Absence of top predators disturbs ocean ecosystems

By Kirtiman Awasthi
Last Updated: Saturday 04 July 2015

ecologists have long predicted that the demise of top predators in an ecosystem could trigger destructive consequences. Researchers, in a first-ever field experiment, have shown that the loss of 11 species of large sharks is affecting smaller organisms such as scallop, oyster and clam, along the Atlantic coast. The study was published in Science (Vol 315, No 5820, March 30, 2007).

Analysing fishery data from 17 scientific surveys, the researchers confirmed decline in large shark population to the point of functional elimination: up to 99 per cent for one of the 11 large shark species. Smaller predators (rays, skates, small sharks), in turn, increased, the most conspicuous of them being the cownose rays, which increased 20 times. The cownose ray consumes large quantity of shellfish (scallops, clams, oysters, and other shellfish). "A rise in the numbers of cownose rays is affecting scallops," says Julia Baum, a researcher.

"This impacts local communities that depend so much on healthy fisheries," says Charles Peterson, a co-author. Besides, he feels that maintaining populations of top predators is critical for sustaining healthy oceanic ecosystems. "Organisms are interconnected and changes at one level affect the entire chain," he adds.

The study has underscored the need to take a more holistic ecosystem-based approach to fisheries management. As a solution, the researchers recommend enhancing protection of great sharks by reducing fishing pressure on all of the shark species and enforcing bans on shark finning. Around 73 million sharks are killed each year for the finning trade.

In 2006, a paper in Science (Vol 314, No 5800) reported that one-third of fish species have collapsed worldwide, meaning, their catch has declined 90 per cent below the historic maximum; seven per cent of them have become extinct. Another paper published in Nature (Vol 423, No 6937, May 15, 2003), revealed a 90-per cent decline in the world's predatory fish, including sharks in the Gulf of Thailand.

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