The Effect of River Discharge on Fish Year Class Strength in the Apalachicola River

Many rivers experience natural fluctuations in flow, including periods of extremely high water levels. As surrounding floodplains inundate, nutrients and habitat become available for numerous aquatic organisms.

Many species of fish have adapted to use these conditions for spawning and nursery habitat over time. Changes to the flow of water within a river system can affect the spawning behavior of adult fish and alter important food sources and refuge for juvenile fish. Water level fluctuations can also impact the number of fish surviving to enter a fishery. These potential problems created an opportunity for freshwater fisheries biologists studying fish populations in the Apalachicola River and associated sloughs in northwest Florida.

In 2005, biologists with FWRI’s Aquatic Habitat and Restoration Enhancement Subsection, the Office of Conservation Planning of the Division of Habitat and Species Conservation and the University of Florida began a project to study the effect of water levels and flow, or floodplain inundation, on year-class strength on native fishes of the Apalachicola River. This research is part of a long-term monitoring study on the sloughs and mainstem of this important north Florida river system.

The Chattahoochee and Flint rivers, with headwaters in Alabama and Georgia, combine to form Lake Seminole on the border of Georgia and Florida. The Apalachicola River originates from the Jim Woodruff Dam at the base of Lake Seminole and flows 106 miles south to Apalachicola Bay. Historic droughts and increased water demands in the upstream portion in Alabama and Georgia have decreased flow in the downstream portions. Over time, the Apalachicola floodplain has been inundated less frequently and to a lesser magnitude than it has historically. Biologists needed improved information on the effects of flow and floodplain inundation on important fisheries in the Apalachicola River system.

Researchers are collecting information on the relationship between water levels and flows and the recruitment of largemouth bass, redear sunfish and spotted suckers. These species were selected based on their feeding and habitat requirements within the river and floodplain. Fishes respond differently to hydrologic conditions based on life history traits, and these species occupy different trophic niches and provide a broad perspective on how discharge may impact fish populations in the Apalachicola River.

Following similar methods established for this research project in 2005, 50 transects between mile markers 80 and 20 in the main channel of the Apalachicola River are randomly selected and electrofished by boat (pulsed direct current) for ten minutes per transect. Randomly selected transects from ten sloughs connected to the main channel are also sampled via electrofishing. All largemouth bass, redear sunfish, and spotted suckers are counted and measured, and sagittal otoliths (largemouth bass and redear sunfish) and asteriscus otoliths (spotted suckers) are removed from a subset of fish. Otoliths, commonly known as “earstones,” are hard, calcium carbonate structures located directly behind the brain of bony fishes. Otoliths help FWRI biologists determine the age of fish as well as the growth rates of various species.

The extrapolated catch per unit effort (CPUE) for age-0 fish of each species is averaged among transects to obtain a mean catch rate. To compare CPUE data among years and perform more robust analyses, researchers collect CPUE at age data for each species from 2005 to present from both main channel habitat and slough habitat. FWC biologists obtain river discharge data from the U. S. Geological Survey on the Apalachicola River near Chattahoochee, Florida, and a linear regression model is used to evaluate the relationship between river discharge and year-class strength of largemouth bass, redear sunfish, and spotted suckers in the Apalachicola River.

Field work is ongoing, and additional statistical analysis will continue for years to come. The FWC has 12 years of data that cover a variety of meteorological and hydrological conditions, and annual field research will incorporate additional meteorological and hydrological conditions and more recruitment information over time. Data has shown that strong year classes for these species on the Apalachicola River system are strongly correlated with extended periods (or days) of floodplain inundation and flows. If more water is provided to Florida, these species will continue to thrive and prosper in this river system.