Over the past decade, smallmouth bass in five Chesapeake Bay tributaries have suffered from fish kills and perplexing illnesses—and nutrient pollution could be to blame.
According to a new report from the Chesapeake Bay Foundation (CBF), excess nitrogen and phosphorous in our rivers and streams could be behind two of the leading problems affecting smallmouth bass: first, the rapid growth of fish parasites and their hosts, and second, the expansion of large algae blooms that can lead to low-oxygen conditions and spikes in pH. When paired with rising water temperatures and ever more prevalent chemical contaminants, nutrient pollution seems to have created a “perfect storm” of factors that are making smallmouth bass more susceptible to infections and death.
Image courtesy Mr. OutdoorGuy/Flickr
In a media call, CBF President Will Baker called the smallmouth bass “the canary in the coal mine for the Bay’s rivers.” Because the fish is sensitive to pollution, problems within the population could indicate problems within the Bay.
Smallmouth bass in the Susquehanna, Monocacy, Shenandoah, Cowpasture and South Branch of the Potomac rivers have seen a string of recent health problems, from open sores and wart-like growths to abnormal sexual development. In the Susquehanna, smallmouth bass populations have plummeted so far that Pennsylvania has made it illegal to catch the fish during spawning season.
“Our fish are sick, our anglers are mad and my board and I—protectors of our [smallmouth bass] fishery—are frustrated,” said John Arway, executive director of the Pennsylvania Fish and Boat Commission. “Our bass, and our grandchildren who will fish for them, are depending on us to fix the problem.”
Image courtesy CBF
While specific causes of smallmouth bass fish kills and illnesses remain unclear, CBF has called on state and local governments to accelerate their pollution-reduction efforts in hopes of improving water quality and saving the driving force behind a $630 million recreational fishing industry. The non-profit has also called on the federal government to designate a 98-mile stretch of the Susquehanna as impaired, which would commit Pennsylvania to reversing the river’s decline.
“This is the moment in time to save fishing in our streams and rivers, as well as the jobs and quality of life that are connected to it,” Baker said.
U.S. Geological Survey researchers have found a possible connection between the occurrence of intersex fish and fish kills and lesions on bass in the Potomac and Shenandoah rivers.
In a recently published study, USGS researchers showed that largemouth bass that were injected with estrogen produced lower levels of a hormone called hepcidin. In addition to regulating iron in mammals, hepcidin is suspected to act as an antimicrobial peptide in fish, frogs and mammals. Antimicrobial peptides are the first line of defense against disease-causing bacteria, fungi and viruses.
“Our research suggests that estrogen-mimicking compounds may make fish more susceptible to disease by blocking production of hepcidin and other immune-related proteins that help protect fish against disease-causing bacteria,” said Dr. Laura Robertson, a USGS genomics researcher who led the study.
The study showed that the estrogen blocked the production of hepcidin in fish that were exposed to bacteria, giving more weight to the theory that estrogen or estrogen-mimicking chemicals could be making fish more susceptible to diseases, according to Robertson.
USGS researchers found intersex fish -- or fish with both male and female reproductive traits -- in the Potomac and Shenandoah rivers several years ago. Studies have shown that estrogen and estrogen-mimicking compounds can cause intersex traits to appear in fish.
Because fish lesions, fish kills and intersex traits have been found to co-occur in the Potomac and Shenandoah rivers, USGS scientists have theorized that estrogen-mimicking compounds could also be involved in lesions and fish kills.
Birth control pills, hormone replacements and hormones from livestock operations are a few possible sources of estrogen and estrogen-mimicking chemicals. These chemicals are found in treated wastewater, as they are not removed during normal sewage treatment processes.
Learn more about this USGS study.
Nutrients are generally considered to be a good thing for humans because they are necessary for our health and strength. But just like anything else, too many nutrients can be too much of a good thing. In fact, excess nutrients are the main cause of the Bay's degraded water quality and aquatic habitat loss.
Nutrients occur naturally in air, water and soil. However, in addition to these natural sources, vehicle exhaust, treated sewage and runoff from urban, residential and agricultural areas all contribute nutrients to the Bay and its tributaries.
Together, these sources send too many nutrients to the water, causing serious problems:
This summer, high nutrient levels were responsible for harmful algal blooms and low DO levels that led to fish kills in several Bay tributaries, including:
Bay scientists have found that reducing nutrient loads to the Bay in spring is critical to improving water quality conditions. The majority of nutrients are washed into the Bay in spring as snow melts and larger amounts of rain fall on the watershed. Planting cover crops in the fall and skipping spring fertilizer are two important ways people can reduce the amount of nutrients that enter the Bay.