The world’s most serious marine invasion began more than a decade ago when lionfish populations exploded in the Atlantic. Scientists are still figuring out what we’re up against and how we can protect our reefs.
There was very little out of the ordinary when the DreadKnot departed Destin, Florida, on the morning of July 8, 2017. As usual, the 28-foot charter boat was carrying a small group of divers to an artificial reef several nautical miles offshore. Their dive gear was attached to scuba tanks and strapped into racks. Aboard were the captain, his brother, a lab assistant and Kristen Dahl, a PhD candidate at the University of Florida. Dahl studies lionfish, the common name for invasive scorpionfish that are disrupting marine ecosystems from Rhode Island to Brazil.
Lionfish can eat almost anything that fits in their stomach and they have no known major predators. Females can spawn two million larvae per year and larvae can ride ocean currents for nearly a month — sometimes traveling great distances and invading far-off reefs. Their rapid population growth and voracious appetite are so concerning that a committee of biologists named the lionfish invasion one of the ten greatest threats to global biodiversity in a report published in 2010.
Dahl was holding onto the only unusual item on the DreadKnot — a dry box containing 25 small black cylinders nestled into a pink foam casing.
“The cylinders are half the size of my pinkie finger,” she said. “And they cost between $500 and $900 a piece.”
For two days last summer, Dahl and her team traveled from reef to reef, catching lionfish and anchoring the small acoustic transmitters into the muscle behind their shoulders. The transmitters are part of an acoustic location monitoring system. When Dahl returns to collect her data, she’ll know exactly where each lionfish was, every second, for up to a year.
This study began the same way a lot of science begins: Dahl observed something she didn’t expect and wanted to figure out what was going on.
“We’d been finding odd things in their stomachs — fish and crustaceans from sandy bottoms — which is different from what other researchers had found,” she said. They also discovered large adults on isolated reefs that had been culled just a week prior. This contradicted the widely held belief that lionfish spend their entire adult lives very close to their homes. While it might seem like a small point, beliefs about how they move underpin ideas about how they mate, hunt and migrate. They also have massive implications for lionfish management. Dahl is not the first to dive into this topic — other researchers have seen large adults swim as far as 656 feet (200 m) from their native reefs — but hers is the largest project to date.
At first, recreational divers and underwater photographers were delighted to see the frilly little fish. From the first verified sighting in 1985 until the mid-2000s, lionfish were just one among many exotic fishes living off Florida.
“Because of aquarium releases, dozens of Indo-Pacific reef fishes can be found on Florida reefs,” according to Mark Hixon, an ecologist at the University of Hawaii, “but only one or a few of each species.” Almost all of them live unimportant lives.
“It’s certainly not a given that an exotic fish will become common,” he said. Most do not, “but a number of factors have caused the lionfish invasion to become one of the most significant marine invasions we’ve ever seen.” Researchers are not sure exactly why lionfish populations exploded when they did, but it appears that other environmental problems — such as the overfishing of large predators — paved the way for the lionfish invasion. Still today, lionfish numbers are lower where grouper populations are healthy.
Exotic reef fish are concentrated in the waters off Miami, a hub in the global aquarium fish trade. It is for good reason that lionfish are among the species most frequently imported from the Pacific. Their bright stripes and gentle movements are mesmerizing. They glide through the water, swaying their slim bodies and holding their fins firm but not stiff. Compared to the Caribbean’s hyperactive prey fish and lethargic predators, the lionfish looks confident — almost cocky — and absolutely out of place.
The world’s oceans changed dramatically about three million years ago when the Panamanian Isthmus emerged from the seafloor, uniting the Americas and closing off an ancient ocean current that once joined the world’s oceans. The lionfish is evidence of the very different paths the two oceans took. In the Pacific, glaciers, volcanoes and shifting tectonic plates caused seas to emerge and disappear, to isolate and re-connect. At times, shifting seas closed off ecosystems, creating new conditions that allowed species to change and emerge. Over the generations, this tumultuous geology created the tremendous biodiversity that still exists in the Indo-Pacific.
Life in the Atlantic has been more relaxed. While Atlantic ecosystems are exquisitely complex, their native species are unaccustomed to many of the behaviors and abilities their cousins evolved to survive the crucible of the Pacific.
Lionfish stand out in the eyes of would-be predators, yet they are invisible to potential prey. Their stripes, spots and frilly fins warn sharks and grouper not to try anything and the venomous spines that run along their backs and bellies protect the fish from predators — and divers — who don’t take the hint.
As predators, lionfish are nearly undetectable. They swim slowly toward prey or float motionless and wait for prey to approach them.
“It’s actually hard to describe how a lionfish eats because they do it in a split second,” Dahl said. But scientists know from watching slow-motion footage that lionfish use a complex series of tactics that no other fish in the world is known to employ. In one moment they are hovering above prey. In the space of an instant, a lionfish flares its fins, fires a disorienting jet of water from its mouth, inhales its prey and returns to a puttering float. The attacks happen so quickly that nearby fish don’t seem to notice.
“It’s actually nice when I’m looking at gut contents,” Dahl said, “because if something has been freshly eaten, it’s in immaculate condition.” One stomach she looked at contained three squid, tentacles and all. Another had six whole arrow crabs. True generalists, lionfish are sometimes found with stomachs full of rocks or smaller lionfish.
Lionfish are fast and powerful, but their biggest hunting advantage is novelty. Atlantic prey fish simply don’t know what’s going on. Biologists call this phenomenon prey naïveté and they believe it is largely responsible for the lionfish’s unprecedented success in the Atlantic.
“When an invasive species is successful, it’s always because of an absence of a coevolutionary history with the native species,” said Dan Simberloff, an ecologist at the University of Tennessee and a giant in the field of biological invasions. The prey fish that evolved alongside lionfish know to avoid them. The Atlantic fish are surprisingly unaware of the danger they pose.
“We’ve watched lionfish swim up slowly to a school of wrasse and seen them pick the prey off, one by one, until they were all gone,” Hixon said.
Since arriving in the Atlantic, invasive lionfish have begun to diverge from their Pacific siblings. With better diets, almost no parasites and few worthy competitors, invasive lionfish are growing bigger. Divers have also reported some becoming wary of humans and aggressive toward divers. The most worrisome change was reported last year by geneticists who suspect the species collectively known as lionfish have begun interbreeding and creating hybrids better suited to life in the Atlantic.
Conservation biologists and wildlife managers have used extreme measures to combat invasive species. When the ash whitefly invaded California in the 1980s and 1990s, the state released a European wasp that feeds on whitefly nymphs. Invasive carp have proven so intractable that managers in the United States have resorted to draining lakes or using rotenone to kill everything and rebuild freshwater ecosystems.
“Each introduced species is unique,” Simberloff said, “and the biology of the lionfish leaves few good management options.”
The best available option is removing lionfish from reefs. While humans have developed many techniques for large-scale fishing, lionfish’s food and habitat preferences leave them invulnerable to most traditional fishing methods. But fortunately for divers, lionfish are so well protected by their spines and stay-away-from-me stripes that they have little fear of dark figures swimming above them.
Divers are extremely effective at harvesting lionfish. YouTube is full of videos showing divers picking them off, much like Mark Hixon has seen lionfish picking off wrasses and gobies. Last year, a three-woman team diving off Pensacola harvested 926 in 24 hours. And, at least for now, divers are the most effective weapon in the fight against lionfish.
Lionfish management is a patchwork. The invasive range spans millions of square miles and includes the territorial waters of more than 30 countries. In the United States, at least a dozen federal agencies have jurisdiction over different regions and different types of life, and responsibility for state waters is often spread across multiple agencies.
While there is no unified management strategy, most lionfish control programs are aimed at getting divers in the water. In Florida, for example, the Fish and Wildlife Conservation (FWC) has a half-dozen incentive programs that offer everything from tshirts to trucks to cash to entice divers to get in the water and harvest lionfish.
The derby is one of the most successful management tools today. At a derby, spearfishing divers spend a day working together to remove as many lionfish as they can. At the larger derbies, organizers award prizes to the teams or individuals who catch the biggest, smallest or most lionfish. Derbies feel like festivals and serve as massive public education events with games for kid, workshops and cooking demonstrations.
“The derbies are a good opportunity to educate people about the lionfish and about the danger of releasing aquarium fish into the wild,” Dahl said. She’s worked and volunteered at dozens of derbies. “If enough people learn about this invasion, maybe there won’t be another lionfish.”
Given how quickly lionfish reproduce, some scientists have questioned whether removing them by the hundreds or the thousands has any significant effect. It has recently become clear that divers can remove them from particular reefs faster than they can repopulate, but those studies also show the limitations of diving as a management tool.
Scientists are paying a lot of attention to lionfish — over 150 studies have been published in the last five years — and many of their findings have important implications for managing the invasion. For one, they have learned that they are not extremely detrimental until their numbers reach a certain threshold. A single lionfish can eat a lot of prey, but they wreak havoc on a reef only when their populations reach a certain density, meaning that keeping lionfish populations low is almost as beneficial as eliminating them from a reef. Another important finding is that after most individuals on a reef have been removed, lionfish are slower to repopulate the reef than scientists had once feared. Removing most lionfish three or four times per year seems to do the trick.
Stephanie Green, a biologist at Stanford, studies how lionfish repopulate reefs after removal.
“I like the analogy of weeding a garden,” she said “You’re certainly never going to eradicate weeds to the point that they’re not coming back — but you can keep them from taking over your garden.”
Constant pressure by divers and fishermen can also help keep reefs clean. At a handful of popular dive sites in the Florida Keys, recreational divers are so diligent in culling invasives that it is unusual ever to see a lionfish.
Working constantly to keep lionfish populations low on a handful of reefs is far from eradication, but it’s a small victory. The divers who remove them from reefs are relieving a significant pressure from ecosystems and providing small oases for fish and crustaceans. But events and incentive programs are expensive and many populations are beyond recreational divers’ reach.
Lionfish: Eat ’em to Beat ’em
Most conservationists think the future of lionfish control lies with commercial fishermen. Even if they do not turn the it into the next Chilean sea bass, the idea is that fishermen can put enough pressure on lionfish to relieve pressure on other fishes and crustaceans. And the fishermen are already joining in. Though reliable statistics are hard to come by, the Florida lionfish market is already large enough to supply hundreds of restaurants and dozens of grocery stores. Kristen Dahl even had to move her study site from Pensacola when fishermen started affecting the population she was studying.
Lionfish has yet to become a thriving fishery. While selling lionfish is a great bonus for recreational divers and some commercial fishermen who double-dip (as intended) by selling their catch and claiming incentives from government programs, margins are too small to attract many large-scale fishing operations. Several agencies and organizations have launched marketing campaigns to increase demand and drive up prices.
Following in the steps of food companies, agencies and organizations write recipes, dispel rumors and work with chefs and restaurants to get lionfish on menus. At the same time, many groups are working behind the scenes to ensure that restaurants and home cooks will be able to buy lionfish when they want. Some organizations in the Caribbean are training fishermen to handle lionfish safely and working with governments to encourage them to harvest lionfish. FWC helps buyers and sellers by maintaining a list of seafood distributors that trade in lionfish and this year the University of West Florida is offering a business course on marketing the fish.
The other big economic problem is that lionfish are expensive to catch. They cannot be easily caught using traditional fishing techniques and huge populations live far offshore and below recreational diving depths. While research has shown that independent fishermen in the Caribbean will be able to turn a profit by selling lionfish to their neighbors and local restaurants, commercial operations that fish at much larger scales will have to find new ways to capture a lot of them quickly and at deeper depths.
Scientists don’t know enough about lionfish reproduction to know whether wholesale fishing can effectively control the population, but FWC is offering grants to encourage entrepreneurs to develop new ways to harvest them from deep waters with traps and remotely operated underwater vehicles (ROVs).
Most of the trap designs in development attract lionfish by simulating habitat rather than offering bait, though one group of researchers hopes to exploit the recent discovery that they are attracted to certain high-frequency sounds. The ROVs simulate divers by puttering up to lionfish and sucking them into chambers or stabbing them with onboard spears. One company hopes to use artificial intelligence to allow its ROVs to hunt lionfish autonomously.
But some biologists fear that a market would cement the lionfish into the cultures and economies in its invasive range. Dan Simberloff is a particularly vociferous critic.
“There’s very little evidence that creating a marketplace has ever helped alleviate the effect of an introduced species,” he said. In the 1990s, Louisiana tried and failed to encourage people to eat nutria, an invasive mammal. The same thing happened with kudzu.
One concern is that commercial markets might not do very much to control the population. In order to have regional effects on ecology, markets must encourage divers and fishermen to remove lionfish more quickly than they can be replaced. Scientists know that the vast majority of larvae don’t make it to adulthood, but they don’t know how quickly adults or juveniles have to be removed to outpace replacement. A fishery will not have an ecologically meaningful effect if fishermen catch fish only to make room for those that would otherwise have died.
Concerned scientists are also worried what a mature lionfish market might look like. While most of today’s lionfish entrepreneurs are dedicated to conservation, most established fisheries are dominated by large commercial fishing operations that make decisions based on profits, not passion.
“If there’s a market for lionfish, then it’s in the interest of the fishermen not to eliminate lionfish or to bring it below a level where they are easy to catch,” Simberloff said. He and other biologists fear that once companies make significant capital investments, they’ll take steps to protect the market, such as by lobbying regulators to manage lionfish stocks like traditional fisheries. In Chile and Argentina, for example, some ecologically detrimental types of trout and deer have become so commercially important that they are managed like native fish and game.
They also fear that lionfish will become so culturally important that future attempts to suppress or eradicate the species will be met with resistance. In Hawaii, for example, invasive wild boar threaten endangered plants and public health, but many Hawaiians strongly oppose efforts to eradicate them.
An Unnatural Future?
Humans have long used selective breeding to manipulate DNA and create new forms of life, such as wheat and the beagle. Scientists have recently learned how to manipulate DNA directly, potentially spelling the end of lionfish in the Atlantic.
In 2012, researchers unveiled a tool called CRISPR that lets scientists cut and paste sections of DNA. For species with genes that are well understood, CRISPR gives scientists the power to change physical traits and even some behaviors. The end result is the same as selective breeding, but the process is faster and more precise. Scientists are already using CRISPR to make drought-resistant crops and to develop new antibiotics.
CRISPR is well suited for cases where scientists control which individuals breed. But out of the laboratory and in the wild, individuals that carry CRISPR-altered genes have to compete with unmodified species in order to survive and reproduce. And as it happens, modified organisms are often less suited for survival than their natural cousins. In order to help CRISPR genes spread in wild populations, scientists have developed a second molecular tool that grants modified organisms a reproductive edge. It’s called gene drive technology and many scientists believe it has the potential to change the future of biological invasions.
Under normal circumstances, only fifty percent of an individual’s offspring inherit a particular gene. Genes are diluted so quickly that only about three percent of an individual’s great, great, great grandchildren will inherit any particular gene. While CRISPR is a powerful tool, most modifications slowly disappear through the generations. But gene drive technology changes everything.
A gene drive interferes with egg and sperm development to ensure that every descendent will inherit a particular gene, causing that gene to spread quickly through a population. Scientists are figuring out how to use this edge to shrink lionfish populations. Kirsten Vacura, a graduate student at Ball State University, recently modeled one possibility. In her scenario, lab-raised lionfish were modified with a CRISPR/gene drive treatment that caused infertility in some female descendants. “As the CRISPR-affected gene spreads through the population, a greater proportion of fish in the population are infertile females and fewer are wild type,” she said. Over time — in the case of her model, about 30 years — lionfish numbers fall below a level where they are environmentally destructive.
CRISPR/gene drive lionfish will not be released into the wild anytime soon. The technology is simply too new. Some fear the technology will be too effective. They include the Pentagon, who fears that enemies could use the technologies to affect agriculture or engineer deadly mosquitoes, and scientists, who fear that efforts to control invasive species could wipe out species in their native range. On the other hand, it’s far from clear that the technologies will work at all. Stephanie Green consulted on Vacura’s project. “There’s a lot of work to be done before we can know whether this is feasible,” she said, “such as overcoming the surprisingly difficult challenge of raising lionfish in captivity.”
The New Normal?
This summer, Kristen Dahl and her team will ride back out to her study site and recover the 30 receiver units that have been tracking the transmitters for the past year. Once the data have been cleaned and processed, Dahl will know how exactly her lionfish traveled around and among artificial reefs in the northern Gulf, giving scientists and wildlife managers better information to plan culls and derbies.
No matter what divers, scientists, managers and fishermen do, lionfish will eventually find a balance in their new ecosystems, a state ecologists call homeostasis.
“There should be a point where lionfish reach carrying capacity,” Dahl said. They might be limited by the amount of food or space. “Perhaps native predators, parasites or pathogens will bring lionfish into a population check.”
It is impossible to say what the new normal will look like or whether it will be something we can live with.
In the worst case, lionfish become so numerous and eat so much that ecosystems collapse under the weight of severed ecological relationships. They eat away entire populations of small reef fish and add further pressure on grouper, snapper and goatfish by eating juveniles. Free from gnawing parrotfish, carpets of algae smother coral. The most prominent fish are lionfish, of course, and species they do not threaten, such as sharks, rays and pufferfish.
The best case isn’t rosy — but there is reason to hope that ecologists have overestimated the damage lionfish will do: a study published last fall showed that populations on dozens of reefs in the Bahamas may be in decline.