Diel Patterns of Fish Use of Open-Water Habitats in a Freshwater, Oligotrophic Wetland

Diel changes in movement and activity of fishes typically occur to reduce predation or competition, resulting in changes in abundance and assemblage structure amongst habitats.  The ability of fish to disperse or migrate on a diel basis is largely dependent upon the level of connectivity across habitats.  The goal of this study was to investigate diel patterns of fish use of open-water habitats adjacent to wetlands that experience seasonal marsh drying.  We used a Dual-Frequency Identification Sonar to quantify fish density and evaluate predation risk for small, prey fish (< 8 cm).  Results revealed large, piscivorous fish (> 8 cm) were more active and densities were higher during the day than at night.  The nighttime decline may be a result of fishes performing nocturnal, horizontal migrations from the open-water to adjacent marsh.  These piscivorous fish are also visual foragers and may reduce foraging efforts under darkness.  Behavioral changes in small fish were also identified.  Schools of small fish were observed exclusively during the day, while at night prey fish were detected individually.  Daytime schooling may be a response to elevated predation risk, but at night schools disband as a result of decreased foraging rates of piscivorous fish.