Bay’s oysters, SAV fall victim to cownose rays’ eating habits
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Already reeling from diseases, oysters remaining in the Bay are being picked off by winged predators that flock into the Chesapeake each summer to snack on shellfish.
The creatures are cownose rays, sometimes called flat sharks, which are perhaps better known for their spiny tails — which can inflict a venomous sting — than for their appetite.
“It seems cownose rays are everywhere, and they’re very big shellfish predators,” said James Wesson, who is in charge of oyster restoration efforts for the Virginia Marine Resources Commission. “They’re a problem that we’re going to have to deal with.”
Cownose rays — the most common type of ray found in the Bay — are problems for more than just oysters. Many shellfish thrive in underwater grass beds which the rays rip to pieces as they flap their wings to expose clams buried in the sediment.
“If a school of rays come in, the area looks like a bomb field,” said Bob Orth, of the Virginia Institute of Marine Science. “There are craters everywhere, and we can see large mats of uprooted grass floating around.”
Conflicts between rays and other resources are not new. As far back as 1815, one Bay resident wrote that cownose rays “are detested by the people who live near the shores, by reason of the damage they do the clams.” In the mid 1970s, a sea grass bed where Orth was conducting research was obliterated when a school of rays descended on it.
Concern that the rays are causing problems for oysters and grasses — critical species that have been targeted for recovery efforts by the Bay Program — has spurred the creation of a study group by the Virginia Sea Grant Program Marine Advisory Program at VIMS to figure out what, if anything, can be done about it. It will be the second time that it has looked at the problem.
No accurate estimate of the cownose ray population is kept. Rays have always been in the Bay, but in the past, their population may have been kept in check because they were often caught in nets targeting other fish, according to a 1979 report by VIMS and Virginia Sea Grant.
In fact, cownose rays in the early part of this century were considered “very rare,” the report said. But the use of pound nets, which caught many rays, peaked in Virginia at 2,262 nets in 1930 and have declined to a fraction of that number. The report also said a slight increase in ocean temperatures may also have caused a greater proportion of the coastal cownose ray population to swim northward into the Chesapeake.
But as the cownose ray population grew, the oyster population has been devastated by disease, and pollution has reduced grass beds to only about one-tenth of their historic area. That essentially means that schools of rays may be zeroing in on what little remains.
“It’s not like the cownose ray has a lot of choices of stuff to eat out there,” Wesson noted.
And they can have voracious appetites. Cownose rays are, after all, relatively large. Adult males average about 35 inches in width and weigh about 26 pounds, with females being slightly larger, averaging about 38 inches in width and 34 pounds. At one aquaculture project, Wesson said, a school of cownose rays ate 60,000 oysters in a single night.
Aquaculture oysters, which typically are grown on trays in the water, can be protected by nets. Wesson’s main concern is that newly constructed oyster reefs and other areas “seeded” with young oysters are vulnerable to predation by the rays. As they flap their wings, Wesson said, the rays often leave any oysters they don’t eat buried in silt.
“We have tremendous amounts of money invested in these oyster restoration and seagrass restoration projects, and the potential cownose rays hold for negating those efforts is really large,” Wesson said. “I think, potentially, they are as big of a problem for us as diseases in getting oyster restoration projects going.”
But figuring out what can be done about the rays is a problem itself. The issue has been studied on and off since the late 1970s as the number of rays — and concerns about them — appears to have grown.
A potential solution is to establish a fishery for cownose rays.
The 1979 Sea Grant/VIMS report suggested that a sportfishing program targeting cownose rays could be implemented, complete with a cownose ray derby and the inclusion of the species in saltfishing tournaments. Persuading people to catch the stinging rays has yet to catch on.
The wings of cownose rays are high in protein and low in fat, raising the potential for a commercial fishery. During a taste test conducted for a 1991 Sea Grant study, many people responded favorably to cownose ray meat. But the study found the processing costs for cownose rays were high — $1.26 a pound — especially for a product with no proven market.
“If it were some kind of of gourmet item, that would be all right, but it’s not,” said William DuPaul, coordinator of the Virginia Sea Grant Marine Advisory Program.
DuPaul said the Bay is not the only place where people have searched, without luck, for a use for cownose rays. “People have worked them in North Carolina, and down in the Gulf they’ve checked it,” DuPaul said. “The point is, it’s a very difficult animal to work with, both harvesting and processing. So it’s not going to be a cheap product.”
DuPaul is hoping that the most recent effort to review the issue, which could kick off in November, will generate some new ideas. “We have nothing particular in mind,” DuPaul said. “But we do have an obligation to rethink the situation because it is obviously a problem.”
Ironically, creating a fishery for rays could create its own set of problems — particularly for the rays. Cownose rays are an elasmobranch — a type of non-bony fish, which includes sharks, whose skeletons are made of cartilage.
Like others in that group, cownose rays mature relatively late, usually between the ages of 5 and 8, and have relatively low levels of reproduction — something that could make them vulnerable to overfishing.
Setting fishing levels would be difficult because no one knows how many cownose rays there are. Because there is no fishery for them, no assessment has ever been made of the overall stock. “We don’t know how much of the population comes into the Bay,” DuPaul said.
Rays are thought to winter off the coast of South America, then migrate north beginning in early March.
In early April, huge schools consisting of thousands of cownose rays are seen passing by Cape Lookout on the North Carolina coast.
These schools fragment, going to different places along the East Coast, with a sizable population reaching the Chesapeake Bay by early May. Schools further fragment, now consisting of anywhere from a few dozen to several hundred individuals each, as they swim up various Bay tributaries in mid to late May.
But, some note, the presence of rays is not entirely bad news. “They might be a symptom that the Bay is getting better, for all we know,” said Steve Jordan, a biologist with the Maryland Department of Natural Resources who heads the Oxford Cooperative Laboratory.
Rays, after all, are benthic feeders, and bottom-dwelling organisms have been among those most stressed by the Bay’s poor water quality in the past. If the Bay is, in fact, supporting more rays, it could be a sign that they are finding more benthic organisms in general — not just oysters — on which to feed.
“Some may say we need more of that kind of stuff, not less,” Jordan said. “You can’t have lots of everything you like and nothing you don’t like. It doesn’t work that way.”
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