Does hunting invasive species make them harder to hunt?

The Indo-Pacific lionfish, as its name implies, is not native to Caribbean waters. It is a dangerous invasive species, threatening various native fish that swim amongst coral reefs, and thus has been targeted for culling in recent years. This is generally considered a good idea: hunt down the thing that doesn’t belong in order to save all that does. But what if those repeated hunts are actually influencing lionfish behavior, making the species harder to find and kill off?

Research published last week in PLoS One suggests just that. Led by a group from Simon Fraser University in British Columbia, the new study compared how lionfish act if they lived in an area where spear-based culling had occurred with fish in areas where no such hunts had taken place. The 16 coral reef patches were all in the Bahamas.

On eight of those patches, divers have been culling the lionfish with “non-projectile, three-prong paralyzer-tip pole spears.” (Which does look about as intense as it sounds.) They have done this at those eight patches each three or six months for two years, and the last of the culls was done three weeks prior to the new study.

The lionfish clearly did learn something about the huge creatures that lunge at them with tridents. On culled reefs, a lower proportion of the fish were active during the day, and they hid themselves much more carefully as well. The investigators assigned a “hiding score” to the fish based on certain behaviors: half of the lionfish on culled reefs achieved the highest such score, compared to only 19 percent of those on the unculled reefs. In this image, it’s pretty easy to tell which lionfish score better on their Avoid the Spearfisherman exam.Lionfish hiding behaviors

This suggests, of course, that if a lionfish survives a cull (the culls achieved success rates ranging from 30 to 100 percent), it becomes more likely to survive the next one as well. It is, in a sense, a very rapid form of natural selection.

Now, such behavior is of course not necessarily limited to invasive species. Anything we hunt, presumably, might alter its behavior to avoid being killed. But it does have implications for how control of invasives is carried out. Culls sometimes set goals of a certain percentage of the invasive species, but if we purposely leave 30 percent of an animal that is causing damage to an ecosystem, it may make it that much harder to get back to 30 percent the next time we give it a shot. This also backs up those invasive species control programs that aim for total annihilation: for example, the leader of an attempt to kill off the invasive brown rat in the sub-Antarctic Island of South Georgia said last year that “killing 99.999 percent is a failure. If we don’t get every last one, we may as well not have gone there in the first place.”

With the lionfish, this study just adds fuel to the idea that aiming for eradication is likely the best approach. In one study from 2012, an increase in lionfish abundance in the Bahamas coincided with a 65 percent drop in the total biomass of the 42 types of fish that it eats. It is likely not possible to completely eradicate the invasive lionfish from Atlantic waters at this point, but plans for individual culls may have to consider the animal’s ability to adapt in order to keep this problem fish’s population down. – Dave Levitan | April 8 2014

Source: Cote IM, Darling ES, Malpica-Cruz L, et al (2014). What Doesn’t Kill You Makes You Wary? Effect of Repeated Culling on the Behavior of an Invasive Predator, PLoS One, 9 (4) e94248. DOI: 10.1371/journal.pone.0094248

Image: Top,, Bildagentur Zoonar, GmbH; Middle, PLoS One, Isabelle Cote