Population Monitoring of the Federally Threatened Okaloosa Darter at Eglin Air Force Base

Science Center Objects

USGS' and Loyola University New Orleans' innovative research techniques played a role in the decision to downlist the Okaloosa Darter, a freshwater fish endemic to northwest Florida, from Endangered to Threatened.

Okaloosa darter, (Etheostoma okaloosae)

Okaloosa darter, (Etheostoma okaloosae)

The Science Issue and Relevance:

Effective management of endangered species requires an understanding of temporal and spatial variability in population size and age structure. Specifically, long-term, quantitative data on population size and age structure are needed to detect the effects of acute and chronic anthropogenic activities on endangered species. Unfortunately, historical data are often absent or of insufficient quality to reliably assess ecological impacts. This problem has proven to be especially critical in the management of freshwater stream fishes in North America. Few quantitative, long-term databases exist to evaluate the natural fluctuations of, or the effects of management actions on, populations of stream fishes. Lack of information frequently results in restrictive limitations on human activities within watersheds that support fishes with special conservation status. The Okaloosa darter (Etheostoma okaloosae), which is endemic to Eglin Air Force Base (AFB) in northwest Florida, is typical of this management dilemma.

The Okaloosa darter resides among vegetation and woody debris along the margins of only six clear flowing streams in northwest Florida. In fact, over 95 percent of the fish’s limited habitat can be found on Eglin Air Force Base, one of the world’s largest conventional weapons testing facilities, just east of Pensacola, Florida. Their limited geographic range, habitat degradation due to historical land use practices, and competition from a darter species thought to be introduced resulted in the inclusion of the Okaloosa darter on the endangered species list.

Researchers used a combination of three-pass removal and point count observations

Researchers used a combination of three-pass removal and point count observations 

Methodology for Addressing the Issue: Using innovative sampling and biometric techniques over the past two decades, researchers from USGS and Loyola University New Orleans used a combination of three-pass removal and point count observations with mask and snorkel to estimate the number of these 50-mm (2-inch) fish in six small stream systems. The research team established that by using visual methods, they were observing an average of three times the number of darters than were captured using the more traditional seine nets. Their data were used by the U.S. Fish and Wildlife Service to develop the rule to downlist Okaloosa darters from endangered to threatened in 2011 and complete a species status assessment for potential delisting in 2019. With an estimated total number of darters around 500,000, and commitments by Eglin AFB to continue habitat restoration efforts, the outlook for the Okaloosa darter appears secure. Monitoring the darter will continue to ensure that these fish remain part of the northwest Florida landscape.

Future Steps: While continuing to monitor the Okaloosa darter with similar methods at historical sites, the team expanded their efforts into habitat restoration areas and sections of stream where a new highway crosses all six streams. Darters were marked with latex dyes to estimate movement, site fidelity, and longevity. Researchers determined that one marked darter lived at least eight years, which is a record for the genus. Fire opens stream canopies, which has proved to be an important factor in increasing numbers of darters. There is also interest in the role beavers play in modifying Okaloosa darter habitat. Techniques developed to sample small stream fishes and analyze collected data could fill information gaps for similar imperiled species in the southeastern United States.

Okaloosa darter on sand substrate

Okaloosa darter on sand substrate

Related Project(s): North America Freshwater Faunal Assessments, Pinewoods Darter Assessment