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Freshwater Fish & Macro-Invertebrate Monitoring Begins

July 7, 2020

After the summer rains begin, ephemeral wetlands begin to fill with water at the same time that the Sanibel River level starts to rise. Eventually, a point is reached where connections between the river and the previously dry swales connect and fish are able to venture out from the river into the previously dry lands. 

These shallow (~12” or less) swales quickly become occupied by small live-bearing fish species, as well as some egg-laying killifish. Mosquitofish (Gambusia affinis) and sailfin mollies (Poecilia latipinna) are usually the first species to penetrate these wetlands and are usually the most dominant species. However, other native species such as flagfish (Jordanella floridae), least killifish (Heterandria formosa), and bluefin killifish (Lucania goodei) arrive soon after and can be locally abundant at certain locations. 

Macro-invertebrates also infiltrate these swales during the wet season. Many insects in the Order Hemiptera (true bugs) and Odonota (dragonflies and damselflies) use this habitat to lay eggs that hatch into nymphs (aquatic larval stages of these insects) until they go through metamorphosis into adults.

SCCF monitors these species at several locations on preserve lands. The fish are caught with Breder traps, small Plexiglas rectangular traps that are placed in shallow water, and checked after an hour. Swale water depth and species abundance are recorded. 

During the hour-long wait, macro-invertebrate sampling occurs using a protocol with a dip net and a sorting tray. Random dips in the vegetated sections of the swales are performed and the contents are sorted and counted. 

Dip netting stops when no new species are captured after three consecutive swipes of the dip net. The species richness and abundances can be compared from previous years to signify changes in the habitat or the lack or abundance of rainfall. By monitoring annually, we can also record the presence of any exotic species that have invaded our freshwater systems, such as the Mayan cichlid (Cichlasoma urophthalmus) that was first documented in 2008 from our efforts.

The SCCF Wildlife & Habitat Management staff strives to provide the necessary habitat for these important animal species to occupy each year. Keeping these swales and wetlands with open canopy is essential for many of the lower vertebrate and invertebrate species. These swales are very important feeding grounds for many native wildlife species on the island, especially wading birds. As the wetlands dry down, the fish, macro-invertebrates, and tadpoles become crammed in the remaining shallow pools and otters, wading birds, and reptiles take advantage of the easy prey.


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