14 DAY TRIAL // What's the link between the fin soup and paella?
Well, less big sharks in the oceans means no scallops and other sea food.
A new Canadian-American team, led by world-renowned fisheries biologist Ransom Myers at Dalhousie University, has discovered that overfishing the largest predatory sharks, like the bull, great white, dusky, and hammerhead sharks, along the Atlantic Coast of the US has provoked a boom on their ray, skate, and small shark prey species. "With fewer sharks around, the species they prey upon - like cownose rays - have increased in numbers, and in turn, hordes of cownose rays dining on bay scallops, have wiped the scallops out," said co-author Julia Baum of Dalhousie.
"This ecological event is having a large impact on local communities that depend so much on healthy fisheries," said co-researcher Charles Peterson, a professor of marine sciences biology and ecology at the Institute of Marine Sciences, University of North Carolina at Chapel Hill.
There has been a steady decline of the sharks in the northwest Atlantic since the mid-1980s.
By investigating a dozen surveys from 1970-2005 along the eastern U.S. coast, the team has discovered that hammerhead and tiger sharks may have decreased by over 97 %; bull, dusky, and smooth hammerhead sharks by over 99 %. "Large sharks have been functionally eliminated from the east coast of the U.S., meaning that they can no longer perform their ecosystem role as top predators. The extent of the declines shouldn't be a surprise considering how heavily large sharks have been fished in recent decades to meet the growing worldwide demand for shark fins and meat", said Baum.
Sharks are targeted or by-catch in fisheries looking for tunas and swordfish.
More than 73 million sharks are killed worldwide yearly for the fin trade, and the number is steadily increasing. "This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops," said Ellen Pikitch, a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science and executive director of the Pew Institute for Ocean Science. "These unforeseen and devastating impacts underscore the need to take a more holistic ecosystem-based approach to fisheries management."
Large sharks' prey appeared to have increased considerably in the past 16 to 35 years. The largest increase was experienced by the cownose rays, a conspicuous fact due to their near-shore migrations. They experienced an increase of roughly 8 % yearly and the east coast cownose ray population numbers now up to 40 million.
These rays can grow over 4 ft (1.2 m) across, and eat huge quantities of bivalves, like bay scallops, oysters, soft-shell and hard clams during summer. In the early '80s, when Peterson monitored the area, most bay scallops in North Carolina survived the summer cownose ray predation, so the population supported fishery activity.
But in the recent years, the migrating rays ate almost all adult bay scallops in the area, except those guarded inside fences that the scientists had put up to protect them from the rays.
By 2004, cownose rays had completely destroyed the scallop population, finishing North Carolina's century-old bay scallop fishery. "Increased predation by cownose rays also may inhibit recovery of oysters and clams from the effects of overexploitation, disease, habitat destruction, and pollution, which already have depressed these species," said Peterson, pointing that there are growing shellfish populations in zones beyond the ray's northernmost limit.
"Despite the difficulty of piecing together ecosystem impacts of overfishing, the real challenge will be to move beyond retrospective analyses and instead prevent ecosystem-wide changes from happening in the first place", said co-author Travis Shepherd of Dalhousie.
"Our study provides evidence that the loss of great sharks triggers changes that cascade throughout coastal food webs. Solutions include enhancing protection of great sharks by substantially reducing fishing pressure on all of these species and enforcing bans on shark finning both in national waters and on the high seas", said Baum.
"Maintaining the populations of top predators is critical for sustaining healthy oceanic ecosystems. Despite the vastness of the oceans, its organisms are interconnected, meaning that changes at one level have implications several steps removed. Through our work, the ocean is not so unfathomable, and we know better now why sharks matter", said Peterson.