Cownose Rays

Every summer, large numbers of cownose rays, Rhinoptera bonasus, make Chesapeake Bay their home. The rays migrate up the Atlantic coast from as far south as Brazil, and may eventually reach as far north as Massachusetts. They generally make a stop in the lower Chesapeake Bay in May, remaining until September or October. The Bay provides an important nursery ground, with females often giving birth to pups early in their stay.

As a member of the elasmobranch group, which includes sharks, skates, and rays, cownose rays are supported not by a bony skeleton like most fish, but rather by a network of cartilage. This makes their muscles extremely energy efficient as they flap their wings to swim through the water or to stir up sediment in their hunt for food. Bottom feeders, cownose rays use their strong dental plates to crush through the shells of their favorite prey — oysters, hard clams, and especially the soft clam, Mya arenaria.

Like other rays, the cownose has a sharp spine at the base of its tail laced with a toxic venom. Although potentially deadly, it is highly unusual for humans to be stung.

But people do have something else to worry about when it comes to cownose rays — their appetite for shellfish.

The rays have earned a reputation as voracious consumers, often decimating shellfish beds, including important oyster reefs. In 2004 cownose rays ate nearly $80,000 worth of oysters set on artificial reefs as part of a restoration project by the Army Corps of Engineers in the Great Wicomico River.

This threat to the region’s already troubled shellfish populations has led some managers and watermen to propose ways to winnow the number of rays in the Bay. A market for ray meat does not exist in the U.S., so Virginia seafood marketers have looked to markets abroad, particularly in Asia where shark meat is already popular. So far, the demand doesn’t justify establishing a commercial fishery and the costly harvesting and processing of the fish. Scientists also point out that female rays mature slowly and birth only one pup a year, so there is a real chance for overfishing.

While some proposed a bounty system, managers rejected that option due to the potential for widespread ecological effects, especially given the cownose ray’s habitat range up and down the Atlantic coast.

To protect oyster restoration efforts, some experts have suggested setting juvenile oysters (spat) on shell before putting them in the Bay. This is a tried and true method, one suggested since the early 1980s by the Sea Grant Extension Program and other experienced oyster growers. Once attached to shell, the small oysters are harder for the rays to get.

Although no one knows for sure how many cownose rays visit the Bay, many think their numbers have significantly increased in recent years and are continuing to grow. In an era marked by dwindling fish populations, this could be seen as good news — but there’s a flip side to the story. A recent study by Canadian and U.S. scientists finds that the boom in cownose rays may be due to the loss of their predators in the ecosystem — large sharks like bulls, hammerheads, and tigers. Scientists report that intense fishing for these top predators has caused their populations to drop more than 97 percent along the East Coast over the last several decades.

Cownose rays have taken this opportunity to thrive. And as they continue to feast on shellfish — often tearing up underwater grasses in the process — we learn another lesson on the importance of balance in the ecosystem.

Useful Links

Plan to harvest cownose rays could be recipe for trouble
The Chesapeake Bay Journal, July/August 2007

Bay's oysters, SAV fall victim to cownose rays' eating habits
The Chesapeake Bay Journal, November 1998