The study, published today in the journal PLOS Biology shows that the average body size and number of sharks and other marine predators—vital to maintaining healthy ocean ecosystems—fell significantly near cities with more than 10,000 people and associated fishing boats.
The minimum distance from people and fishing which had no measurable effect was 1,250 kilometers. This is far 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.
Lead author Dr. Tom Letessier said, ''Human activity is now the biggest influence on sharks' distribution. Just 13% 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 kilometers 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 overuse on our seas. ''
To collect their data, the team analyzed videos taken at 1,041 sites across the Indian and Pacific Oceans, selected to test the biggest possible range of conditions and habitats. Sharks and other free-swimming predators were studied using cameras attached cannisters(炮筒)filled with bait. In total, the team recorded 23,200 animals representing 109 species. These included 841 individual sharks from 19 different species.
Dr. Letessier added, ''Our study also found that shallower water environment, of depths less than 500 meters, were vital for marine predator diversity. We therefore need to identify sites that are both shallow and remote and protect them. However, there are still numerous shallow hot spots near human markets that are not appropriately protected, and this must change. ''
