Decline in Large Shark Populations Cascades through Oceanic Food Web

A tiger shark (Galeocerdo cuvier)

The loss of large sharks in the northwest Atlantic has cascading effects leading to a sharp decline in scallop, clam, oyster, and other shellfish populations, researchers report in the 30 March issue of Science.

Due to their exploitation to meet the worldwide demand for fins and meat, populations of large sharks, considered to be an apex predator because of their location at the top of the oceanic food web, have sharply dropped.

By analyzing a compilation of independent research surveys and fisheries records, the authors found sharp declines in 11 species of large sharks. The big sharks usually eat smaller sharks, rays and skates, but with the predator numbers in decline, the researchers found that populations rose for most of the smaller species they examined. That, in turn,

has led to a decrease in the populations that they prey on—shellfish and other animals found lower in the oceanic food web.

A cownose ray (Rhinoptera bonasus)

"As abundances of all 11 great sharks... fell over the past 35 years, 12 of 14 of these prey species increased in coastal northwest Atlantic ecosystems," wrote Ransom Myers, lead author and a marine biologist at Dalhousie University in Halifax, Nova Scotia, Canada. [Myers, 54, died 27 March of a brain tumor at a hospital in Halifax, Nova Scotia.]

"Effects of this community restructuring have cascaded downward from the cownose ray (an intermediary species), whose enhanced predation on its bay scallop prey was sufficient to terminate a century-long scallop fishery," Myers wrote.

The research found population declines of 87% for sandbar sharks; 93% for blacktip sharks; up to 97% for tiger sharks; 98% for scalloped hammerheads; and 99% or more for bull, dusky, and smooth hammerhead sharks.

A scalloped hammerhead shark (Sphyma lewini)

The authors believe studying the cascading effects of rapidly increasing and decreasing populations—shark populations affecting ray populations, in turn, affecting shellfish populations—is essential to help protect ecosystems from degradation.

"Illuminating the operation of indirect species interactions within marine and other environments brightens the future for... ecosystem-based management to achieve sustainability of natural living resources," they wrote.

Kathy Wren and Benjamin Somers

30 March 2007