Alewife floaters, a freshwater mussel. Photo by DRBC.

Freshwater mussels are bivalve mollusks that live along, and in, the sandy bottoms of rivers, streams and lakes.

They are considered one of, if not the, most at risk animal group in the United States, with many species considered threatened or endangered by state and federal governments.

There are over a dozen species of freshwater mussels which are native to the Delaware River Basin, but their populations have greatly declined over time.

The reduced population of freshwater mussels in the Delaware River Basin is consistent with what's happening nationwide. Habitat and water quality degradation are the primary factors for declining and imperiled mussel populations.  

Freshwater Mussel Life Cycle

Freshwater mussels have complicated reproduction strategies dependent on fish hosts.

Female mussels internally fertilize their eggs with the sperm of nearby males. Once fertilized, the larvae are released as fish pass by.

The larvae attach to the fish’s gills until they are larger. They then drop off and settle on the stream bottom; this can be quite a distance away from where the female mussel released her eggs. 

It is believed that each mussel species relies on different fish species to host their young.

Why Care about Freshwater Mussels?

They provide valuable ecosystem services by increasing water clarity, enriching habitats, and stabilizing bed erosion.

Most importantly, as filter-feeders, freshwater mussels clean the water in which they live, improving water quality.

They suck water in, trap solids (dirt, algae, and other pollutants) in their gills, and then release clean, filtered water back into the water column.

Each mussel can filter 10 gallons of water or more every day.

Mussels are sensitive to pollution and also erosion and sedimentation; they are more likely to be found in cleaner waterways. They also can live a long time - 50 years or more. 

These factors make freshwater mussels good long-term indicators of water quality and habitat conditon.

DRBC staff surveying for freshwater mussels. Photo by DRBC.

The importance of freshwater mussel populations within an ecosystem has driven several projects within the Delaware River Basin.

In the early 2000s, the United States Geological Survey - Northern Appalachian Research Laboratory (USGS-NARL) conducted comprehensive, qualitative mussel surveys of the Upper Delaware Scenic and Recreational River and the Delaware Water Gap National Recreation Area.

  • USGS: Qualitative and Quantitative Surveys of Native Freshwater mussels in the Upper and Middle Delaware River (2000-2002)

In 2013, to build on this previously-completed survey work, DRBC and USGS-NARL, partnered to conduct a study to provide a snapshot of mussel distribution and density in the Lower Delaware Scenic and Recreational River, the river section just below the Delaware Water Gap. Surveys were conducted over a period of seven weeks.

  • Read more: DRBC Completes Lower Delaware River Mussel Survey

The surveyors found more than 25,000 mussels! While the most common species was by far the eastern elliptio, six other species were found, as well as juveniles.

  • View Report: Freshwater Mussel Community Composition and Relative Abundance in the Lower Delaware River (pdf 691 KB; 2014)

Eastern floater. Photo by DRBC.	Eastern elliptio. Photo by DRBC.	Yellow lampmussel. Photo by DRBC.

In September 2019, the DRBC installed 24 mussel cages in the Delaware River across six locations, three upstream of the Lehigh River (Phillipsburg, N.J.; Sandts Eddy, Pa. and Belvidere, N.J.) and three downstream of the Lehigh (Raubs Island, Pa., Riegelsville, N.J. and Riegelsville, Pa.). The cages were checked in July 2020 and pulled in September 2020.

The objectives of this pilot study were to 1) quantify growth and survival of freshwater mussels upstream and downstream of the Lehigh River and 2) evaluate the feasibility of installing mussel cages in a large, rocky river. Mussels for the project came from the Partnership for the Delaware Estuary (PDE).

• View Study Summary: Growth and Survival of Caged Mussels in the Non-Tidal Delaware River (pdf; January 2022)

DRBC and PDE staff measure and tag each mussel with an identifier at the beginning of the study. Photo by DRBC.	One of the cages of freshwater mussels deployed in the Delaware River for this study. Photo by DRBC.	DRBC staff install one of the freshwater mussel cages in the river. Photo by DRBC.

Continued, improved and coordinated monitoring, expanded studies and enhanced efforts to protect, conserve and restore mussel habitat are all needed to ensure the sustainability of the Basin's freshwater mussel populations.

• Delaware Wetland Monitoring and Assessment Program: Delaware's Freshwater Mussels
  
  
• Fairmount Water Works: Freshwater Mussel Hatchery
  
  
• Partnership for the Delaware Estuary: Freshwater Mussels
  
  
• Pennsylvania Fish and Boat Commission: A Field Guide to Pennsylvania's Freshwater Mussels