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Cownose Rays: What Effect Are They Having?

There have been cownose rays in the Chesapeake since before Captain John Smith landed on the Bay’s shores, but recently, they have gotten some bad press. Is it warranted?

Some people think that the population of cownose rays (Rhinoptera bonasus) in the Chesapeake Bay is growing. And they are concerned that these rays—which eat shellfish—may hurt oyster restoration and shellfish aquaculture efforts. But are numbers of cownose rays actually growing, and are they having a significant effect on restoration and aquaculture? And if they are, what—if anything—should we do about it?

cownose ray feeding; photo thanks to Bob Fisher, VIMSScientists around the Bay are doing research to help answer these questions. For example, the NOAA Chesapeake Bay Office (NCBO) has funded research at the Virginia Institute of Marine Science that is helping to develop a better understanding of cownose rays. This information will help resource managers understand the effects of their decisions. The Virginia Institute of Marine Science researched how cownose rays grow and reproduce, as well as what they eat—including whether some things are too big for them to eat.

Researchers found that cownose ray populations are slow growing because they reproduce at low levels. Female rays generally don’t have offspring until they are eight or nine years old and only have one pup at a time. Some people have suggested that a commercial fishery for cownose rays should be started so that people can eat this shellfish predator, instead of it eating oysters. But because cownose ray populations grow slowly, we need to make decisions very carefully. Effects from these decisions will last a long time because if the population is reduced too much, it would take a long time to recover.

School of cownose rays; photo thanks to Bob Fisher, VIMSResearch shows that cownose rays prefer to eat thin-valved shellfish like soft-shell and razor clams, but will also eat oysters and hard clams. But they aren’t able to produce the force needed to crush and eat large oysters. So if restoration efforts and aquaculturists can protect smaller oysters, those oysters could eventually grow to be too big for cownose rays to eat.

NCBO does not currently support developing a fishery or using other methods to reduce the number of cownose rays in the Bay. More science is needed to determine if a sustainable fishery would be possible given the cownose ray’s biology, population numbers and distribution, and reproductive capacity.

In the meantime, focus can be put on developing ways to protect oysters and other shellfish from cownose rays that do not involve removing rays from the Bay ecosystem.