Hurricanes sent Florida lionfish swimming for the Bahamas

The venomous Indo-Pacific lionfish is a beauty, but the fearsome predators are turning the marine ecosystems off the Florida coast and in the Caribbean upside down. In their native range, the lionfish’s bite is controlled because it’s got its own predators to worry about. But in the Atlantic Ocean and Caribbean Sea they swim about, perturbed only occasionally by the humans who put them there in the first place. It isn’t clear just what ecological ramifications the lionfish invasion will have, but it probably won’t be good. They gobble up all sorts of fish and crustaceans, including the economically important snappers and groupers.

Lionfish were first noticed in South Florida’s waters in 1985, and by 2004 they began turning up across the Florida Straits in the Bahamas. The puzzling thing is that adult lionfish are fairly sedentary; they don’t migrate from reef to reef. In their larval stages the fish can disperse, but they would likely do so along ocean currents. As drifters, they wouldn’t be able to navigate clear across the northerly current sweeping through the Florida Straits towards the eastern seaboard. And genetic data rules out the possibility that the two populations in Florida and in the Bahamas represent two separate introductions. Somehow, the two populations have constructed a genetic bridge across the sea.

Nova Southeastern University’s National Coral Reef Institute researchers Matthew W. Johnston and Sam J. Purkis think they know how: hurricanes. They believe that intense storms (for these purposes, that means hurricanes in addition to named tropical storms) are powerful enough to alter the ocean’s currents enough to divert lionfish larvae from the Florida coast across to the Bahamas. Strong storms don’t just alter the course of surface water: in 2004, a category 3 storm called Hurricane Ivan altered the flow of water as deep as 84 meters. As the crow flies (or, uh, as the lionfish swims) the shortest possible distance from southern Florida to the Bahamas is 87 kilometers. Water flowing eastward will allow a floating critter to make that journey in as little as 24 hours, and the water during Hurricane Ivan flowed more than twice that fast.

To test their idea, the researchers used wind and current information to create computerized simulations of water flow. In general, water flows in the direction of the wind above it. In the Northern Atlantic, storms rotate counter-clockwise. That means that any storm whose eye is north of the Florida Straight could deflect the water towards the east. Their simulation confirmed that storms passing close to that area would indeed open up windows for lionfish larvae to flow east from Florida to the Bahamas. Real-world hurricane data suggests that those windows could remain open as long as ten days. And that, write the researchers, is “ample time” for lionfish larvae to traverse that distance in less than a day.

Between 1992 and 2003, thirteen storms that passed through the area could possibly have opened what the researchers call “transport windows” for lionfish larvae. Of those, six had trajectories that would have shifted high-velocity water off the Florida coast towards the Bahamas: Erin, Mitch, Harvey, Irene, Gabrielle, and Michelle. The years 1998 and 1999 alone hosted a series of such storms in such rapid succession that those years alone could have been responsible for the initial establishment of lionfish in the Bahamas. Given that lionfish probably occupy an ecosystem for several years before they are formally reported, the researchers suspect that sometime between 1998 and 1999 is when the lionfish successfully breached the Florida current.

After they had made their way to the Bahamas, additional storms would have facilitated the growth of the population, spreading the fish both farther east and farther south. For that, based on natural history data and their computer simulations, the researchers blame the 2004-2005 storm season for spread the invasion to the Turks and Caicos and to the island of Hispaniola.

The lionfish are just another in a long list of invasive species aided by storms in the Caribbean and Gulf of Mexico – African locusts, cheesewood trees, and Asian soybean rust in particular – but they’re the first marine species we know of to show storm-mediated dispersal.

USGS records report 33 invasive marine fish and one crustacean off the Florida coast. Most concerning is perhaps the Asian tiger shrimp, an introduced species first reported in 2011. That the critters have already established breeding populations in the Atlantic and Gulf of Mexico suggests that they could become invasive. While the shrimp have not yet been reported in the Bahamas, they reproduce just like the lionfish, which means that strong storms could help them spread farther as well. If they do, the consequences to native ecosystems could be severe, as the Asian tiger shrimp are voracious predators of deep-sea invertebrates and compete with the commercially important pink shrimp.

Warming oceans are poised to become even more storm-intensive in that part of the world, which could even further exacerbate the consequences of marine invaders. “It is disconcerting that more intense storms may become a common transport mechanism between Florida and the Bahamas for non-native species residing in the Florida Straits,” caution Johnston and Purkis. “Going forward, the community should be aware that lionfish will likely not be the last invasive marine species to benefit from altered water circulation instigated by hurricanes.” – Jason G. Goldman | 15 May 2015

Source: Johnston, M. W., & Purkis, S. J. (2015). Hurricanes accelerated the Florida–Bahamas lionfish invasion. Global Change Biology 21, 2249–2260. DOI: 10.1111/gcb.12874.

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