Mar 29 (IPS) – Major declines in large sharks along the U.S. coast have in turn triggered declines in shellfish and reduced water quality, proof that the ocean’s food web is collapsing, a groundbreaking new study reveals.
With the virtual elimination of large sharks along the U.S. east coast, such as black tip and tiger sharks, the species they used to eat — small sharks, rays and skates – have boomed in numbers. Cownose ray populations increased 20-fold since 1970 and as a direct consequence, shellfish like scallops that the cownose ray eats have been nearly wiped out despite major conservation efforts.
The cascade of impacts resulting from overfishing large sharks goes further still, marine scientist Ransom Myers and coauthors document in a paper published Thursday in Science. The loss of scallops has reduced water quality because scallops and other shellfish filtre sea water. And the cownose ray is now feeding voraciously on other shellfish, like oysters and clams.
“We’ve also seen large seagrass beds the rays have dug up looking for shellfish,” says co-author Charles Peterson of the Institute of Marine Sciences at the University of North Carolina.
Seagrass is considered the “nursery” for many fish species, but Peterson says there is no data available on what the impacts the rays are having on seagrass beds.
The cownose ray is just one of 12 species of rays, skates and small sharks that have sharply increased in numbers.
“We have no idea what impacts the other 11 are having,” Peterson told IPS. “What we do know from this study is that sharks play a crucial role in the ocean ecosystem.”