Humans pose health risk to sharks

| 7th August 2019
Scientists have discovered that the supposedly fearsome creatures cannot thrive near large human populations.

Human activity is now the biggest influence on sharks' distribution, overtaking every other ecological factor.

Human predators are a threat to sharks' wellbeing, research suggests.

Scientists have discovered that the supposedly fearsome creatures cannot thrive near large human populations and fish markets.

Researchers also found the average body size of sharks and other marine predators fell dramatically in these areas, where sharks are caught and killed intensively for their meat and fins.

Over-exploitation

The study indicates the average body size and number of sharks and other marine predators fell significantly in proximity to cities with more than 10,000 people and associated fishing fleets.

Lead author Dr Tom Letessier, of the Zoological Society of London's Institute of Zoology, said: "Human activity is now the biggest influence on sharks' distribution, overtaking every other ecological factor.

"Just 13 percent 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 1,250 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 over-exploitation on our seas."

Published in the journal PLOS Biology, the research suggests the minimum distance from people and fishing which had no measurable effect was 1,250 kilometres.

Tropical

This is much further than previous studies have suggested and probably reflects the increased distances fishing boats can now travel.

As a result, sharks were only observed at 12% of sites monitored, scientists say.

According to the research, sea surface temperature also had a strong influence on predators' average body size, with a marked decrease at more than 28C.

While this is consistent with what is already known about many smaller species living 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 1,041 sites across the Indian and Pacific Oceans.

Remote

Sites varied in proximity to fish markets and human populations, with some close to cities and others up to 1,500 kilometres away.

Sharks, and other free-swimming predators, were studied using cameras attached to canisters filled with bait.

The team recorded a total of 23,200 animals representing 109 species, including 841 individual sharks from 19 different species.

Dr 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.

Prey

"Existing large, no-take Marine Protected Areas (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 those populations healthy by removing sick or injured animals, and transport nutrients between loosely connected habitats over vast distances."

This Author

Nina Massey is the PA science correspondent.

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