In pre-Ian times, SCCF’s wildlife team documented a high diversity of freshwater fish species on Sanibel, resulting from both natural and human-induced transport to the island. Changes in salinity from repeated storm surges have drastically altered that diversity.

Salinity, or how much salt is in the water, is measured in units described as ppt (parts per thousand), which is the mass of the total amount of solid material (salt), or PSU (practical salinity units) that are based on the conductivity ratio.

Both units are roughly equal but are used in different applications. Pure freshwater is given a ppt or PSU of 0, whereas standard seawater is typically 35 ppt or PSU. Sanibel “freshwater” is usually in the 1-4 range, even though it is technically slightly brackish.

However, our freshwater fauna, such as fish, amphibians, reptiles, and mammals, were able to drink and live in it without harm.

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“The storm surge from Hurricane Ian in 2022 had major repercussions on the fish and wildlife that lived in the freshwater bodies on the island,” said Wildlife & Habitat Management Director Chris Lechowicz. “Most of our slightly-brackish-tolerant wildlife species showed declines in population when the salinities exceeded 12 ppt in most areas and went well over 20 ppt in other areas.”

Quick disappearance of freshwater game fish

Introduced freshwater gamefish such as largemouth bass (Micropterus salmoides), pictured above, and bluegill (Lepomis macrochirus) disappeared almost overnight and have not been seen since due to the sustained high salinities in permanent waterbodies on the island.

These species, as well as the warmouth (Lepomis gulosus) and red-ear sunfish (Lepomis microlophis), were the dominant predatory fish in the Sanibel Slough and other freshwater lakes on the island until the introduction of the exotic Mayan cichlid (Mayaheros urophthalmus) sometime in the early 2000s.

These fish infiltrated the east basin of the slough and eventually made it into the west basin. It is unknown how this fish arrived to Sanibel, but it is considered a nuisance species in Florida, especially right off island in Fort Myers.

“It may have been released on the island intentionally or possibly swam across the bay during heavy rain/storm events when the salinity was really low,” said Lechowicz. “Just prior to Ian, Mayan cichlids appeared to be the dominant predatory fish on the island.”

Hurricane Ian transformed our near freshwater bodies into highly brackish basins where only Mayan cichlids and the live-bearing fish, such as mosquitofish (Gambusia), sailfin mollies (Poecilia latipinna), and other similar small fish species seemed to be able to tolerate it.

Other fish such as snook (Centropomus undecimalis), sheepshead (Archosargus probatocephalus) and even tarpon (Megalops atlanticus) that were trapped in these waterbodies after the overwash are still present in many of these permanent waterbodies.

There has recently been discussion about whether these freshwater game species should be reintroduced to the island, given that we can go several years without major storm surges.

The last full island overwash pre-Ian was 1926; however, there have been three since 2022 (Ian: 2022, Helene: 2024, Milton: 2024) and one partial overwash (Debbie: 2024). Lower salinities are expected in these waterbodies with more years of no overwashes and substantial rainy seasons.

While invasive freshwater fish are back on the island, game fish won’t return unless they are reintroduced.

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Native to Florida, but not to the islands, both bluegill and bass are not considered invasive or exotic, but whether they are viable in the long term and should be reintroduced is up for more discussion.

These game species were originally introduced by the J.N. “Ding” Darling National Wildlife Refuge in 1959 to provide residents with another type of fishing opportunity and to supplement the prey base for wildlife species, such as American alligators (Alligator mississippiensis) and river otters (Lontra canadensis), as well as raptors and wading birds.

“Time will tell if we should consider this,” said Lechowicz. “Future storm surges are inevitable, making the chance of their survival questionable.”