Rising water temperatures off the North Carolina coast are good news for the expansion of tropical fish and for divers. But they may also add to an underwater menace that could ultimately threaten reefs in the Atlantic.
In a study that’s the first of its kind, researchers from the National Oceanic and Atmospheric Administration and UNC Wilmington found temperature patterns that could support an expanded range for invasive species such as the lionfish. In the past, the poisonous predator couldn’t inhabit these locales because of cold winter temperatures.
The seven years of research involved 40 native fish species on natural as well as artificial reefs in the Onslow Bay area, between Cape Lookout and Cape Fear. Paula Whitfield, a research ecologist based in Beaufort for NOAA’s National Centers for Coastal Ocean Science, said the study will lay the groundwork to help gauge future water temperature changes and their impacts.
“We were really able to look at what is the relationship between where these fish are located, what those species are and the temperature,” said Whitfield, the study’s lead author. The research, completed in 2010 but only recently released, following an analysis of the data, was reported in the September issue of Marine Ecology Progress Series.
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Collecting the data
“We had to do some decompression-type diving because of the depths,” Whitfield said. “Divers observed the fish and counted them, and used temperature (data) loggers.” Data analysts combined previously recorded year-round bottom water temperature figures with 2006-2010 fish community surveys in water depths from 15 to 150 feet off the North Carolina coast.
They found that fish were primarily tropical in the deeper areas surveyed (122 to 150 feet), with a mean winter temperature of 69.8 degrees Fahrenheit. Many of these native tropical fish that were usually abundant in shallow, slightly cooler reefs were often staying in the deeper, warmer water – evidence that temperature is a key to determining their distribution.
“This temperature gradient that sets up in the wintertime, that’s what’s controlling the fish communities,” Whitfield said. “If that temperature gradient shifts and it becomes warmer, we’re going to see a shift in fish communities, near-shore to offshore, as well as seeing them disperse northward, changing up and becoming more tropical. That’s the expectation.”
Jonathan Hare, an oceanographer with NOAA who worked on data analysis on the project, said “this tropicalization is consistent” with global warming trends, but is not direct evidence of it. “We don’t have the baseline data to absolutely say that this is due to global warming,” Whitfield said.
She added it’s not necessarily accurate to infer from this that deeper water is always warmer: “The way you have to think about it is near-shore, shallow waters to offshore, deeper waters.
“Say you’re at the beach and it’s January. You know that the water’s cold. But if you just continue offshore and get into deeper, deeper water, the water temperature on the bottom gets warmer and warmer. So if you go out diving to 150 feet on that same January day, it’s going to be 60 or 65 degrees. That’s that moderating influence of the gulfstream that’s year-round in this area offshore.”
The lionfish threat
“We know that there’s always tropical species offshore, including the lionfish,” she said. “They are primarily found in the warmer areas that stay warm year-round because of the gulfstream influence.”
Whitfield talked about two ways that lionfish wreak havoc on the ecosystem: They reduce fish recruitment and biomass (the total mass of organisms in a given area or volume); and their rampant predatory ways – Hare calls them an “ambush predator” – cause a negative chain reaction that results in decreased coral cover on coral reefs.
Dwarfing most other fish species, lionfish (usually about 15 inches long) gobble up herbivorous fish that consume algae. When algae overgrows coral reefs, it can reduce habitat for corals and sponges.
“It’s not that they’re actually causing the decrease in the sponges,” Whitfield said. “They’re inadvertently causing it because they’re eating the herbivores that consume the algae.”
Their recent emergence and spread underscores how dramatically fast nature can change. “The reality is, they arrived in North Carolina in 2000,” Whitfield said. “We were the first to investigate them.
“That’s when we first started observing one here and one there. What we didn’t realize was, they had actually been sighted in one location off West Palm Beach, Fla., since the late ’80s. They were successfully reproducing, and their larvae rapidly dispersed via ocean current. From there they spread northward via the Gulf Stream, which is a super-fast-moving current, and then across the Gulf Stream, through the Bahamas in 2004 and southward through the Gulf of Mexico and Caribbean.”
But how were they introduced in the Atlantic in the first place? “We’re never going to know exactly how this happened, because we can’t go back and observe it,” Hare said. “The hypothesis with the strongest support is that it was a release from the aquarium trade” – when, reportedly, aquarium owners in southern Florida dumped a few of the fish into the Atlantic.
Answers are limited
Whitfield said there are no major control efforts for lionfish in North Carolina, though some are ongoing in the Florida Keys. “The scary thing is that other than local control – for people to go out and try to kill as much as they can – there’s nothing that’s really going to control them, except for temperature,” she said.
Added Hare: “The depressing thing is that once these things get out and start growing in their population size, it’s an unstoppable invasion. They’ve had some success with bounty programs and contests in different parts of the Caribbean to control the population – but to wipe them out or to end the invasion, as far as I can tell, is impossible.”
Whitfield and Hare said projection and prevention are useful approaches.
Asked how the lionfish explosion could affect the future management of protected areas, Whitfield said: “The thing that we really need to do next is use these data to project how we think the fish communities might change based on these temperature tolerances that we found. We can project the water temperature changes based on the climate models. And then that data will probably need management and policy-level decisions.”
Hare said that if the introduction of lionfish in the Atlantic was indeed the result of an aquarium dump, knowledge of this “may help us try to be more cautious on other species which could be introduced by the same mechanism. So are there things that can be done in the aquarium trade to prevent the release of another species that will become invasive?”