Male bass in the northeast are developing female reproductive traits, and researchers aren’t sure why.

More than 85 percent of male smallmouth bass tested and 25 percent of male largemouth bass tested have developed female reproductive traits in nineteen wildlife refuges in the northeast, reports New Jersey Public Television. The U.S. Geological Survey tested nearly 300 fish, including some from the Wallkill River, part of which is designated as a national wildlife refuge.

Scientist think that something in the water is affecting the hormones of these fish, though they don’t fully understand the phenomenon. Fred Pinkney, a co-author of the study, said “it resonates with the public when you see things like this that scientists frankly don’t fully understand.”

National Geographic reports that 60 to 100 percent of the smallmouth, depending on the wildlife refuge tested, contained eggs within their testes. This, however, is not an isolated incident. Similar cases have been found in 37 different species in lakes and rivers around the world.

According to Pinkney, this study “is just pointing attention to the widespread nature of this problem.”

This condition is called intersex and, though it’s not fully understood, is associated with man-made chemicals, particularly those found in birth control pills. Researchers suspect that the contaminants in the Wallkill originate from either an upstream water treatment plant or agricultural runoff. Scientists are especially concerned with the appearance of intersex fish in wildlife refuges, which are meant to protect wildlife from pollution.