Photograph by George Grall, Nat Geo Image Collection
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Bluegill, popular for their taste and ease of catch, may be more likely to get hooked if they are sociable.

Photograph by George Grall, Nat Geo Image Collection

Fishing May Be Making Fish Less Sociable, More Aggressive

By removing animals with certain personality traits, angling could have unforeseen effects on fish populations.

Nice fish may not finish last, but they may be more likely to get caught, new research shows.

The bluegill, a fish found throughout the U.S. Midwest and South, is not terribly difficult to catch and many consider it quite tasty. But not all bluegill are created equal: A paper published recently in Animal Behaviour found that bluegill that more readily take the bait tend to be more sociable, and less aggressive, than those that don’t.

Bluegill are relatively gregarious fish and tend to hang out in groups. The more social individuals play an important role in helping the group find food, locate choice habitat, and avoid predators, says Mike Louison, a behavioral biologist at McKendree University in Lebanon, Illinois, who completed the study as part of his doctoral studies at the University of Illinois.

But if you remove these individuals from the school, “that can lead to dysfunction in the whole group,” Louison says. (Read how diverse personalities help animals survive.)

Several studies in some fish and other animals such as birds and fruit flies suggest that sociability—as well as aggression—has a significant genetic component. If more sociable animals are removed from a population, it stands to reason the population could possibly shift toward being less social.

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“If you’re selectively targeting individuals with certain behavioral traits, the population [may] evolve away from that,” Louison says. Those left behind could be “angry little fish,” he jokes.

That being said, it’s unclear how changeable, or plastic, these behavioral traits are, and whether or not removing such fish would actually have a meaningful impact on the population’s genes, says study co-author Cory Suski, a fisheries biologist at the University of Illinois at Urbana-Champaign and Louison’s former Ph.D. advisor.

Pond Scrum

For the study, Louison and colleagues caught bluegill in a pond in Illinois, in which all the animals had been tagged. Animals that were caught were immediately returned to the water. Later, the pond was drained, and the animals—including uncaptured fish—were then taken alive into the lab.

Next, the researchers conducted a series of experiments with 19 fish that had been caught, and 19 that had evaded capture. They fashioned a large aquarium with two sections, separated by a transparent window. In one side, they placed six randomly selected bluegill from the pond. In the other, they deposited each of the 38 fish, solo.

While there, the lone animal could move freely, either lingering close to the window and “interacting” with the bluegill on the other side, or hide in a shelter that was provided on the opposite end. (Find out how we know that animals think and feel.)

The researchers videotaped each animal’s movement in two different trials, measuring how much time each spent close to the window acting social and how much time it spent hiding or hugging the far end of the tank. The results showed that the fish caught in the pond were indeed more social than the uncaptured fish.

The scientists note that while bluegill more likely to be caught were significantly more social than those that had escaped capture, these social fish were only slightly less aggressive, though an upcoming study provides further support for this correlation.

Why might sociable fish be more likely to get caught? Social fish are more likely to hang out in groups, and if you cast a lure or a baited hook into a pond, you’re more likely to encounter a large school than a solitary individual, Louison reasons.

Competition could also play a role. At the moment a potential food item lands amidst a group, there could be a rush to get at it, even if its identity is unclear, Louison says. “There’s no time for those individuals to sort of think about it,” he says.

But “if you’re a loner, you have time to check it out”—and possibly avoid the bait, Suski adds.

Unintended Consequences

The paper adds to a growing body of work on the many biological impacts of angling. Some work shows, for example, that fish genetically predisposed to grow faster are significantly more likely to be caught. This, in turn, selects for slower-growing fish.

Other work shows that largemouth bass that are less likely to be caught are worse parents. Like bluegill, male bass protect nesting sites, and those which are more likely to defend such spots are more likely to be caught—presumably because they see fishing lures as potential predators, Louison explains. (See pictures of all-star animal dads.)

Though each species has its own idiosyncrasies, results from bluegill may apply to other fish with similar social systems, says Shaun Killen, a biologist at the University of Glasgow who wasn’t involved in the paper.

Next, Louison and colleagues hope to study how this phenomenon might impact fish in the wild, which is as yet unclear.