That trophy largemouth you caught might come from a long line of bass that take the bait.

And in the same pond, there are hundreds of fish you'll never see. You may never see their offspring, either.

A new, 20-year study, led by University of Illinois at Urbana–Champaign ecologist David Philipp, "provides the first direct experimental evidence that vulnerability to angling is a heritable trait," the authors wrote.

But Philipp says the discovery amounts to more than just gee-whiz genetics. Previous research has documented that commercial fishing can change gender ratios and drive down fish size by selecting for the largest. But the new study is the first to suggest recreational fishing can cause evolutionary changes, too.

"This type of selection experiment, which we propose has been going on in all bass lakes since the inception of angling, has the potential to alter, perhaps quite significantly, the behavior and even the life history of individual fish in those populations," the study said.

With the help of anglers, Philipp and his colleagues tagged and released largemouth bass in a state park pond in central Illinois beginning in the mid-1970s. Many fish were caught time and again, they found—up to 16 times in a single year. The researchers drained the pond in the 1980s and discovered that 200 of about 1,700 fish had never been hooked.

From this stock, they have since bred the separate groups of "low-vulnerability" and "high-vulnerability" bass, and through three generations the offspring have stayed true to their parents' susceptibility—or aversion—to getting caught.

The researchers aren't sure which inherited behavior is causing the differences, but they speculate that a wariness of anglers' hooks may be passed on to offspring. And largemouth bass reproductive strategies, combined with angling pressures, might amplify the evolutionary selection.

Female largemouths swim away from their eggs after laying them; it is the male that guards them for their first month of life. The aggressive males are best at protecting their fry from predators—but they also may strike more readily at lures in their territories, making them more vulnerable to being caught.

Most times of the year male and female fish are caught in equal numbers. But during spawning season, males are caught the most. And hanging on to a caught male for longer than a few moments during nesting season could spell death by predation for the fry.

"Anglers may be negatively impacting the populations without knowing it," Philipp says.

He says the findings, published recently in the journal Transactions of the American Fisheries Society, carry lessons for the timing of recreational angling seasons, particularly tournaments, during which bass are kept on boats for up to eight hours before being released.

To maintain healthy bass populations across North America, he says that management agencies need to protect the nesting males during the spawning season (early April through mid-June in Illinois).

Meanwhile, the research team is studying other aspects of the evolutionary implications of largemouth bass behavior.

Because the readily caught fish are more aggressive, "they likely have higher mating success in terms of the number of eggs," Philipp says. But the researchers suspect that those fish often have lower reproductive success, because when they're snared they lose babies. They're curious whether the hook-avoiding fish prove to be worse dads, because they're less aggressive against predators and provide less protection for their young.

Philipp, an angler himself, says "the vast majority of anglers are conservationists" and are likely unaware of their impacts on fish populations.

"Showing anglers that removing nesting males may be creating populations of bass less adept at raising their young," he says, "should help them change how they fish in the future."