Updated: Apr 24, 2020
The Susquehanna River drains nearly 71,000 square Kilometers making it the largest freshwater source to the Chesapeake Bay. The Susquehanna River has become a source of increasing focus over the years as the Chesapeake Bay is (at the time of writing) determined to be impaired for aquatic life by the United States Environmental Protection Agency based on high sediment, nitrate, and phosphate loading. Despite not being impaired for aquatic life, in 2005 large numbers of Susquehanna River young smallmouth bass Micropterus dolomieu were dying within their first year of life, and infections were cropping up on adult smallmouth bass in ways never seen before.
Smallmouth bass like the one shown above were becoming a common scene with white lesions, sores, and eroded fins plaguing these struggling smallmouth bass. In response, the PA Department of Environmental Protection quickly mobilized to figure out what was happening to these fish. Embarking on a scientific adventure to address the 14 different hypotheses they posited, they quickly got to work collecting fish.
Using a fleet of electroshock fishing boats (like the tow-behind version shown below) were used to canvas streams and rivers across the Susquehanna River watershed. Anodes and cathodes on the vessels create an electrical field that attracts fish from the depths so they can be netted, measured, and cataloged before being returned to the streams.
The study investigated and eliminated potential causes such as high flow, intraspecific competition, fatty acid food quality for young smallmouth bass, mortality from temperature, pH, mortality form dissolved oxygen, ammonia, and toxic chemicals.
Pathogens, Parasites, and Disease
Propagated by a combination of other stressors it was concluded that environmental conditions such as habitat loss and higher temperature in shallow spawning areas, and dissolved oxygen concentrations could increase the proliferation and infections from warm water bacteria (e.g. Aeromonas hydrophila, Flavobacterium columnare) (Smith et al., 2015). Viruses like large mouth bass virus that do not generally cause mortality among infected hosts can have a greater influence on immune-stressed fish.
Influence of Endocrine Disrupting Chemicals
Commonly found in Pharmaceuticals and Pesticides the effects of these chemical vary by organisms and by concentration (Vanderberg et al., 2012). However, many negative effects on fish including compromised immune responses and even some linkage to sterile fish, or the development of ovaries in male fish could cause poor egg quality and increased susceptibility to disease.
The Lethal Cocktail
Even bacteria and viruses that are generally not harmful to fish can have lethal effects or other negative impacts on the development of young fish when their immune system is compromised and can lead to death if the fish are stressed by warmer water, poor spawning grounds, and chemicals that compromise their immune response. The study also identified uncertainty among the following as they may have contributed to the stress and population decrease including interspecific competition, thiaminase food quality for young-of-year, algal and bacterial toxins, and the presence of pathogens and parasites.
What caused this?
Ultimately climate change is imposing unknown effects and ecosystem stress. Causing variable flow such as high flow events in the spring (e.g. high one-day rainfall) and dry hot summers. While endocrine disrupting chemicals are common in household products, pharmaceuticals, and agricultural sprays the applications of these are expanding. In an effort to reduce the sediment loading impairment of the Chesapeake Bay and other basins with a high amount of agricultural land use new techniques such as no-till plowing are being employed to reduce erosion. However, these techniques foster an increased dependency on herbicides that contain these endocrine disrupting chemicals.
For more information you can view the PA DEP report here