New research challenges cascading effects of shark declines
TALLAHASSEE, Fla. — New Florida State University research appearing today in Scientific Reports, a Nature journal, challenges a 2007 study published in Science claiming that shark declines led to rising populations of cownose rays, which were responsible for the collapse of oyster and shellfish industries along the Atlantic coast.
The new research is significant since the previous study led in part to the creation of fisheries and bow-fishing tournaments for cownose rays such as the "Save the Bay, Eat a Ray" campaign that could put ray populations in jeopardy.
"Our research presents clear evidence that directly refutes the 2007 Science study," said Dean Grubbs, lead author of the new study and associate director of research at the FSU Coastal and Marine Laboratory (FSUCML). "For instance, the declines in predatory sharks presented in the 2007 study are not nearly as severe as was reported, and the increases in cownose ray populations are biologically unrealistic."
Grubbs' mention of shark declines is important because the 2007 study bases its conclusions on the notion that over the past several decades apex predatory shark populations along the Atlantic coast have been drastically reduced. With dwindling numbers of predators available to keep cownose ray populations in check, they purportedly grew too numerous for existing shellfish populations to support.
In addition, Grubbs and colleagues argue that not only were shark populations not nearly as depleted as was reported, due to successful management, but most species have recovered or are recovering. They also show it is physiologically impossible for cownose rays to increase at the rates suggested by the 2007 study. Rays are slow-maturing fish that do not reproduce until they are at least six or seven years old, and females only produce one offspring per year.
"One of our primary concerns that led to this study was the effect of a fishery with no harvest limits on a species with low reproductive rates, particularly as this was being incorrectly touted as an 'environmentally responsible' seafood choice," said FSUCML faculty member Charles Cotton, a co-author on the study. "Evidence-based management decisions are critical in preventing overfishing, as we've seen many times in the past with unregulated fisheries for other species of sharks and rays."
Grubbs' research also notes that the declines of shellfish stated in the 2007 study do not match the ray population increases it claimed.
"The timing just does not match," Grubbs said. "Oyster and bay scallop populations declined dramatically decades before the supposed increase in cownose rays due to many factors including overharvesting, disease, pollution and habitat loss. Cownose rays have been used as a convenient scapegoat and while they may negatively affect some efforts to restore harvestable shellfish stocks, they were not the cause of the collapse of shellfish fisheries as the 2007 study claimed."
The FSU study was conducted in collaboration with National Oceanic and Atmospheric Administration researchers John Carlson, Tobey Curtis, Camilla McCandless and David McElroy, U.S. Geological Survey researcher Jason Romine and Virginia Institute of Marine Science researcher John A. Musick.