By Katharine Sucher, Science Writing Intern
A research tool more often linked to forensic science is helping to crack the case of a marine mystery: how many populations of cownose rays are out there? Using DNA analysis, Virginia Institute of Marine Science (VIMS) researchers have found that the population on the East Coast may not be as geographically extensive as originally assumed. Instead, there are at least two populations of cownose rays: one on the East Coast and one in the Gulf of Mexico.
These populations, known as stocks, are genetically distinct from one another and do not interbreed, says Jan McDowell, VIMS researcher and co-author of the National Oceanic and Atmospheric Administration-funded study. Although industry has argued for a ray fishery in the Chesapeake Bay, overfishing the East Coast stock could risk wiping it out for good.
McDowell explains, “There’s no mixing between stocks. If we hammer the Chesapeake Bay, it isn’t going to be repopulated with rays from the Gulf of Mexico.”
The threat of this possibility supports Bob Fisher’s continued search for non-lethal ways to keep rays out of shellfish beds—especially because the East Coast stock itself could be smaller than originally assumed. Fisher, VIMS extension staff affiliated with Virginia Sea Grant, was the co-author of the study. He and McDowell agree that the East Coast stock might be made up of multiple, smaller stocks.
Fisher says, “There’s a lot of work to be done” before researchers understand the cownose ray population, but he sees investigating this possibility as the next step.
“It’s all about breaking down the question piece by piece,” he says.
To differentiate between the East Coast and Gulf of Mexico stocks, Fisher and colleagues collected tissue samples from the pectoral fins of female and newborn rays in each location. They focused on females, because females are thought to return over and over again to the same nursery grounds. While males have a greater straying range, some males were also analyzed. Genetic differences were found between the two study areas in both males and females.
Repeating this same process in Sandy Hook Bay, Delaware Bay, Chesapeake Bay, and other potential nursery grounds along the East Coast would reveal if there were multiple East Coast stocks. Extensive tagging studies of the more far-ranging males would be the final link in understanding the cownose ray population. This information would be valuable to resource and fishery managers, who depend on informed understanding of species’ population size and role within an ecosystem to make management decisions.
Despite the DNA clues and ideas for future research, McDowell says that the mystery of the cownose ray population size will take a while to solve.
“What this research did was open our eyes to how many unanswered questions there are,” McDowell says. “We’ve just scratched the surface.”
