Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida

Invasive Freshwater Fish are Back!

April 30, 2024
Juvenile Mayan Cichlid

Before Hurricane Ian, the lakes and marshes of Sanibel were filled with fish. The native sunfish and bass were there but in the last 20 years, they had become outnumbered by non-native, invasive Mayan cichlids (Mayaheros urophthalmus) and blue tilapia (Oreochromis aureus).

“At nearly any freshwater spot on Sanibel you could observe large nests along the shorelines made by tilapia and the smaller holes cleared by the cichlids,” said SCCF Research Associate Mark Thompson. “Typically, if you thought you saw a sunfish, you were actually looking at a cichlid.

Also making a comeback: Toxic cane toads >>

Mayan cichlids gather in groups as juveniles near submerged limbs and detritus.

Mayan cichlids and tilapia out-compete the island’s native fish because they:
• Can exist in low-oxygen conditions
• Will eat almost anything
• Protect their young by holding them in their mouths until they get old enough to fend for themselves
• Begin spawning when they are 3 months old
• Are aggressive (and easy to catch)

“If you came to Sanibel within a few days of Ian’s surge event, you noticed the mud covering everything and lots of dead fish,” said Thompson. “It was truly stunning the number of dead tilapia and cichlids that paved the island’s roads and yards.”

Since Sanibel’s lakes contain so much food, including algae and phytoplankton, these fish could sustain “bloom” levels. The sudden replacement of fresh water with salt water during Ian’s surge event killed most of the island’s freshwater fish.

Since Ian, our waterbodies have contained estuarine fish, mosquitofish, and killifish. Lakes and marshes that contain large volumes of water may take years to become fresh water again. However, many shallow ponds and the Sanibel Slough are now barely fresh enough to support the non-native Mayan cichlid, which can also tolerate slightly salty water.

“Fish watchers around the island have been wondering when we would see them return — knowing that Mayan cichlids would be the first,” said Thompson. “Now that they are here, the only thing that will limit their range is how salty the lakes are.”

Last weekend, Thompson and his cast-netting son Yuan pulled in living proof that Mayan cichlids have returned to the Sanibel Slough. In addition, a blue tilapia was also found in the Slough near Casa Ybel Road.

The Sanibel Slough is currently about 7-9 practical salinity units (PSU). For reference, fresh water is 0-2 PSU and Gulf water is 30-36 PSU. Some of the biggest lakes on Sanibel are still 25-36 PSU, but where there are low salinity pools – you can bet Mayan cichlids will find a way to get there.

Small, shallow ponds such as this one on a City of Sanibel preserve are becoming fresh more quickly than larger, deep ponds.
Freshwater fish will repopulate these ponds before the larger ones.


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