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Freshwater Fishes Page

Threats to Aquatic Habitat

Georgia’s aquatic species are threatened by a myriad of factors, including urbanization, poor land use practices associated with historic and current agricultural and forestry operations, reservoirs, chemical pollution, and invasive species. Below we explain each of these factors and provide recommendations on how to minimize their impact on aquatic resources. The purpose of this discussion is not to impugn any one land use type or economic sector but to show that we are all connected to aquatic habitat degradation and that we can all help to correct and prevent these problems.

Runoff from impervious surfaces (e.g., sidewalks, storm drains, rooftops,etc.) carries toxic chemicals into streams and causes erosive flooding that degrades stream habitat quality.


Urbanization negatively affects stream ecosystems in a multitude of ways. Impervious surfaces (e.g., rooftops, parking lots, roads) accelerate runoff after rainfall events, leading to erosive flooding and degradation of instream habitats. This runoff also carries many pollutants into streams. Because water rapidly runs off, instead of infiltrating into the groundwater more slowly, there is less water available in streams during low-flow periods. The effect of urbanization on aquatic communities has been well documented by scientists, who have discovered that some fish species are becoming extirpated (i.e., locally extinct) at very low-levels of impervious cover (< 5% of watershed area). Everyday citizens can help by minimizing or avoiding the use of fertilizers and pesticides in their yards, maintaining vegetated buffers along streams, and by constructing a rain garden to promote infiltration of runoff on their property.

Runoff from unimproved roads often flushes undesired sediments into stream, threatening water quality and aquatic species. 

"Sedimentation is one of the leading reasons for declines in many aquatic species. Sediment runoff from agricultural fields, developing areas, and unimproved roads all contribute large volumes of sediment into adjacent streams and rivers, in turn covering stream bottoms where many fish spawn and all mussels live. This video shows the sediment runoff into a small creek in Early County, Georgia during a 10 minute storm in August 2007. Notice the colorless water from the naturally forested watershed upstream of the drainage ditch in contrast to the brown color of water washing in to the creek from the ditch."

Agriculture and Forestry

Agricultural activities also threaten fish populations, primarily through run-off of sediment, nutrients, and pesticides. These impacts are most extreme in areas where cultivation occurs right up to stream banks or when livestock are allowed access to streams for watering. Sedimentation, which can also result from forestry practices and construction activities, is an almost ubiquitous problem in Georgia streams. Suspended sediment decreases water clarity and interferes with sight-feeding and other visually oriented fish and mussel behaviors (e.g., territorial displays, lures used by mussels to attract their fish hosts). Sediment also fills in the spaces between larger rocks, which eliminates habitats used for spawning, feeding, and shelter. Nutrient pollution from such sources as fertilizer runoff and animal waste can lead to algal blooms, which in turn may affect water clarity, oxygen levels, and also the condition of other aquatic plants that are beneficial to fishes (e.g., Riverweed). Farmers can help protect streams by maintaining wide forested buffers along creeks and streams. There are also many technical assistance and cost-sharing programs that can help farmers implement best management practices, such as restricting cattle access to streams.

Cattle access point (lower left) contributes to sedimentation and nutrient pollution (from Cattle waste). This site also lacks an adequate riparian forest buffer, which provides shade, filtering of runoff, wildlife habitat, and a food source (leaves) for aquatic insects. There are now many programs to help farmers reforest stream buffers with native plants and to develop off-channel watering areas for cattle. 

Depending upon the size of the operation and its location, forestry operations may negatively impact streams and their fishes. Mechanisms of impact include sediment runoff associated with vegetation removal, soil disturbance from forestry equipment, and stream bank destabilization from poorly designed stream crossings. Additionally, the removal of trees from sensitive habitats such as cypress swamps, floodplains, or other wetlands contributes to sedimentation and increases in water temperature. The Georgia Forestry Commission has developed best management practices to minimize the impacts of forestry on streams.


Because most of our native riverine aquatic species cannot survive or complete their life-cycles in deep or slow-flowing habitats, large amounts of habitat are destroyed when reservoirs are created. Reservoirs and smaller impoundments (e.g., farm ponds) also fragment stream ecosystems and prevent the movement of animals between upstream and downstream habitats. Such movements are necessary for spawning migrations and feeding, and for colonization when disturbances (e.g., drought, chemical spill, etc.) eliminate upstream populations. Depending upon their size and outflow structure, reservoirs may also change the quality and quantity of downstream waters. 

One of the best ways to minimize the impacts of reservoirs on native fishes is to adopt water conservation practices that prevent the need for additional reservoirs. American Rivers released a report in 2008 showing how water efficiency measures decrease the need for additional reservoirs and also conserve significant taxpayer dollars. If water demand cannot be met entirely through efficiency measures or existing water withdrawals, then there are ways to minimize the impacts of new reservoirs.

For example, expanding the size of an existing reservoir is usually much less detrimental than building a new reservoir. Similarly, locating a reservoir in a stream with existing downstream reservoirs will fragment fish populations less than building new reservoirs in free-flowing systems. Some new reservoirs are off-channel, meaning that they are built on smaller tributary streams but are filled by pumping water from a larger river. Although it may be more costly to filter water during floods, filling these reservoirs during high flow events is much less harmful than removing water during normal or low-flow periods. 

As part of an effort to develop a Habitat Conservation Plan for federally listed fishes, a technical committee developed a protocol for evaluating water-supply reservoir options for effects on imperiled stream fishes. This protocol helps identify the relative impacts of different reservoir locations on rare aquatic species. Our program also routinely reviews proposals for new reservoirs and can help applicants identify areas that would be least detrimental to state and federally listed fishes. Early coordination with us and other resource agencies will usually result in a better process and outcome for the applicant and for natural resources. 

Carter's Lake Dam on the Coosawattee River.While dams and reservoirs may be important for flood control, water supply, hydropower, and recreation, they also negatively impact many of our native fishes. Impacts include loss of free-flowing stream habitat, barriers to migration, and alteration of downstream water quality.

Chemical Pollution

The effect of pesticides and other chemicals originating from industrial sources may cause lethal effects in rare cases but more frequently are associated with subtle effects which may ultimately have similar consequences. Chemicals may reduce immune system function making aquatic species more susceptible to diseases. Pesticides often affect the nervous system and cause altered behavior which can make an aquatic species less effective at feeding or hiding from predators. A number of chemicals have been shown to interfere with the endocrine system of fish, which regulates reproduction, growth, development and other activities. 'Intersex' fish, usually male fish with eggs in their gonads, have been found in rivers across the country, including Georgia, often near treated sewage effluents. These 'feminized' males can have reduced ability to reproduce successfully.

Invasive Species

Non-native organisms may negatively affect Georgia’s native fishes. Potential impacts include direct predation, competition for habitat and food, and disease. These factors are a natural part of aquatic ecosystems and our fishes have adapted to them throughout their shared evolutionary history with other native species. However, our fishes may be poorly adapted to these factors when they are associated with introduced species. For example, native bullhead catfishes (Ameiurus spp) maintain healthy populations in the face of flathead catfish predation where both species naturally occur, but suffer large population declines where flathead catfish have been introduced. Another important impact of non-native species is the loss of unique genetic diversity when closely related species interbreed. For example, the red shiner (Cyprinella lutrensis) readily hybridizes with other shiners in the genus Cyprinella where it has been introduced outside of its native range.

Once a non-native species has become established, it is very difficult or perhaps impossible to eradicate them. Thus, strategies to prevent the accidental introduction of non-native species are critical to the protection of our native species. Anglers, recreational boaters, and aquarists have a very important role to play in preventing additional introductions and the further spread of species that have already been introduced. Please visit our invasive species page (under construction) to learn more about what you can do. You can also click on the icons below for more information.



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