Generalist No More

Yazoo Darter as a Microhabitat Specialist

Although the Yazoo Darter (Etheostoma raneyi) was previously thought to be a habitat generalist able to thrive in a variety of conditions, U.S. Forest Service scientists Kenneth Sterling and Melvin Warren guessed differently.

“Literature suggested the darter was a generalist, but details at small spatial scales were lacking for its sand-bottom stream habitat in the upper Gulf Coastal Plain,” Sterling explains.

This small benthic headwater fish is categorized as a Tier 1 species of greatest conservation concern by the Mississippi State Wildlife Action Plan. Scientific studies have covered headwaters with upland warmwater and coldwater fish, but Coastal Plain stream habitats for non-game fish have rarely been quantified.

yazoo-darter-fish
The Yazoo Darter is a small, near-threatened insectivore found in northcentral Mississippi. They feed primarily on insects. Photo by Fredlyfish4, courtesy of Wikimedia.

“The geology, hydrology, land use history, and available habitat in Coastal Plain streams are much different than upland streams,” says Warren.

To quantify their results, the scientists compared habitat use to the available habitat. Over the three-year study, Yazoo Darters were sampled in streams of the Little Tallahatchie River, Yazoo River basin, in Mississippi, where the species is endemic.

Electrofishing and dip nets were used to capture the fish, and capture locations were marked with small anchored floats. Environmental conditions such as water depth, water velocity, cover, substrate, and distance to bank were recorded.

In order to categorize available habitat, random points were surveyed for the same conditions. Size and sex of fish were also noted.

Once quantified, the results showed nonrandom habitat use. “Cover type and the extent of cover were identified as the most important variables for the Yazoo Darter’s habitat use,” Sterling points out.

While the fish used all available cover, it most commonly used wood that helped provide complex cover. “This shows how important cover stability is. In fact, they completely avoid areas with no cover.”

The study also showed significant shifts in available habitat. “But these shifts are actually small,” says Warren. Perennial streams experience limited disturbance, but the Darter’s habitat is specific, making it sensitive to large changes.

tallahatchie-river-bridge
A truss bridge over the Tallahatchie River, the primary habitat of the Yazoo Darter. Photo by cmh2315fl, courtesy of Flickr.

Interestingly, the microhabitat used by Yazoo Darters does not vary with season, sex, or size. “The Darter only exists in a narrow range of habitat,” Sterling claims. “It needs a range of depth between 20 and 30 centimeters, water velocity of greater than about 0.25 meters per second, stable complex debris piles, rooted aquatic plants, and coarse substrates.”

These data are imperative for small-scale habitat enhancement projects. Headwater fish provide a large amount of fish biodiversity in the southeastern U.S., so keeping their environment stable is critical.

The results suggest that adding large wood, rooted aquatic plants, and rock rubble could create stable cover and debris piles to sustain the microhabitat of the Yazoo Darter.

The Yazoo Darter is closely related to a diverse group of snubnose darters — many of which are imperiled — suggesting that their habitat requirements at different spatial scales should also be considered in conservation planning.

Read the full text of the article.

For more information, email Kenneth Sterling at kennethsterling@fs.fed.us or Melvin Warren at mwarren01@fs.fed.us.

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