Risk Factors Associated with Mortality of Age‐0 Smallmouth Bass in the Susquehanna River Basin, Pennsylvania

Abstract

Evidence of disease and mortalities of young of the year (age‐0) Smallmouth Bass Micropterus dolomieu has occurred during the late spring and summer in many parts of the Susquehanna River watershed since 2005. To better understand contributing factors, fish collected from multiple areas throughout the watershed as well as out‐of‐basin reference populations (Allegheny and Delaware River basins; experimental ponds, Kearneysville, West Virginia) were examined grossly and histologically for abnormalities. Tissue contaminant concentrations were determined from whole‐body homogenates, and water contaminant concentrations were estimated using time‐integrated passive samplers at selected sites. Observed or isolated pathogens included bacteria, predominantly motile Aeromonas spp. and Flavobacterium columnare; largemouth bass virus, and parasites, including trematode metacercariae, cestodes, and the myxozoan Myxobolus inornatus. Although these pathogens were found in age‐0 Smallmouth Bass from multiple sites, no one pathogen was consistently associated with mortality. Chemicals detected in tissue included polychlorinated biphenyl (PCB) congeners, organochlorine, and current‐use pesticides. Pyraclostrobin, PCB congeners 170 and 187, cis‐chlordane and trans‐nonachlor were detected in all Susquehanna watershed samples but rarely in samples from the reference site. The findings support the idea that there is no single cause for disease of age‐0 Smallmouth Bass; rather the cumulative effects of co‐infections and potential immunomodulation by environmental stressors during a sensitive developmental life stage may lead to mortality. Identifying the most important risk factors will be necessary for more in‐depth analyses of individual stressors and better management of the habitat and fish populations.

Spatial and temporal variability of myxozoan parasite, Myxobolus inornatus, prevalence in young of the year smallmouth bass in the Susquehanna River Basin, Pennsylvania

“…Coinfections with bacterial, viral and parasitic pathogens have been suggested to contribute to the mortality of YOY smallmouth bass and consequent declines in abundance (Walsh et al., ). However, the underlying environmental conditions contributing to increased pathogen prevalence/virulence or immunomodulation of the host fish are not fully understood.…”

“…Ruptured cysts can release spores into the tissue eliciting inflammation (Walsh, Blazer et al., ). Subsequent research identified myxozoan cysts with similar morphology and tissue tropism, in the mouth, opercula and musculature (Walsh et al., ).…”

“…The discovery of M. Inornatus in Susquehanna River YOY smallmouth bass was one of many stressors identified as potential risk factors for YOY disease and mortality (Smith, Blazer, Walsh, Iwanowicz, & Sperry, ; Walsh et al., ). Multiple pathogens and parasites have been identified including bacteria (e.g., multiple aeromonads, Flavobacterium columnare (Bernardet and Grimont); Starliper, Blazer, Iwanowicz, & Walsh, ), largemouth bass virus (Boonthai et al., ; Smith et al., ; Starliper et al., ), trematodes (Smith et al., ) and cestodes (Walsh et al., ).…”

“…The discovery of M. Inornatus in Susquehanna River YOY smallmouth bass was one of many stressors identified as potential risk factors for YOY disease and mortality (Smith, Blazer, Walsh, Iwanowicz, & Sperry, ; Walsh et al., ). Multiple pathogens and parasites have been identified including bacteria (e.g., multiple aeromonads, Flavobacterium columnare (Bernardet and Grimont); Starliper, Blazer, Iwanowicz, & Walsh, ), largemouth bass virus (Boonthai et al., ; Smith et al., ; Starliper et al., ), trematodes (Smith et al., ) and cestodes (Walsh et al., ). In addition to pathogens and parasites, water quality concerns including elevated water temperatures, suboptimal dissolved oxygen concentrations and contaminants have also been investigated in regard to the overall health of YOY smallmouth bass in the Susquehanna River Basin (Blazer et al., ; Chaplin, Crawford, & Brightbill, ; Smith et al., ; Walsh et al., ).…”

“…Although it is not understood what potential role M. Inornatus may be playing in the mortality of YOY smallmouth bass, it was a common finding throughout the Susquehanna River Basin between 2007 and 2012 (Walsh et al., ). However, investigation of how M. Inornatus parasite prevalence varies temporally and spatially across sites in this system has not been evaluated.…”