Fishes in the forested flood plain of the Ochlockonee River, Florida, during flood and drought conditions

Abstract

A study of hydrologic conditions, vegetation, and fishes was made in the Ochlockonee River and flood plain in northwest Florida from 1987 to 1989. The study was conducted by the U.S. Geological Survey in cooperation with the Florida Game and Fresh Water Fish Commission to document the importance of the flood-plain environment to fishes. A total of 8,225 fish belonging to 15 families and 48 species was collected during the study in flood-plain, backwater, and main channel habitats by five different sampling methods. During floods, three-quarters of the fish species known to occur in the main channel were collected in the flood plain. The most frequently collected species in the flood plain during floods were weed shiner (Notropis texanus), redbreast sunfish (Lepomis auritus), brown bullhead (Ictalurus nebulosus), bluegill (Lepomis macrochirus), gizzard shad (Dorosoma cepedianum), mosquitofish (Gambusia affinis), and redear sunfish (Lepomis microlophus). The study period included a drought during which flood-plain ponds were isolated from the river for 10 to 14 months. Thirteen species found in isolated flood-plain ponds had survived low dissolved-oxygen concentrations the preceding fall. Flier (Centrarchus macropterus), pirate perch (Aphredoderus sayanus), mosquitofish, and warmouth (Lepomis gulosus) were the most frequently collected drought pond species. Seven common flood-plain species that were rare or absent in the main channel apparently depend on flood-plain, backwater, and tributary habitats for their survival in this river system: flier, banded pygmy sunfish (Elassoma zonatum), bluespotted sunfish (Enneacanthus gloriosus), least killifish (Heterandria formosa), taillight shiner (Notropis maculatus), redfin pickerel (Esox americanus americanus}, and brown bullhead. The annual flood season of the Ochlockonee River typically occurs in January, February, March, and April. Summer floods are common but usually of shorter duration than winter-spring floods. Typical range in stage is 16 feet from the low to the high for the year. Alluvial flood-plain processes such as erosion and sediment deposition create topographic relief on the flood-plain surface that allows diverse habitats to develop. Riverbank levees and terraces serve as berms during low and medium river stages, separating still-water habitats in ponds and depressions from the flowing waters of the main channel. During high river stage, all areas of the flood plain are covered with flowing water. Tree communities were distinctly different in each topographic zone, reflecting the varied hydrologic conditions existing within the flood plain. High terraces supported a mixture of canopy species including spruce pine (Pinus glabra), sweetgum (Liquidambarstyraciflua), live oak (Quercus virginiana), and water oak (Quercus nigra), and were flooded an average of 2 to 3 weeks a year from 1931 to 1989. Low terraces, dominated by a canopy of swamp laurel oak (Quercus laurifolia), were normally flooded a total of 2 to 4 months a year. The dominant canopy species in depressions was Ogeechee tupelo (Nyssa ogeche), with bald-cypress (Taxodium distichum) the most common associate. Depressions were typically flooded by the river a total of 3 to 5 months a year, but water was often trapped in these areas long after floods receded. Live and dead vegetative structure in the flood plain, and the amount of time it was flooded, varied with topographic zone and season. The fluctuating hydrologic conditions of the river-flood-plain system, along with the variety of alluvial features and tree communities, contribute to diversity in the fish community. During low water, flood-plain ponds support a fish community distinctly different from that of flowing waters of the main channel. During high water, flood-plain ponds mix with flood waters, and all river-flood-plain fishes gain access to diverse forest habitats.