Changing Freshwater Flows Affect Fish Populations in the Potomac River


Smallmouth Bass School of Fish

From 1975 to 2017, some fish species that require stable or predictable river flows for spawning – like these smallmouth bass – have declined in abundance in the Potomac River. Photo credit: Gretchen Hansen

Millions of people rely on the Potomac River for drinking water and recreational opportunities. The Potomac is Maryland’s most popular freshwater fishing destination, and the second largest river that enters the Chesapeake Bay. Restoring fisheries is also an important goal for the Chesapeake Bay Partnership restoration efforts.

The USGS worked with Maryland Department of Natural Resources (MD DNR) to analyze fish population trends in the non-tidal Potomac River using data collected from 1975-2017. The aim of the study was to determine whether fish populations were increasing or decreasing, and if so, whether these changes could be explained based on species spawning strategies.

Primary findings:

  • Thirteen of the 28 species in the analysis had changed in abundance over the 43-year period of data.
  • The species that showed a decline in population included smallmouth bass and other fish species that require stable or predictable river flows for spawning. The species that showed a population increase included banded killifish, mosquitofish, and other species that thrive in unpredictable river flow conditions.
  • River flows during the spring spawning season have become less predictable over recent decades, and this has hindered some species while enhancing others.

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