By Katharine Sucher, Science Writing Intern
You know you’re at the heart of an explosive issue when someone suggests using dynamite as a solution. That’s how one exasperated waterman recently proposed reducing the cownose ray population to Bob Fisher, Virginia Institute of Marine Science (VIMS) extension staff affiliated with Virginia Sea Grant.
For years, cownose rays have been gobbling up oyster crops—and industry wallets have taken a hit. But despite lingering temptations to blow or fish rays out of the water, Fisher says, “Even the industry has mostly realized that a reduction fishery is not an option.”
Now researchers are focusing on non-lethal ways to fortify and keep rays out of oyster beds. In a recent NOAA-funded study, Fisher and VIMS researcher, Roger Mann, covered oyster seed with a layer of extra shell.
This protective armor didn’t hold up against hungry rays, but Mann says, “Even though it didn’t necessarily provide us with the answer that we needed, the problem hasn’t gone away.”
Fisher has been trying to keep rays out of shellfish beds since 2006. That year he tested if magnets could form a protective perimeter around shellfish beds. Like sharks, rays can sense the heartbeat of their prey by detecting weak voltages. Using powerful magnets, Fisher successfully over stimulated this sense and kept rays away from prey in tank experiments. At $370 per four-inch magnet, however, these magnets were too expensive for commercial use. Fisher tested cheaper alternatives, but rays adjusted to the weaker magnetic fields.
Next, Fisher tried using a bubble curtain, a stream of bubbles that creates a visual barrier, to manage ray appetites. Fisher says the curtain worked great, “until the rays figured it out.” After only a few minutes in the tank, hungry rays discovered they could pass through the bubbles and swim to their meal.
Looking forward, Fisher and SharkDefense researcher Eric Stroud hope to field-test a chemical extracted from decomposed ray tissue. According to one theory, this chemical tricks rays into avoiding areas with the chemical smell because rays avoid other dead rays. The experiment was effective in tanks and Fisher thinks it has potential to work just as well in the field.
The chemical is not shown to affect surrounding aquatic life—an important quality of any management solution and one of the main arguments against a reduction fishery. Although they are portrayed as pests, Mann reminds us that cownose rays are a native species and “part of a food chain that’s been around for a long time.”
As Fisher says, “The cownose ray has been a victim of negative attitudes, but it has its place in our ecosystem.”
