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Cownose rays

The cownose ray (Rhinoptera bonasus) is the most common type of ray found in the Chesapeake Bay. The rays can grow quite large; adult males average about 35 inches in width and weigh about 26 pounds while adult females are slightly larger with 28 inches in width and an average weight of 34 pounds.

Photo of four cow nose rays just under the water surface.Picture: A school of cownose rays in the Chesapeake Bay. Picture courtesy of Mary Hollinger, NODC biologist, NOAA. outside linkTo the original photo.

The cownose rays are benthic feeders, which means they eat bottom-dwelling organisms like oysters and clams. The ray's favorite food source is the soft-shelled clam (Mya arenaria), a common inhabitant of the Chesapeake Bay's muddy and sandy soil.

To eat the clams, cownose rays first flap their wing-like fins against the bottom sediments to expose the clam; then they crush the shell between two strong dental plates in their mouth. This flapping behavior exposes the clams but, unfortunately, also destroys the underwater grass beds where the ray is looking for its food.

Because of this behavior and because the rays also eat many oysters, many people are concerned about their growing number. While their population has been growing, the oyster population has suffered from diseases and pollution has reduced grass beds to only about one-tenth of their historic area.

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Photo of a cow nose ray just under the water surface of the Chesapeake Bay.Picture: A cownose ray near the surface, near the mouth of the Patuxent River. Picture courtesy of Mary Hollinger, NODC biologist, NOAA. outside linkTo the original photo.

 

 

The Virginia Sea Grant Marine Advisory Program has considered commercial fishing as a possible solution to the damage the cownose rays are causing.

Commercial fishing, however, posed so many problems that it has not yet been established. First of all, there is not a market for the rays and though participants in a taste test enjoyed the cownose ray meat, the harvesting and processing of the fish is difficult and would make the fish too expensive to sell.

A fishery for rays could also cause problems. No one really knows how many cownose rays there are, which makes it hard to set any fishing levels. In addition, like all rays and sharks (which are all elasmobranchs, a type of fish whose skeleton is made of cartilage instead of bone) the rays mature relatively late and have low levels of reproduction. Both these characteristics would make them vulnerable to overfishing. Although the cownose rays are causing some problems, they also are a part of the ecosystem.

The presence of the rays in the Bay can also be a good sign. Their increase may mean that they are successful in finding bottom-dwelling organisms as a food source other than the oyster. This would be a good sign for the Bay because these benthic organisms have suffered from poor water quality. Maybe things are getting better.

References and further reading

Most information for this program comes from the article Bay's oysters, SAV fall victim to cownose rays' eating habitsoutside link, by Karl Blankenship. Published in the Bay Journal, Vol. 8, Number 8, November 1998. The Bay Journaloutside link is a publication of the Alliance for the Chesapeake Bayoutside link.

The Virginia Sea Grant Marine Advisory Programoutside link.

Life in the Chesapeake Bay, by Alice Jane Lippson & Robert L. Lippson. The Johns Hopkins University Press, Baltimore and London. 1984. ISBN: 0-8018-3012-5. This illustrated guide to fishes, invertebrates, and plants is an excellent resource for anyone who want to learn more about life in the Chesapeake Bay.