FORT MYERS, Fla. - David Ceilley and his son Connor weren't surprised last week when they pulled two clear-plastic fish traps from Owl Creek in East Lee County.
Total catch: 21 African jewelfish (also known as jewel cichlids and African cichlids), four dollar sunfish and one sailfin molly.
Although not surprising, the catch was disturbing because African jewelfish are an exotic (nonnative). Dollar sunfish and sailfin mollies are natives; in other words, the exotic species outnumbered the natives 4.2 to 1, at least in these two traps.
The African jewelfish is one of 23 exotic (nonnative) freshwater fish species that have become established in Florida.
"African cichlids are the worst," said David Ceilley, a senior ecologist for Johnson Engineering. "They're having a terribly negative impact. The biomass of jewel cichlids is massive. They can dominate a water body. They're prevalent in the Peace River and Caloosahatchee River and areas in between."
For the past few years, an exotic saltwater fish species, the lionfish, has been in the news because it reproduces rapidly and eats huge numbers of juvenile native fish, including snappers and groupers.
But how harmful are Florida's freshwater exotic fish species?
"We don't have many good scientific studies on how non-native freshwater fishes are affecting native habitat and native species," said Pam Schofield, an exotic-fish expert for the U.S. Geological Service. "We have observational data that show when a non-native fish becomes abundant in an area, we see changes."
Blue tilapia, for example, arrived in Florida in 1961 when 3,000 were stocked in a series of Hillsborough County phosphate pits for aquatic plant control experiments and have been documented in 35 counties.
They're so abundant, in fact, that a commercial fishery has grown up around them: In 2004 and 2005, the only years for which data are available, commercial fishermen in Florida harvested 5.8 million and 6 million pounds of blue tilapia with a dockside value of $1.79 million and $1.84 million.
"When blue tilapia become abundant, native fishes decrease, or vegetation decreases," Schofield said. "But there's no smoking gun, no scientific studies that show they are the causative agent."
Aside from the common carp, which was introduced from Europe into the Hudson River in 1831 and reached Florida in 1889, the first freshwater exotic fish documented in Florida was the Rio Grande cichlid, first reported in the Tampa area in the 1940s and Lee County in 2010 (it hasn't been reported in Collier County).
Many freshwater exotics got into Florida waters when they were released from aquariums or escaped from fish farms.
Pike killifish became established in the state after several specimens raised for medical research at the University of Miami's Department of Medicine were released into a local canal in 1957.
This species, which has been documented in Collier County but not Lee County, is thought to compete for food with juvenile snook and has been reported to reduce populations of mosquitofish, which help control area mosquitoes by eating mosquito larvae.
African jewelfish were first documented in Hialeah in the 1960s, probably the result of aquarium releases; the species was first reported in Collier County in 2002 and Lee County in 2003.
From October 2006 to February 2011, Ceilley was principle investigator for a study of freshwater fish communities in man-made and natural waterways across Babcock Ranch in Charlotte and Lee counties.
Ceilley's team collected 9,059 fish from 26 species and found that the two dominant species were native mosquitofish and African jewelfish; a single 12-inch-by-six-inch-by-6-inch trap would sometimes catch 150 African jewelfish.
The study showed that African jewelfish not only ate native fish, but they also bit off the tails of natives, causing fungal infections that led to death.
One positive discovery from the study is that African jewelfish are very cold-sensitive and suffered mass die-offs during major cold events in 2009 and 2010.
Unfortunately, many survived in the slightly warmer water of canals, and the population quickly recovered.
"They're a bad actor," Ceilley said. "They're the lionfish of the freshwater ecosystem. They make good aquarium fish. That's where they belong. The only thing that knocks them back is cold, but then they come back."
Jeff Schmid, environmental research manager at the Conservancy of Southwest Florida, has been studying the gut contents of Mayan cichlids from the tidal creeks of Estero Bay — this species was first documented in Everglades National Park in 1982, another probable aquarium release; Mayan cichlids appeared in Lee County in 1998 and in Collier County in 1999.
Now they can be found in neighborhood retention ponds and tributaries, from the ditches along Tamiami Trail to protected waterways like the Estero River.
"Our study is showing that in this estuary system, they're not eating fish; they're eating invertebrates, mussels, snails, terrestrial and aquatic insects,
and a lot of organic matter, stuff they probably chew off mangrove roots," Schmid said. "They might not have any direct effects on native fish communities, but they might have indirect effects."
Because Mayan cichlids feed heavily on mussels, they might be competing with native mussel-eaters, but Schmid doesn't know of any natives that rely on mussels.
"If there aren't estuary fish feeding on mussels, it appears that Mayan cichlids have found an unoccupied niche," Schmid said. "That's probably what's allowing them to persevere: They've tapped into a food source not used by other fish."
Exotic species are rarely good for the environment, but some non-native freshwater fish species have become popular angling targets.
Oscars, for example, rank second to largemouth bass in popularity among South Florida freshwater fishermen. This species and Mayan cichlids, are very aggressive, tough fighters and are highly prized for their white, flaky fillets.
Although Oscars and Mayan cichlids are not good for the ecosystem, they are good for a fish fry. And unlike native fish, some of whose populations are struggling in Florida, there is no ecological reason for not taking as many as possible; FWC has no daily bag limit on exotic fish.
Exotic freshwater fish species have been swimming in Florida's waterways for more than 100 years.
While none of the state's established 23 non-native species has become evil-incarnate like the lionfish, some are causing problems for local fish populations.
At the same time, others seem to be fitting in with the natives.
"We sure wish none of them was here," said Kelly Gestring, director of the Florida Fish and Wildlife Conservation Commission's Non-Native Fishes Laboratory. "Now it's up to us to try to develop ways to cope with these guys. There's no way we can get rid of them, so it's important to stress prevention: Don't release them into the environment. Once they're out there, there's not a lot we can do."
Information from: The (Fort Myers, Fla.) News-Press, http://www.news-press.com