And as CBS News national correspondent Dean Reynolds reports, those fish swim where millions get their drinking water.
The fish are biting on Lake Pepin this fall, and that's good news for Minnesota fishing guide Loren Waalkens.
Many of Waalkens' customers catch small mouth bass, which, along with their large-mouth cousins, are big business for fishermen and fishing guides. They've hooked anglers at tournaments as hyped as the Super Bowl.
The small mouth is also a fish of special value to researchers who suspect it may tell us something alarming about our water.
In Columbia, Mo., the U.S. Geological Survey is keeping smallies in some artificial ponds, investigating why so many males are showing female characteristics.
"Because it's male, you're seeing sperm here and here," said USGS Dianna Papoulias while examining a fish. "But oddly, you're also seeing eggs. Small, undeveloped eggs."
"It is an abnormality," she said. "In bass we would not expect to see eggs in a male."
Abnormal - but increasingly common. In the upper Mississippi River where Loren Waalkens fishes, more than 70 percent of the male smallmouth bass had female characteristics.
In South Carolina's Peedee River, the ratio was even higher - 9 out of 10.
And in one section of the Potomac River near Washington, every smallmouth bass had the same condition.
In fact, a recent USGS study found the phenomenon in virtually every watershed in the country. And it's and not just bass. Some carp, catfish and sturgeon have the same odd make-up.
The suspicion is that hormone-disrupting chemicals in the water - pesticides, pharmaceuticals including birth control pills, or even household detergents - may be prompting the feminization of the fish.
And that matters because in controlled experiments like those in Columbia, which duplicated the chemicals found in U.S. rivers, entire populations of fish simply collapsed, unable to spawn.
What's more, tens of millions of Americans get their drinking water from rivers - an estimated 18 million from the Mississippi river alone.
Waalkens wonders if his beloved bass could be the proverbial canary in the coal mine.
"Are there other species and other types of animals that this may be occurring in?" he said. "You know, there's a lot of unanswered questions."
Those questions' answers may lie somewhere beneath the surface of the water we drink.