Hormone-altering compounds and herbicides are likely weakening the immune systems of Susquehanna River smallmouth bass, making them more susceptible to diseases, and that, in turn, has caused a population collapse for the bass, the river’s most popular recreational fish, a new study has concluded.

The multi-year study, which involved dozens of scientists from multiple state and federal agencies as well as universities, said that exposure to endocrine-disrupting compounds and herbicides, along with infections from parasites and pathogens, were the “most likely” reasons that few young smallmouth bass in the river have survived to become adults since 2005.

Several studies have found evidence of endocrine disruptors, which interfere with the hormone system in animals and fish, around the Bay watershed. Sometimes those findings were associated with isolated fish kills, but the Susquehanna report is the first time those compounds have been identified as the likely culprit in causing a widespread population collapse in the region.

Smallmouth bass once attracted anglers from around the world to the Bay’s largest tributary, but during the last decade the number of young fish that survive to become adults has plummeted in the middle section of the river from Sunbury, where the river’s West and North branches merge, to York Haven, a few miles north of the Maryland line. The collapse has also been observed on the lower portion of the Juniata, one of the Susquehanna’s largest tributaries. Altogether, about 100 miles of river are impacted.

The Pennsylvania Fish and Boat Commission has banned recreational harvest of the fish, but the population has not recovered and remains at less than a third of what it was a decade ago.

In addition, adult smallmouth bass have turned up with a variety of maladies, such as lesions, sores and dark blotches. A recent study found a surprisingly high rate of intersex — a condition in which adult males contain types of cells usually found only in females.

The poor health of smallmouth bass in much of the river has alarmed anglers, environmentalists and the Fish and Boat Commission, all of whom have called on the state Department of Environmental Protection to list the river as “impaired” — a designation that would ultimately require the development of a cleanup plan.

The DEP has resisted the designation because it could not identify what, if any, pollutants were causing the problem, though in recent years it has supported increased monitoring. It also helped coordinate the scientific review that was aimed at narrowing down the likely factors from a long list of suspects.

“This study does not identify a single smoking gun but it does point the way toward likely causes, which we will continue to pursue,” DEP Secretary John Quigley said in releasing the findings Dec. 14.

The scientists started with a list of 14 potential contributing factors, but ruled out eight, such as changes in river flow, changes in food quality and several others considered unlikely to cause fish mortality based on available information.

Several others were considered “uncertain” causes because of a lack of data, but scientists thought they could be contributing to the problem by stressing fish. Those included things such as increased water temperatures in some years, low dissolved oxygen, increases in harmful algae and habitat degradation.

But available information, the scientists said, pointed to two factors as the most likely causes:

  • Endocrine-disrupting chemicals and herbicides: Both have been shown to suppress the fish’s immune system and were found at higher levels within the study areas than in other areas around the state. Endocrine disruptors stem from several different sources including wastewater treatment plants, pharmaceuticals and a variety of chemical products. Many herbicides also act as endocrine disruptors, and the study said two common herbicides — atrazine and metolachlor — were at higher concentrations in the problem area than in other areas.
  • Pathogens and parasites: Several types were found in infected fish. Most of those found in the river typically do not kill fish — and, in fact, are found in other Pennsylvania rivers — but the study suggested that the weakened immune systems of the Susquehanna fish made them more vulnerable to the infections.

While the study identified the chemicals as a “most likely” contributor to the problem, it stopped short of proving the link. The study said it is hard to test small fish for exposure to those chemicals, but noted that the presence of intersex in adult fish, which has been linked to endocrine disruptors and certain herbicides, suggests those chemicals are present at levels that would affect smaller fish.

The next steps in the study will focus on determining the sources of chemicals, and whether the factors listed as uncertain causes are also contributing to the problem.

While the research continues, Quigley said the department may take some actions such as compiling an inventory of herbicides used near areas where smallmouth bass problems have been identified and developing outreach materials about herbicide use.

He also said the department’s upcoming plan to “reboot” the state’s lagging Chesapeake Bay nutrient control efforts could help.

“We will be emphasizing among other things, the installation of low-cost, high-impact BMPs [best management practices] like riparian buffers that can substantially improve water quality and, by filtering out some, but not all, endocrine-disrupting compounds, they can also help smallmouth bass populations,” Quigley said.

But he said the findings from the report do not necessarily mean the DEP would list the river as impaired. That decision will not be made until February, when the state finalizes the list of impaired waters it submits to the EPA, something which happens every three years.

John Arway, executive director of the Fish and Boat Commission, which is an independent agency, said the study’s findings support the commission’s longstanding call for an impairment listing.

“We’ve gone on for 10 years already,” he said. “We’ve finally come to a decision point where we admit that bass are sick, the river is in trouble and we have some causes that we can begin working on to try to fix.”

Arway said the DEP was using a different standard for the Susquehanna than it does for other rivers because impairment decisions are usually made based on evidence of biological impacts without waiting to determine the exact cause. “It is a little unusual for the river that they needed to know the sources and causes of the impairment before they could make an impairment decision,” he said.

Arway also said that he expected that continued research would implicate other factors that are causing problems that stress the fish, such as low dissolved oxygen levels caused by nutrients.

“Endocrine disruptors have been identified as one of those causes, but in my opinion you are going to eventually see some of those uncertain causes be identified as likely causes, too,” he said. “We always felt it wasn’t just one thing. It was multiple stressors.”

Harry Campbell, Pennsylvania executive director for the Chesapeake Bay Foundation, also called for the DEP to list the river as impaired, saying it would help revive its world-class smallmouth bass fishery.

“An impairment listing will help start the healing process, so that the waterway that millions of Pennsylvanians depend upon, and that provides half of the freshwater to the Chesapeake Bay, can benefit from an unwavering level of restoration, resource investment and pollution study,” he said.

As recently as 2007, smallmouth bass accounted for about two-thirds of the angler catches in the middle Susquehanna and lower Juniata, generating $3.35 million in economic activity, but that’s been greatly reduced.

“All you have to do is drive around the watershed and look at the parking lots on the first day of bass season,” Arway said. “You don’t see many boats and trailers in those parking lots.”

Besides the Susquehanna, intersex has been seen in smallmouth bass and in some other fish in the Potomac and Shenandoah rivers over the last decade. A study by U.S. Geological Survey and U.S. Fish and Wildlife Service scientists published in December found intersex in fish at all 27 National Wildlife Refuge sites tested from the Chesapeake Bay to Maine.

The Bay Program’s recently completed management strategy for toxics research identified endocrine-disrupting compounds as a key area needing more research, and the USGS in 2014 launched a five-year study to understand the impact of endocrine disruptors in the watershed. Several studies are also examining the effectiveness of various best management practices in controlling the runoff of those compounds.