In an experiment spanning over 20 years, researchers at the University of Illinois have found that vulnerability to being caught by anglers is a heritable trait in largemouth bass.
The study began in 1975 with the resident population of bass in Ridge Lake, an experimental study lake in Fox Ridge State Park in Charleston. The fishing was controlled. For example, anglers had to reserve times, and every fish that was caught was put into a live well on the boat. The fish were measured and tagged to keep track of how many times each fish had been caught. All fish were then released.
“We kept track over four years of all of the angling that went on, and we have a total record – there were thousands of captures,” said David Philipp, ecology and conservation researcher at U of I. “Many fish were caught more than once. One fish was caught three times in the first two days, and another was caught 16 times in one year.”
After four years, the pond was drained, and more than 1,700 fish were collected. “Interestingly, about 200 of those fish had never been caught, even though they had been in the lake the entire four years,” Philipp said.
Males and females from the group that had never been caught were designated Low Vulnerability (LV) parents. To produce a line of LV offspring, these parents were allowed to spawn with each other in university research ponds. Similarly, males and females that had been caught four or more times in the study were designated High Vulnerability (HV) parents that were spawned in different ponds to produce a line of HV offspring. The two lines were then marked and raised in common ponds until they were big enough to be fished.
“Controlled fishing experiments clearly showed that the HV offspring were more vulnerable to angling than the LV offspring,” said Philipp.
This selection process was repeated for several generations over the course of the 20 year experiment.
“As we had predicted, vulnerability was a heritable trait,” he said. Philipp went on to explain that with each generation, the difference between lines in angling vulnerability grew even larger.
“Most of the selection is occurring on the LV fish – that is, for the most part, the process is making that line of fish less vulnerable to angling. We actually saw only a small increase in angling vulnerability in the HV line,” Philipp said.
Male bass are the sole caregiver for the offspring. Females lay eggs and leave. The male guards the nest against brood predators for about three to four days before the eggs hatch and another eight to 10 days after they hatch, before they become free-swimming. Even after the baby bass start to swim, the dads stay with them for another three weeks while they feed and grow, protecting them from predators.
Philipp explained that the experiment sped up what’s actually happening in nature. “In the wild, the more vulnerable fish are being preferentially harvested, and as a result the bass population is being directionally selected to become less vulnerable. We selected over three generations, but in the wild the selection is occurring in every generation.