse of the oceans do sharks aggregate? How much fishing takes place in those chosen habitats? This knowledge is lacking, even though it will be crucial to selecting sites to conserve sharks.

The Movement ecology and conservation of marine predators research group led by Professor David Sims coordinated a global collaboration of over 150 scientists from 26 countries to tackle this knowledge gap by collating movement data from nearly 2,000 sharks tracked with satellite transmitter tags.

Researchers mapped shark positions and revealed ‘hotspots’ of space use in unprecedented detail. They found multi-species pelagic shark hotspots were mostly located in frontal zones, 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.Strikingly, results showed that the longline fishers efficiently ‘shadowed’ the shark hotspots in both space and time. For instance, commercially exploited sharks such as North Atlantic blue and shortfin makos sharks have on average 76% and 62% of their space use, respectively, overlapped by longlines each month. Even internationally protected species such as great white and porbeagle sharks had overlap values exceeding 50%.

“Our results show major high seas fishing activities are currently centred on ecologically important shark hotspots worldwide” says Professor David Sims, who led the study as part of the Global Shark Movement Project based at the Marine Biological Association Laboratory in Plymouth, UK.

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” said Dr Nuno Queiroz, a lead researcher in the study from the University of Porto in Portugal.

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.