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The Big Picture
‘Human activity is the biggest influence on sharks’ distribution’
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Georgina Mills reports on a recent study that looked at how fishing and human activity affect shark populations

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Large marine predators – and sharks in particular – play a unique and irreplaceable role in the ocean ecosystem

Sharks are much rarer in habitats near large human populations and fish markets, new research has found.

The study, led by researchers from the Zoological Society of London (ZSL), shows that the average body size and number of sharks fell significantly in proximity to places with more than 10,000 people and associated fishing fleets.

In fact, the study found that the minimum distance from people and fishing which had no measurable effect on sharks was some 1250 kilometres, far further than previously thought.

The researchers also found that sea surface temperature had a strong influence on predators’ average body size, with a marked decrease at temperatures higher than 28°C. While this is consistent with normal biogeographic patterns – it is known that many smaller species live in tropical waters – it could become a problem as global temperatures continue to rise.

To collect their data, the team analysed video footage taken at over 1000 sites across the Indian and Pacific Oceans, selected to test the biggest possible range of conditions and habitats. The sites varied in proximity to fish markets and human populations, with some close to cities and others up to 1500 kilometres away. Sharks – and other free-swimming predators – were studied using cameras attached to canisters filled with bait. In total the team recorded 23,200 animals representing 109 species. These included 841 individual sharks from 19 different species.

Lead author Tom Letessier, from ZSL’s Institute of Zoology, said: ‘Human activity is now the biggest influence on sharks’ distribution, overtaking every other ecological factor. Just 13 per cent of the world’s oceans can be considered “wilderness” but sharks and other predators are much more common and significantly larger at distances greater than 1250 kilometres from people. This suggests that large marine predators are generally unable to thrive near to people and is another clear example of the impact of human overexploitation on our seas.’

One way of conserving species such as sharks is the implementation of large, no-take marine protected areas (MPAs), but such efforts require knowledge of the critical habitats for predators, both across shallow reefs and the deeper ocean. It is hoped that work such as this will help to find the best locations for MPAs.

Letessier added: ‘Our study also found that shallower water habitats, of depths less than 500 metres, were vital for marine predator diversity. We therefore need to identify sites that are both shallow and remote and prioritise them for conservation. However, there are still numerous shallow hotspots in the vicinity of human markets that are not appropriately protected, and this must change. Existing large, no-take MPAs need to be better enforced and extended to focus on the last refuges where these extraordinary animals remain abundant.

‘Large marine predators – and sharks in particular – play a unique and irreplaceable role in the ocean ecosystem. They control populations of prey species, keep populations healthy by removing sick or injured animals, and transport nutrients between loosely connected habitats over vast distances.’

The study is published in PLOS Biology and can be found at https://bit.ly/2ZY9Xls

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