Ever given up on a favorite fishing spot because they're not biting anymore? Maybe it's just "fished out"?

A recent study by the University of Illinois offers an equally logical but more humbling explanation: they may just be avoiding you.

After studying controlled largemouth bass populations in a 20-year experiment, researchers determined that the genetic makeup of an individual bass influences its likelihood of ending up on your line. In other words, if a fish's parents were caught by anglers, it will probably be plucked from its habitat one day as well.

That being the case, Darwin's "survival of the fittest" concept favors more passive bass in heavily-fished areas -- those that don't have the genetic moxie to attack your bait.

The experiment was engineered to simulate a natural selection process that occurs in the wild, when bodies of water experience frequent and prolonged human fishing activity.

"We were mimicking what's happening in lakes when fish are constantly harvested," said David Philipp, one of the study's leading researchers. "People go to a lake and say 'fishing used to be great here and now it's difficult,' or 'fishing was better here years ago.' The reality is not that there were better fish, but different fish."

Starting in 1975, researchers monitored bass that inhabited an experimental study lake in Charleston's Fox Ridge State Park. Anglers were invited to fish there if they reserved four-hour time slots and dumped all catches into a livewell where the bass could be tagged and re-released. Scientists marked all apprehended fish with fin clips or notches in their dorsal spines, ensuring that they could be easily identified years into the future.

After four years, scientists drained the lake and inspected more than 1,700 bass for the number of times they were reeled in. Interestingly enough, some fish had never been caught within the entire four-year period while others had been hooked multiple times.

Bass that had escaped tagging were identified as "low vulnerability" (LV), and those caught at least four times were dubbed "high vulnerability" (HV). The two categories were then placed in ponds where they could only breed among themselves.

After the distinct sets of offspring had matured, they were moved to common ponds where they became susceptible to angling.

The study's results, published in the journal "Transactions of the American Fisheries Society," revealed that offspring from LV parents were measurably less susceptible to the lure and showed a dramatic decrease in vulnerability. While the HV offspring were not caught decidedly more than their predecessors, they did maintain a tendency to be much easier to attract with fishing lures.

"What we found is that long-term recreational fishing can have an impact on fish populations," said Philipp. "If we're harvesting fish, we're changing their behavior."

He explained that evolution in heavily fished ponds favors smaller, non-aggressive inhabitants that mature earlier and are "less prone to taking risks." Larger, bolder, and faster-maturing fish are often removed before they can mate, which also removes their traits from the gene pool.

Rick Lorsen, area fisheries manager for the Pennsylvania Fish and Boating Commission, said it's biologically important to maintain genetic variety in fish species.

"We don't want to have a population out there with only slower-growing, less aggressive fish," he said. "Catch rates would be down, and you'd end up with potential survival problems for the whole population."

Philipp agreed.

"We may be impacting bass in a way that is not long-term beneficial to their reproductive success," he said. Since male bass are responsible for guarding their young, capturing an aggressive male leaves his entire brood vulnerable to predators. And a suffering bass population can jeopardize the entire ecosystem.

"If bass populations are decimated, it would have ramifications down the predatory line," he said. "Other species will move in to fill the niche, and the ecosystem will reach a new equilibrium. We can't just walk away when that happens -- it may take too long rebound."

Lorsen suggests "selective harvest" as a solution, which allows humans to favorably influence bass natural selection.

"You would release some of the larger fish to keep them in the gene pool," he said. "It provides a [genetic] mix for the fish that are reproducing. You end up having the larger, faster-growing genetic material remain in populations."

In addition to calling for mandatory catch-and-release regulations (which are critical during the spawning season), Philipp also proposed that hard-hit places use "spawning sanctuaries" as "a good way to guarantee reproductive success." As special breeding areas closed to fishing, they generate offspring that can be exported elsewhere to replenish dwindling or genetically static populations.

Calling himself a "devoted bass fisherman," Philipp emphasized that he does not want his research to "close opportunities" for other anglers or convince them to quit the sport altogether. Instead, he wants to make sure that such opportunities will continue to exist in the future.

"What we're trying to do is maintain the long-term health of the population for our grandchildren," he said.