High diversity of freshwater fish on Sanibel
The high diversity of freshwater fish species on Sanibel is a result of both natural and human-induced transportation to the island. Small live bearing species such as mosquitofish (Gambusia holbrooki), sailfin mollies (Poecilia latipinna) and least killifish (Heterandria formosa) likely arrived in pockets of freshwater on floating mats of vegetation from the Caloosahatchee River.
Other diminutive species such as marsh (Fundulus confluentus), bluefin (Lucania goodei), rainwater killifish (Lucania parva) and flagfish (Jordanella floridae) are all egg layers and likely arrived the same way. However, two of them are true brackish water species that could have swam across during periods of heavy rain events when bay waters were more fresh than average seawater. Florida gar (Lepisosteus osseus) are common in the permanent, deeper water bodies and have some brackish water tolerance They may have traversed the bay near the surface during low salinities.
There were no freshwater game fish, such as bass and sunfish, native to the island. Residents convinced the Sanibel refuge to release several species of gamefish from the Welaka National Fish hatchery on June 1, 1961. The fish released were described as copperhead bream and largemouth bass, but they were likely a hodgepodge of several Centrarchid (sunfish) species. The fish were released in a couple of sections of the Sanibel River and a lake. They were reported to be widespread on the island by 1964 by sport fisherman. During annual Sanibel-Captiva Conservation Foundation surveys, several sunfish species have been identified over the last two decades such as largemouth bass (Micropterus salmoides), bluegill (Lepomis macrochirus), red-eared sunfish (Lepomis microlophus) and warmouth (Lepomis gulosus).
Exotic fish are also part of the diversity in the Sanibel River and wetlands, such as the walking catfish (Clarius batrachus) that was reported as early as the 1970s. The southeast Asian fish actually cross roads during heavy rainstorms to expand their range by using their pectoral fins as pivots as they wiggle in a snakelike motion. The Mayan cichlid (Cichlasoma urophthalmus), an extremely invasive species in South Florida, was first documented by SCCF during its annual swale fish surveys in 2008. A small juvenile showed up in a Breder trap in the east basin of the Sanibel River. Every sampling effort after that revealed more of the species. They started showing up in the west basin sampling effort in 2012, which was traced back to the opening of the weir for flood control a couple of years prior. Sampling efforts on SCCCF conservation lands have shown increased detections of Mayan cichlids over the last decade and a slight decrease in game fish.