Expanding Biologists’ Toolbox for Assessing the Status of Southeastern Freshwater Fishes

USGS Cooperative Research Unit Corner

Expanding Biologists’ Toolbox for Assessing the Status of Southeastern Freshwater Fishes

A research team consisting of scientists and managers from the USGS Mississippi Cooperative Fish and Wildlife Research Unit (MS Unit), Mississippi State University, USGS Columbia Environmental Research Center (CERC), U.S. Fish and Wildlife Service (USFWS), and University of Wisconsin-La Crosse are developing a tool to help USFWS biologists assess imperilment of freshwater fishes in the Southeastern U.S. that have been petitioned for review under the Endangered Species Act.

Candy darter

The Southeastern U.S. is a global hotspot for freshwater biodiversity. The region contains the highest diversity of fishes in the U.S. (around 589 described species) and the highest diversity of crayfishes (around 221 described species) and freshwater mussels (around 234 described species) on Earth. However, freshwater ecosystems are among the most imperiled ecosystems in the world, and the Southeastern U.S is no different. The freshwater species in the Southeast face numerous threats, including habitat modifications (dams, stream channelization, riparian vegetation removal, etc.), pollution, climate change, and introductions of non-native species.

In 2010, over 400 freshwater species were petitioned for review and listing as Threatened or Endangered in the Southeast, and many of these species still await listing decisions by the USFWS.

Once freshwater species are petitioned for listing under the Endangered Species Act, the USFWS is tasked with compiling available data to determine species’ “viability”, or their ability to sustain populations in the wild over time.

This information is compiled in a document called a Species Status Assessment, which characterizes viability under the “3 R’s framework”, based on species’ Resiliency, Redundancy, and Representation.

  • Resiliency is the ability of each population to withstand frequent, yet moderate disturbance events (e.g., floods, droughts, etc.) and is usually assessed based on the severity of threats and population characteristics such as evidence of decline, abundance of individuals in a population, and reproductive success.
  • Redundancy is the ability of a species to withstand infrequent catastrophic disturbances (e.g., earthquakes, chemical spills, etc.) by spreading risk among many connected resilient populations.
  • Representation is a species’ long-term adaptive potential, based on the persistence of populations exhibiting unique geographic, genetic, or ecological characteristics across the species’ distributional range.

Research and monitoring are limited for most freshwater species in the Southeast, meaning that data useful for determining imperilment of these species is often scarce, and it is difficult to ensure that the risk assessments informing Species Status Assessments are performed consistently across species. Moreover, simultaneously conducting Species Status Assessments for hundreds of species strains limited resources of state and federal natural resources management agencies.

Recognizing the need to streamline and standardize the risk assessments that inform Species Status Assessments, we are building a statistical model that evaluates imperilment risk for freshwater fishes by harnessing information from diverse sources such as population monitoring data, knowledge from closely related species, and input from species experts. This type of model (called a “Belief Network") is particularly useful as a decision-support tool for managing species in which data are sparse or outright missing.

The research team is constructing the freshwater fish risk-assessment model based on the USFWS’s three R’s framework, which will allow USFWS biologists the flexibility to customize the model for specific stressors affecting each species’ viability. The team is also developing an online interface that will allow USFWS biologists to implement the model rapidly, thereby helping alleviate the heavy workload of reviewing hundreds of freshwater species.

Researchers used existing data and Species Status Assessments of three species from across the Southeast to develop initial versions of the model: Candy Darter Etheostoma osburni (listed as Endangered in 2018), Carolina Madtom Noturus furiosus (listed as Endangered in 2021), and Ozark Chub Erimystax harryi (not listed former candidate for listing).

After fine-tuning the model, USFWS and the research team will use the model as a tool to complete the Species Status Assessment for Piebald Madtom Noturus gladiator in 2023, marking the first use of the model in support of an ongoing Species Status Assessment.

In the future, the MS Unit plans to adapt the risk-assessment model to allow its use for imperilment-risk assessments for other imperiled freshwater groups in the Southeastern U.S. like crayfishes and mussels. These decision support tools along with tireless work by natural resources professionals will help manage the tremendous freshwater biodiversity unique to the Southeastern U.S.

The ONB features articles from Cooperative Fish and Wildlife Research Units, U.S. Geological Survey. The Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about.


Logan Sleezer, Mississippi Cooperative Fish and Wildlife Research Unit; Mississippi State University, Department of Wildlife, Fisheries, and Aquaculture

Corey Dunn, U.S. Geological Survey, Mississippi Cooperative Fish and Wildlife Research Unit; Mississippi State University, Department of Wildlife, Fisheries, and Aquaculture

Michael Colvin, U.S. Geological Survey, Columbia Environmental Research Center

David Schumann; University of Wisconsin-La Crosse; Department of Biology and River Studies Center

Photo Credit
Corey Dunn
March 13, 2023