A habitat is the natural environment in which plants, animals and other organisms live, feed and breed. Many habitats are shared by numerous living things, forming what is called an ecosystem. Ecosystems range in size and can be as tiny as a patch of dirt or as large as the Chesapeake Bay watershed.
Sometimes, different species within the same ecosystem are forced to compete for resources like food, water and shelter. Dominant species and environmental stressors can take their toll on lesser plants and animals.
Rapidly increasing human development contributes to this environmental stress: as our population rises, so does our demand for the same resources that many plants and animals also depend on to survive. We build dams to control stream flow and capture energy, develop wilderness into urban hubs and use our finite freshwater resources at an alarming rate.
Migratory fish are particularly sensitive to ecosystem changes because they rely on certain migration routes between connected habitats to reach their breeding grounds. Dams, road culverts and other blockages that fragment waterways can act as barriers to fish passage.
In an effort to better understand the effect that dams and other manmade structures have on fish passage, Steve Minkkinen, project leader at the U.S. Fish and Wildlife Service (USFWS) Maryland Fisheries Office, has teamed up with the U.S. Geological Survey (USGS) and the U.S. Army Corps of Engineers to conduct a 10-year survey of American eel populations in the Susquehanna River.
“We learned quite a bit in 2013. We collected 300,000 juveniles [eels] and transported them above the [Conowingo] Dam. The dam has been blocking the [eels’] migration up the Susquehanna River,” Minkkinen explained. “There has been a lot of work [to open] upstream passage for shad and river herring,” Minkkinen continued. But that work has only focused on adult fish, and as Minkkinen pointed out, the dam’s flow is too fast for younger eels to travel through.
Monitoring American eels is important: at historic levels, they made up 20 percent of the freshwater biomass along the Eastern seaboard. However, the introduction of dams and other structures has blocked eel populations from important migration routes, changing eel populations.
Researchers capture and inject chips known as passive integrated transponders, or PIT tags, into the eels. These tags can be detected in future surveys and help the team track eel populations by letting them know if they are encountering a new eel or one that was caught during a previous survey.
The American eel is the only catadromous fish in the Bay region, which means they spend most of their lives in fresh water but migrate to the ocean to spawn. Spawning takes place in late January when the fish swim out of the Bay and into the Sargasso Sea, a portion of the Atlantic Ocean east of the Bahamas.
Eel larvae drift in ocean currents for nine to 12 months before reaching fresh water and swimming upstream. Monitoring allows scientists to study the migration habits of juvenile eels and learn how to aid their upstream journey.
Minkkinen and his team believe that if fish passage to the upper Susquehanna opens, both American eels and freshwater mussels would thrive. This bivalve relies on fish to store their eggs in their gills until the mussels turn into microscopic juveniles and drop off. Mussel populations in the upper Susquehanna are, for the most part, comprised of older, larger individuals. Because mussels are natural filter feeders, Minkkinen’s team believes that a rise in freshwater mussels will lead to cleaner water and a healthier ecosystem.
“Our hopes are that we can develop passage and restore eel and mussel habitat to that [upper Susquehanna] portion of the watershed,” Minkkinen said.
To view more photos, visit the Chesapeake Bay Program Flickr page.
Images by Steve Droter. Captions by Jenna Valente.
More than 40 miles of the Patapsco River will be opened to the annual migrations of herring, alewife and American shad once the waterway’s lowermost dam is removed.
Bloede Dam has blocked the passage of migratory fish for close to a century. It has also posed a public safety hazard, creating strong currents that have killed a number of swimmers. Its removal is the next step in the Patapsco River Restoration Project, and will be funded by a $3.57 million grant from the National Oceanic and Atmospheric Administration (NOAA)’s Restoration Center to American Rivers.
American Rivers has worked on the Patapsco project for the past five years with NOAA, the Maryland Department of Natural Resources (DNR) and the Friends of the Patapsco Valley State Park. The river’s Union and Simkins dams were removed in 2010 and 2011 in order to create better habitat for fish and a safer swimming hole for people.
“Removing one dam can make a major difference in the health of a river and its fisheries. But removing multiple dams… is really a game-changer,” said Serena McClain, director of river restoration at American Rivers, in a media release.
The Chesapeake Bay Program tracks the opening of fish passage as an indicator of Chesapeake Bay health, and has achieved 91 percent of its goal to open more than 2,800 miles of fish passage by 2014. Because dams, culverts and other barriers can disrupt the natural flow of rivers, their removal can mean a boost in habitat, a drop in pollution and improved protection from flooding.
Learn more about the removal of Bloede Dam.
American eel numbers are up in the headwater streams of Shenandoah National Park, following the 2004 removal of a large downstream dam.
Significant increases in upstream American eel populations began two years after the Rappahannock River's Embrey Dam was removed and have continued nearly every year since, according to a report released last week by the U.S. Geological Survey (USGS), U.S. Fish and Wildlife Service (USFWS) and National Park Service (NPS) researchers.
Image courtesy EricksonSmith/Flickr.
Dams can act as travel barriers to American eels, which undertake long-distance migrations from their ocean spawning grounds to freshwater streams along the Atlantic coast. While American eels can surpass substantial natural barriers--like the rapids of the Potomac River's Great Falls, for instance--dams pose a more difficult obstacle and have contributed to the widespread decline in American eel populations. Dam removal, therefore, could have long-term benefits for eel conservation.
"Our study shows that the benefits of dam removal can extend far upstream," said USGS biologist Nathanial Hitt. "American eels have been in decline for decades and so we're delighted to see them begin to return in abundance to their native streams."
Embrey Dam, which once provided hydroelectric power to Fredericksburg, Va., was breached in 2004 following years of work by nonprofit organizations and city, state and federal government agencies. Its removal was intended to benefit more than the American eel, however, as dams can impact a number of fish that must migrate up rivers to spawn.
"Shad, herring and striped bass are also using reopened habitat on the Rappahannock River," said Alan Weaver, fish passage coordinator for the Virginia Department of Game and Inland Fisheries. "It's exciting to see a growing number of species benefiting from dam removal in Virginia."
Learn more about American eel abundance in Shenandoah National Park.