Sometimes the best news about nature can show up in tiny traces. NOAA Fisheries says scientists have found signs that river herring are using habitat in Maryland’s Patapsco River that had been closed to them. The turning point came with the removal of the large dam in 2018. Years later, tiny traces of DNA and two adult fish show herring finding their way back. It's a small but hopeful sign that rivers can recover if we clear the way.
A dam that blocked the Patapsco River
The Bloede Dam was located on the Patapsco River near Baltimore. For over 100 years, it was the first obstacle that fish met when swimming upstream from the Chesapeake Bay. The old dam had also been a significant threat to public safety for many years, officials say. In 2018, NOAA and partners removed the dam, along with American Rivers and other organizations. The agency reportedly spent more than $9.3 million on the project and monitoring there. Other funding sources included the National Fish and Wildlife Foundation and the U.S. Fish and Wildlife Service.
How scientists tracked fish they couldn't see
River herring are the collective name for two small, silvery fish, the alewife and the blueback herring. Millions of fish migrate to spawning and rearing habitat every year. Barriers such as dams and culverts can block them. If fish cannot get to their habitat, they cannot reproduce, and their populations can decline. Scientists used eDNA, or environmental DNA, to see if the herring were returning. Fish constantly shed scales, tissue, and other genetic material into the water. Researchers take a sample of water and test it for recent visitors. A team from the Smithsonian Environmental Research Center and the University of Maryland Center for Environmental Science has used this method at the site.
What the study found
The results came from a 2023 study in PLOS ONE. The study found that in the four years before the removal of the dam, no adult river herring, eDNA, or eggs were found upstream of the dam, even with a fish ladder. A fish ladder is basically a structure built to help fish pass a dam. After the dam came down, it was different. Within three years, the chance of finding alewife eDNA upstream went from 0 to 5 percent. For blueback herring, it went from 0 to 13 percent. In 2021, biologists with the Maryland Department of Natural Resources recorded two adult river herring upstream, one alewife and one blueback herring.
A hopeful start, but not a finished story
{{/usCountry}}The results came from a 2023 study in PLOS ONE. The study found that in the four years before the removal of the dam, no adult river herring, eDNA, or eggs were found upstream of the dam, even with a fish ladder. A fish ladder is basically a structure built to help fish pass a dam. After the dam came down, it was different. Within three years, the chance of finding alewife eDNA upstream went from 0 to 5 percent. For blueback herring, it went from 0 to 13 percent. In 2021, biologists with the Maryland Department of Natural Resources recorded two adult river herring upstream, one alewife and one blueback herring.
A hopeful start, but not a finished story
{{/usCountry}}The research showed early signs that herring were making use of the new habitat above the old dam. However, this was the case for only a small proportion of the fish population in the river. No change in egg numbers was noted, and no tagged fish were detected upstream after removal. The authors note that continued long-term monitoring is needed to understand any change in the population. So that’s an early win; it’s not proof that herring numbers have come back.
Bigger than just one river
Bloede wasn't the only dam that was removed. Previous NOAA-supported projects removed Union Dam and Simkins Dam further upstream in 2010. The three removals, together, have opened up more than 65 miles of habitat to blueback herring, alewife, American shad and hickory shad, and more than 183 miles for American eel. NOAA’s community-based restoration program has reopened fish passage in over 4,400 miles of streams and rivers. In December 2022, the agency also announced funding for 36 new fish passage projects around the country. For migrating fish, an open river is only the beginning, but it’s the part that needs to come first.