The Everglades is a vast, slow-moving sheet of water that creates one of the most complex and biologically diverse aquatic ecosystems in North America. This unique habitat supports a remarkable array of fish species, spanning from those that require pristine freshwater to others that thrive in full-strength ocean salinity. These fish serve as the foundational food source for iconic wildlife like alligators and wading birds. Understanding the fish life here requires appreciating the sharp contrast between the shallow, inland marshes and the productive coastal estuaries.
The Unique Everglades Aquatic Environment
The Everglades environment creates distinct aquatic zones defined primarily by water movement and salinity. Hydrology, the pattern of water flow, is the most important factor shaping this environment, controlling both water depth and the concentration of salts. The landscape naturally transitions from a freshwater marl prairie in the north to a saline scrub mangrove forest along the coast.
Seasonal fluctuations between the wet and dry seasons impose stress on fish populations. During the wet season (May to October), high water levels allow fish to disperse widely across the marsh. Conversely, the dry season forces fish into isolated, deeper water refuges like alligator holes and canals, concentrating them for predators.
A profound salinity gradient exists, moving from zero parts per thousand (ppt) salt in the interior to 35 ppt in the Gulf of Mexico and Florida Bay. The brackish water ecotone, where freshwater and saltwater mix, is a highly variable zone. Salinity here can fluctuate dramatically due to tidal influence and freshwater runoff, demanding that fish possess a high tolerance for wide salinity ranges.
Key Native Freshwater Fish Species
The interior of the Everglades is home to “primary freshwater fishes,” species that cannot tolerate salty water and are restricted to the marshes. The Florida Largemouth Bass (Micropterus salmoides floridanus) is the region’s most famous primary freshwater species, prized for its potential to grow substantially larger than its northern counterparts, sometimes approaching 20 pounds.
Smaller native species form the base of the food web, providing sustenance for larger predators. The Eastern Mosquitofish (Gambusia holbrooki) is one of the most common freshwater fish, feeding heavily on insects, including mosquito larvae, and breeding rapidly during high-water periods. The Least Killifish (Heterandria formosa) is another minute species, reaching only about one inch in length, making it the smallest fish in North America.
Native sunfish species are abundant throughout the freshwater system, including the Bluegill, Redear Sunfish, and Warmouth. These species, along with the Golden Shiner, thrive in the dense vegetation and open water of the freshwater marshes. The powerful Florida Gar (Lepisosteus platyrhincus) is another indigenous predator adapted to the warm, oxygen-poor shallows, often seen near the surface of canals and marsh edges.
Estuarine and Coastal Native Species
Moving toward the coast, the fish fauna shifts to species that manage the dynamic salinity of the mangrove estuaries and near-shore waters. The Tarpon (Megalops atlanticus) is a sport fish, sometimes exceeding 100 pounds, which migrates into the Everglades’ estuarine rivers during spring and summer.
The Common Snook (Centropomus undecimalis) is highly tolerant of fluctuating salinity and is often found hiding among the mangrove roots and tidal creeks. Snook use the protective cover of the estuary as a nursery and feeding ground before moving offshore. The Red Drum (Sciaenops ocellatus), or Redfish, is also a sought-after estuarine species that forages in the shallow, muddy bottoms of the mangrove edge.
Other smaller fish are also well-adapted to the variable salinity, including various species of Killifish and Mojarra. The Sailfin Molly is a euryhaline fish capable of living in a wide range of salinities, from pure freshwater to full-strength saltwater. The Sheepshead Minnow and the Gold-Spotted Killifish are two other native estuarine species that demonstrate high tolerance for the rapid and acute salinity changes that characterize the coastal Everglades.
The Impact of Non-Native and Invasive Species
The Everglades ecosystem has been impacted by the introduction of non-native fish, many of which originated from the tropical fish trade or aquaculture releases. Over 30 non-native fish species have established populations, posing a threat to native fauna through competition and predation. The Mayan Cichlid (Mayaheros urophthalmus), a South American species, is highly invasive because it can tolerate a wide range of salinities, allowing it to spread from freshwater canals into brackish estuaries.
Some invaders exhibit unique adaptations that enable them to outcompete native fish, especially during dry periods. The Asian Swamp Eel (Monopterus albus) is an air-breather that can survive low water levels and even migrate short distances overland. This nocturnal predator has been linked to severe localized declines, including the near-complete loss of some native fish and crayfish populations in parts of the Everglades.
Other notable non-native species include the Walking Catfish (Clarias batrachus), which uses lung-like sacs to breathe air and can walk across land to find new water sources. Various species of Tilapia are also well-established, feeding on algae and plants, which can disrupt the natural food web dynamics. These non-native fish often replace native species, fundamentally restructuring the aquatic communities of the wetland.