A fishing rod and reel aren't just gear for human recreation: they're the tools of evolution. The difference between fish we pull out of lakes or commercial fisheries by their lips and those we leave behind can drive change in entire fish populations. And that change may be for the worse. In at least some species, the fittest fish are the ones that end up on our hooks.
The largemouth bass (Micropterus salmoides) lives all over the United States and is a popular target of recreational anglers. You wouldn't guess it from the bug eyes and mailbox mouth, but M. salmoides males are also doting dads. For several days after the female lays her eggs and before they hatch, the male stays close to the nest. He fans the eggs with his tail and chases off any other creatures who come looking for a snack.
The more aggressively a male largemouth bass attacks intruders near his nest, the more of his offspring are likely to survive. But when fending off threats, a bass doesn't necessarily notice the difference between a hungry fish and a dangling lure. So the fish that guard their nests most aggressively might also be the most likely to bite into a fishhook.
David Sutter, a PhD student at the University of Illinois at Urbana-Champaign, and other researchers used specially bred largemouth bass in artificial ponds to investigate this question. Their fish had been bred into two lines: one that's especially likely to attack a fishhook, and one that's especially unlikely.
(Vulnerability to being caught on a rod and line is heritable, Sutter explains—a kind of personality trait for fish. Largemouth bass were first bred this way starting in the late 1970s. Researchers kept the bass in a reservoir where any fish that got caught were marked and rereleased. Eventually researchers drained the reservoir, revealing some fish with multiple marks and others that had never been caught. These extremes were separated into new ponds where the experiment was repeated. After a few generations of this, the researchers had created one line of fish that was consistently catchable and a second line that was consistently hard to catch.)
Sutter and his colleagues put the two kinds of male bigmouth bass together in a pond, along with ordinary females. Once the fish had spawned, snorkelers visited their nests to count their eggs. By spying on the fish remotely, researchers could observe how attentive the fish dads were to their nests—and how aggressive they were to "intruders," which in this case were hookless fishing lures.
The fish bred for catchability, as expected, attacked the fishing lures more often than other fish. In addition to chasing these imaginary predators away from their eggs, they also spent more time hanging out near the nest and fanning their eggs with their tails. By comparison, the fish bred to ignore fishhooks spent more time away from the nest and didn't bother chasing away intruding lures. Overall, as the authors report in PNAS, the easiest-to-catch largemouth bass are also the best dads.
Hooking one of these fish from a body of water obviously removes him—and his good-dad DNA—from that population's gene pool. It also dooms any eggs he was guarding, which carry those same genes, to become food for roving predators.
There's another way fishing fouls up the next generation of fish. The researchers found that females spent more of their eggs on aggressive males, especially large ones. Apparently the female fish can tell which males will guard a nest well. When these sexy bass are removed from a pond, they leave behind more eggs (and more potential young) than a smaller and less aggressive male would.
All this means that humans can drive the evolution of a whole fish population. Thanks to our fishing lines, male largemouth bass (or species with similar behaviors) can become less aggressive, less capable as parents, and less attractive to females. And, of course, less catchable by us. The authors suspect these changes are already happening in popular angling spots, though it hasn't been studied yet.
Sutter says fisheries might be able to prevent the problem by leaving largemouth bass alone during their spawning season. "It would probably be best to allow fish to successfully reproduce and minimize disturbances," he says—by disturbances he means fishhooks—"especially in areas where fish only have limited opportunities to reproduce." Otherwise, the only fish that are left alive may be the deadbeats.
David A. H. Sutter, Cory D. Suski, David P. Philipp, Thomas Klefoth, David H. Wahl, Petra Kersten, Steven J. Cooke, & Robert Arlinghaus (2012). Recreational ﬁshing selectively captures individuals with the highest ﬁtness potential PNAS : 10.1073/pnas.1212536109 Image: (Not a largemouth bass) by Blake Facey (Flickr)