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Male Smallmouth Bass Exhibiting "Characteristics of the Opposite Sex," Scientists Say

Male smallmouth bass in and around waters of the Northeast are exhibiting characteristics of both sexes, and scientists are stumped.

A U.S. Geological Survey has shown that 85 percent of male smallmouth bass living in waters in or near national wildlife refuges in the Northeast are now showing characteristics of the opposite sex.

Though this mystery only deepens with study, the strongest suspicion leads to things we wantonly throw down the drains of our homes, runoff from farms, and waste from businesses that add up over time:

  • Prescription drugs
  • Birth control pills
  • Mood altering drugs
  • Chemical pesticides such as atrazine
  • Weed killers with glysophate

These endocrine disruptors are apparently doing some real damage.

With this information in mind, smallmouth bass in Virginia, West Virginia, and Maryland had eggs where their testes should be.

Luke Iwanowicz, a USGS research biologist, said "It is not clear what the specific cause of intersex is in these fish. This study was designed to identify locations that may warrant further investigation."

Iwanowicz said smallmouth bass are "like the canary in the coal mine" Among the 19 wildlife refuges in the bass study were Blackwater and Patuxent in Maryland, Mason Neck and Rappahannock River Valley in Virginia. Also included were Great Swamp in New Jersey, Cherry Valley in Pennsylvania and Moosehorn in Maine. Maybe chief among them is the Chesapeake Bay region.


Although bass were the main issue of the study, the effects have been shown in other animals including mammals. It was also determined that the chemical BPA, which is widely used in plastics manufacturing, had a similar effect on a fish known as the Japanese medaka. Researchers follow the medaka because it reproduces quickly compared to the slower reproducing smallmouth.

The sex-changes in male fish have been documented in some 37 different species. At this time no one single chemical or cause can be identified as responsible for the transformation.


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Male Smallmouth Bass Exhibiting "Characteristics of the Opposite Sex," Scientists Say