Intersex occurrence in freshwater fishes was evaluated for nine river basins in the United States. Testicular oocytes (predominantly male testes containing female germ cells) were the most pervasive form of intersex observed, even though similar numbers of male (n = 1477) and female (n = 1633) fish were examined. Intersex was found in 3% of the fish collected. The intersex condition was observed in four of the 16 species examined (25%) and in fish from 34 of 111 sites (31%). Intersex was not found in multiple species from the same site but was most prevalent in largemouth bass (Micropterus salmoides; 18% of males) and smallmouth bass (M. dolomieu; 33% of males). The percentage of intersex fish per site was 8–91% for largemouth bass and 14–73% for smallmouth bass.The fact that no IS largemouth bass were found suggests two alternative possibilities: the first, that the IS condition reduces lifespan in this species; the second, that something new has happened in the last 5 years.
Intersex was not found in largemouth bass older than five years and was most common in 1–3-year-old male largemouth bass. The cause(s) of intersex in these species is also unknown, and it remains to be determined whether the intersex we observed in largemouth and smallmouth bass developed during sex differentiation in early life stages, during exposure to environmental factors during adult life stages, or both.
Signs of jumbled hormones that trouble creatures from turtles to alligators have now turned up in a superstar of sportfishing - the largemouth bass.The rates differ markedly geographically, suggesting a variable common to these IS "hotspots". But pollution per se doesn't appear to be responsible. A particular chemical may be. Or some other common environmental factor. And the causal mechanism is unknown, but appears species-specific.
Almost one in five male largemouths from rivers around the country had microscopic egg cells inside their testes, said Jo Ellen Hinck, a biologist at the U.S. Geological Survey who worked on a nine-year project to test fish from more than 100 locations.
Bass were among just a few kinds of fish where the study found those characteristics, which researchers labeled as intersex features.
And though she doesn't know why, Hinck said bass in the Southeast were especially affected.
The Apalachicola River was the only Florida waterway checked. Sixty percent of the bass tested at Blountstown were intersex.
While other scientists have tested obviously polluted rivers for damaged wildlife, this was the most comprehensive fish research yet, said Hinck.
His last research was published last month in the journal Aquatic Toxicology.
And it raised new questions, because bass in remote areas that seemed to have little pollution had the same issues as fish caught in cities.
Science is like that sometimes, more questions than answers.