Introducing MAHI: A new tool for quantifying freshwater mussel health

Photo Credit

courtesy of Scott Sharaga, National Park Service

Freshwater mussels are important members of stream ecosystems. MAHI can help people understand how land management decisions affect freshwater mussel communities and streams in general.

A huge mussel study in 92 streams across 13 states is underway. As part of that study, researchers needed an objective way to assess mussel health in each stream – so they developed the Mussel Assemblage Health Index and made it available as a spreadsheet tool. 

Resource managers can use MAHI scores in several ways. Higher numbers indicate better health, so lower scores can mean that conservation action is needed. By tracking MAHI scores over time, people can detect mussel declines early, in time for proactive conservation.

MAHI can also help evaluate the effectiveness of conservation actions.

“For any kind of management or conservation action, we need to know if it works,” says USDA Forest Service researcher Wendell Haag. “Managers can use MAHI scores to objectively assess how an action is affecting mussel assemblage health. This is a critical thing for which no comprehensive and objective method existed previously.”

MAHI scores also account for differences in mussel faunas across different regions and habitats, which means scores can be compared across eastern North America.

“MAHI works across a very large region using data that are generally collected in mussel surveys,” says Traci DuBose. DuBose is an Oak Ridge Institute for Science and Education postdoctoral research fellow who worked on MAHI with Haag and others.

For mussel surveys, people – usually state or federal resource managers or university researchers – place a square PVC frame, called a quadrat, on the river bottom and search the area within the frame for mussels. “Then, they collect the mussels, count them, measure their size, and identify to species. And return the mussels unharmed,” says DuBose. “There are other ways people might survey mussels, but this a common approach.”

With mussel survey data and a few other pieces of information, MAHI calculates four metrics: abundance (an estimate of the population size), recruitment (the amount of reproduction over the last ten years), population growth (change in abundance over time), and species loss (what proportion of the original mussel community is still present).

“Each of these metrics represents a fundamental aspect of mussel population or assemblage health,” says Haag. From the four metrics, MAHI provides an overall score from 0-10, with higher numbers indicating better health.

Conceptually, MAHI is similar to the Index of Biotic Integrity (IBI), which has been used for more than 30 years to assess stream health based on surveys of aquatic insects and fishes.

“However, IBI does not tell us anything about the health of mussel communities,” says Haag. "For example, some streams with high IBI scores have lost their mussels, and vice versa. This tells us that mussels are responding to environmental conditions differently than insects and fishes, and including mussels provides a more comprehensive assessment of stream health.”

Freshwater mussels are in serious decline. The declines are enigmatic, often occurring even in streams that have good water quality and healthy populations of fishes and aquatic insects.

Photo Credit

Sarah Farmer, USDA Forest Service

In streams, sediment is considered a pollutant and it can harm mussels and other aquatic creatures. The image shows clay-colored, sediment-laden waters flowing into the North Toe River in North Carolina. 

Haag, DuBose, and others, including partners from American Rivers, are conducting a huge study designed to identify the cause of enigmatic mussel declines.

Freshwater mussels are filter feeders that eat plankton, microbes, plant particles, and other detritus. In this way, large populations of mussels can have a dramatic, positive effect on water clarity and quality.

“Mussels are unassuming, hardworking animals. They’re the unsung heroes in rivers,” says DuBose. “MAHI is a great example of how we can use data to work better at big spatial scales – which are the scales we can use to conserve freshwater mussel assemblages.”

Read more about MAHI in the journal article in Freshwater Science.

For more information, email Wendell Haag at wendell.haag@usda.gov or Traci DuBose at tracipdubose@gmail.com.