For simple animals that seem to be driven purely by instinct, individual fish can be surprisingly unique. Biologists are learning that fish have what can only be described as personalities, which stay fairly consistent throughout their lives. A recent study from the University of Illinois at Urbana-Champaign is giving sunfish anglers a better idea of how different personality traits might affect a fish’s chances of being caught.


This particular study focused mainly on personality traits that affect how sunfish act in a group. The researchers defined sociability as the tendency of an individual to associate with conspecifics, or members of the same species. When comparing sociability and catch rates, they noticed that the most social sunfish were the ones that were most likely to be caught.


When intrinsically sociable fish group together, they create a dense population that experiences higher levels of competition for food. A separate study from 2011, a joint project from the University of Missouri and the United States Geological Survey, revealed that sunfish suffer from reduced growth rates at high densities. Even when fed to satiation, meaning that each fish could eat as much as they wanted, these sociable sunfish lost valuable energy to the elevated stress and activity inherent in large groups.


To make up for this loss, sociable sunfish eat more than loner sunfish. They are also more likely to approach unfamiliar food sources in an attempt to eat before their buddies figure out there’s food around. Combine these dynamics with the fact that social sunfish are far easier to locate than loner sunfish, and you have a group of fish that is disproportionately caught with a hook and line.


The study went on to explain that this uneven angling pressure may lead to “fisheries-induced evolution.” If personality traits are inheritable in fish, anglers may be unknowingly pushing sunfish populations away from sociability; fewer social fish around to mate means fewer social fish added to the community.


Like many fish, sunfish have evolved to live in groups for very good reasons. More fish means more eyes to spot predators or find food. The trouble with this kind of fisheries-induced evolution is that certain individuals in the sunfish hierarchy are integral to group cohesion. This decreased cohesion could mean loss of social function, interrupted or altered mating behaviors, and lowered nest defense.

When targeting sunfish and other fish that school together for safety, like perch, a good rule of thumb is to return the biggest fish to the water and to harvest only midsize or small individuals. This will lessen the effect angling has on evolution and help to preserve the social functions of fish schools.


For anglers that want more of a challenge, locating loner panfish can be extremely rewarding. Away from the stress of dense groups, these reclusive fish can grow to astronomical sizes.  They have no conspecifics to compete with and they often don’t expend any energy on mating. They’re the fish equivalent of hermit bucks: old, big, and too smart for that mating nonsense. And like those bucks, they’re one of the most exciting trophies a sportsman can target.


Originally published in Outdoor News Minnesota.

M.J. Louison et al. Animal Behaviour 142 (2018) 129-137