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Agriculture & the Decline of Smallmouth Bass

(Posted by Robert A. Bachman.)

In a resolution passed unanimously at its winter quarterly meeting on January 10, 2010, the Pennsylvania Fish and Boat Commission made the following declaration:

…The Board of Commissioners of the Pennsylvania Fish & Boat Commission hereby expresses deep concern for the health of the Susquehanna River’s smallmouth bass fishery and asserts there is evidence showing that the water quality of the Susquehanna River has become increasingly impaired to the level that it is seriously impacting important elements of the Susquehanna River fishery, especially its nationally-reputed smallmouth bass fishery.”

The resolution was prompted by annual smallmouth bass surveys showing a decline in the number of smallmouth bass in the middle and lower Susquehanna River, a declining proportion of small bass in the population, cries of alarm from anglers about the decline in the their catch of bass and other warmwater fish such as rock bass and red breast sunfish, and most alarming of all, a lack of any good (above the long year average) year class of smallmouth bass since 2002. Even though water conditions in several of the intervening years were conducive to a good hatch, and typical numbers of fry hatched in those years, large numbers of diseased, dead and dying juvenile bass were found to be infected by Columnaris, a common bacteria found in river systems such as the Susquehanna, which does not effect the fish unless they are exposed to stress.

Columnaris
Columnaris in a Chinook salmon. Credit: U.S. Fish and Wildlife Service

At the same time anglers and biologists noted an increase in abundance of filamentous algae known as Cladophora covering large sections of the river. Dissolved oxygen concentrations in the microhabitat of the juvenile smallmouth bass were observed to drop to 4 parts per million at night, levels that by themselves are not lethal but stressful to small bass.

Cladophora
Cladophora

Although low oxygen concentrations may not be the only stressors that may contribute to the susceptibility of the juvenile smallmouth bass to Columnaris infection, the only known cause of night-time dips in dissolved oxygen is respiration (uptake of oxygen) by plants, especially mats of algae such as Cladophora and other aquatic plants known as periphyton—algae growing on the bottom and rocks.

Although measurements of total phosphorus have been steadily declining, reports by the U.S. Geological Survey and the Susquehanna River Basin show that dissolved inorganic phosphorus, the limiting nutrient for algal growth in fresh water, has been increasing for the past ten years in all seven sections of the Susquehanna monitored for these nutrients. The increase in dissolved inorganic phosphorus (DIP) and the increase in filamentous algae in the river conspicuously coincide with the decline of smallmouth bass recruitment. DIP has had a significant upward flow-adjusted trend at each of the major Susquehanna Basin sites including the West Branch Susquehanna and Juniata Rivers .

So what can be the cause of an increase in dissolved inorganic phosphorus when total phosphorus is continuing to decline? Dissolved phosphorus such as that found in manure quickly binds with soil particles and is rapidly taken up by growing plants. Soil samples saturated with phosphorus are beginning to show up on farm fields through the watershed. Dissolved phosphorus applied to such saturated soils either runs off or enters the groundwater after a heavy rain or snow melt. Application of manure on frozen ground, especially if applied on top of a few inches of snow can run off rapidly into nearby streams even though the application may meet BMP standard set backs as happened in a small tributary of the Conestoga River in Lancaster County on a sunny day in February 2009. See attached photos.

It is not only possible, but likely that other chemicals, such as endocrine disrupters that cause eggs to develop in the testes of smallmouth bass and other fish also contribute to the stress of juvenile smallmouth bass. Atrazine, a common herbicide banned in Europe and an agricultural pollutant used in the Susquehanna River Basin is a powerful endocrine disrupter, as are many pharmaceuticals discharged into the river from sewage treatment plants.

The question is not whether the decline of smallmouth bass is due to impaired water quality, but rather which of these many and variable pollutants, if any, is primary cause of the decline of smallmouth bass and other warmwater fishes? Or is it a perfect storm of many stressors that is causing the rapid decline? And what effect are these pollutants having on the Chesapeake Bay?

If we are to have any hope of restoring these recently-abundant and highly popular fisheries, the answers to these questions have to be found and remedial measures must be implemented as soon as possible.

For more information, read:

“Susquehanna River Basin Technical Report Pub. 272: Nutrients and Suspended Sediment Transported in the Susquehanna River Basin, 2009, and Trends, January 1985 Through December 2009,”

“U.S. Geological Survey Open-File Report 2007-1372 Changes in Streamflow, Concentrations, and Loads in Selected Nontidal Basins in the Chesapeake Bay Watershed, 1985-2006,” By Michael J. Langland, Douglas L. Moyer, and Joel Blomquist

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