By Janet Krenn
Long before cownose rays were observed disrupting oyster restoration efforts, they likely played an important role in helping oysters spread throughout the Chesapeake Bay.
A study published in April’s “Journal of Shellfish Research” indicates that cownose rays’ mouths aren’t strong enough to crush and eat larger oysters, but this physical limitation doesn’t stop rays from trying. The result? Cownose rays pick up and swim away with large oysters, but eventually drop them after failing to crack the shells open. This behavior could help disperse large, reproductively mature oysters throughout the Bay.
Bob Fisher, Virginia Sea Grant Extension Agent and lead author of the study, says industry and other shellfish growers have confirmed his findings. “I had given multiple talks to industry and oyster gardeners on what I was seeing,” Fisher says. “Some oyster growers started to look at their oysters that remained after the rays and come in and fed, and they say that what I found was true—that their larger oysters weren’t eaten, but were moved away from where they were originally planted.”
Cownose rays are native seasonal residents that migrate in groups into the Chesapeake Bay during summer months. Ever since 2003 when a group of rays was seen descending on an oyster bed and eating all but a few of the newly planted oysters, industry and oyster restoration groups alike have been trying to find ways to keep rays out. According to reports, in a couple of hours those rays ate more than 1 million seed oysters, which were about the size of a fingernail.
Fisher’s study, looking at the ray’s ability to crush oysters of various sizes, came as a response to shellfish growers’ concerns. Fisher began his research hoping to find some clue in how the cownose rays pick up and crush oysters that may help protect the investment by the shellfish industry, which generated more than $3M in oyster sales in 2009.
According to the study, the key to preventing oyster predation is in the shell depth. Shell depth is a measurement of oyster thickness, the distance from highest point of one shell half to the highest point on the other. The thicker the oyster, the less likely a ray can wrap its mouth around the shell and crush it.
“If there was a way to breed oysters so that they would quickly increase their shell depth, more could escape predation,” says Fisher. Until then, “The good news is that you can safely put out broodstock without fear of predation…The bad news, is if you put out unprotected seed, and rays come by, they’re going to get eaten.”
Fisher’s research with cownose rays has included everything from developing seafood products from ray meat to basic biological assessments, such as this. He’s also involved in testing cownose ray repellents. For more information, check out some of Fisher’s reports or look through our YouTube and Flickr galleries below.
[tubepress video=”nit5Jz5H83A”]
Want more? Watch rays use their specialized fins to manipulate oysters as they eat on our YouTube channel.
Want more? Check out our collection of cownose ray photos on Flickr.
