The nontidal Potomac River is Maryland's most popular freshwater fishery, largely due to the multiple angling opportunities and accessibility. Increasing popularity of the Muskellunge Esox masquinongy fishery has prompted further investigation of Muskellunge seasonal movement patterns and summer habitat use during thermally stressful periods. Radiotelemetry was used to investigate seasonal movement and summertime habitat use by Muskellunge in the Potomac River from 2017 to 2020. Movement was greatest during spring, with reduced movement during summer, fall, and winter. Potomac River main-stem temperatures during summer often exceeded levels considered stressful to Muskellunge. During thermally stressful periods, Muskellunge sought thermal refugia and movements ceased. A generalized linear mixed model predicted that (1) when daily average temperatures reached 24.0 degrees C, over 50% of the Muskellunge habitually occupied thermal refugia and (2) at 26.0 degrees C, greater than 90% of tagged Muskellunge were located in refuge sites. Aggregation of Muskellunge in thermal refugia during summer has implications for increased angling vulnerability and fishing mortality in a warming climate, which could potentially limit the quality of these sport fisheries in southern river systems.

Decades of poor reproductive success and young-of-the-year survival, combined with adult mortality events, have led to a decline in the smallmouth bass (SMB; Micropterus dolomieu) population in sections of the Potomac River. Previous studies have identified numerous biologic and environmental stressors associated with negative effects on SMB health. To better understand the impact of these stressors, this study was conducted at the confluence of Antietam Creek and the Potomac River from 2013 to 2019 to identify temporal changes associated with SMB reproductive health. Surface water samples were collected and analyzed for over 300 organic contaminants, including pesticides, phytoestrogens, pharmaceuticals, hormones and total estrogenicity (E2Eq). Adult SMB were collected and sampled for multiple endpoints, including gene transcripts associated with reproduction (molecular), histopathology (cellular), and organosomatic indices (tissue). In males, biomarkers of estrogenic endocrine disruption, including testicular oocytes (TO) and plasma vitellogenin (Vtg) were assessed. Numerous agriculture-related contaminants or land use patterns were associated with gene transcript abundance in both male and female SMB. Positive associations between pesticides in the immediate catchment with TO severity and E2Eq with plasma Vtg in males were identified. In males, the prevalence of TO and detectable levels of plasma Vtg, liver vitellogenin transcripts (vtg) and testis vtg were high throughout the study. Peaks of complex mixtures of numerous contaminants occurred during the spring/early summer when spawning and early development occurs and to a lesser extent in fall/winter during recrudescence. Management practices to reduce exposure during these critical and sensitive periods may enhance reproductive health of these economically important sportfishes.

Using transverse cross-sectional transects, a survey of 31 km of the non-tidal Potomac River was conducted from White’s Ferry, Virginia, to Brunswick, Maryland, USA, between June and September in 2013 through 2015 to assess a recurring benthic cyanobacteria bloom. Abundant benthic cyanobacteria blooms were detected during the 2014 and 2015 sampling seasons and the primary taxon was identified morphologically and molecularly as Planktothrix cf. isothrix . When present, P . cf. isothrix blooms were concentrated from river center to the Maryland shoreline. This pattern was correlated with significantly greater benthic chlorophyll- a and phycocyanin concentrations. In an apparent response to the P. cf. isothrix blooms in the study site, aquatic macroinvertebrate community assemblages were significantly different between areas with extensive benthic cyanobacterial growth compared to areas without cyanobacterial growth. Within the P. cf . isothrix mats, the percentage of pollution sensitive taxa was lower, and the percentage of pollution tolerant taxa was greater. These data suggest that P . cf. isothrix can act as an ecosystem disruptor through direct impacts to the aquatic macroinvertebrate abundance and community structure within this section of the freshwater, non-tidal Potomac River.

Abstract Life history theory provides a framework to understand environmental change based on species traits that capitalize on stable, predictable, or stochastic environmental conditions. We evaluated life history strategies and temporal trends in abundance from 1975 to 2017 for 28 fish species within the Potomac River of eastern North America. Multivariate analysis identified life history strategies defined by small‐bodied species with extended spawning seasons and early maturation (opportunistic strategists), large‐bodied species with delayed maturation and long lifespans (periodic strategists), and intermediate‐sized species exhibiting parental care (equilibrium strategists). Linear mixed models detected temporal trends in abundance for 13 species (46%), of which nine increased and four decreased over time. Increasing species were characterized by opportunistic life history strategies (e.g., banded killifish, Fundulus diaphanous; mosquito fish, Gambusia holbrooki), whereas decreasing species were characterized by periodic or equilibrium strategies (e.g., smallmouth bass, Micropterus dolomieu; river chub, Nocomis micropogon). Recent introductions can account for temporal increases in some cases, but most increasing species are native to the study area. Observed increases in opportunistic native species and decreases in periodic and equilibrium species indicate that environmental conditions have become less stable and less predictable over time, consistent with observed increases in spring peak flows in the study area. Our study indicates the importance of environmental stochasticity for fish community responses to land use and climate change and demonstrates the utility of life history theory in this regard.

Detecting temporal changes in fish abundance is an essential component of fisheries management. Because of the need to understand short-term and nonlinear changes in fish abundance, traditional linear models may not provide adequate information for management decisions. This study highlights the utility of Bayesian dynamic linear models (DLMs) as a tool for quantifying temporal dynamics in fish abundance. To achieve this goal, we quantified temporal trends of Smallmouth Bass Micropterus dolomieu catch per effort (CPE) from rivers in the mid-Atlantic states, and we calculated annual probabilities of decline from the posterior distributions of annual rates of change in CPE. We were interested in annual declines because of recent concerns about fish health in portions of the study area. In general, periods of decline were greatest within the Susquehanna River basin, Pennsylvania. The declines in CPE began in the late 1990sprior to observations of fish health problemsand began to stabilize toward the end of the time series (2011). In contrast, many of the other rivers investigated did not have the same magnitude or duration of decline in CPE. Bayesian DLMs provide information about annual changes in abundance that can inform management and are easily communicated with managers and stakeholders.

Intersex (specifically, testicular oocytes) has been observed in male smallmouth bass (SMB; Micropterus dolomieu) and other centrarchids in the South Branch of the Potomac River, USA, and forks of the Shenandoah River, USA, during the past five years. This condition often is associated with exposure to estrogenic endocrine-disrupting chemicals in some fish species, but such chemicals and their sources have yet to be identified in the Potomac. In an attempt to better understand the plausible causes of this condition, we investigated the reproductive health of bass sampled up-and downstream of wastewater treatment plant (WWTP) effluent point sources on the Potomac River in Maryland, USA. Smallmouth bass were sampled from the Conococheague Creek and the Monocacy River, and largemouth bass (LMB; Micropterus salmoides) were collected near the Blue Plains WWTP on the mainstem of the Potomac River. Chemical analyses of compounds captured in passive samplers at these locations also were conducted. A high prevalence of intersex (82-100%) was identified in male SMB at all sites regardless of collection area. A lower prevalence of intersex (23%) was identified in male LMB collected at the Blue Plains site. When up-and downstream fish were compared, significant differences were noted only in fish from the Conococheague. Differences included condition factor, gonadosomatic index, plasma vitellogenin concentration, and estrogen to testosterone ratio. In general, chemicals associated with wastewater effluent, storm-water runoff, and agriculture were more prevalent at the downstream sampling sites. An exception was atrazine and its associated metabolites, which were present in greater concentrations at the upstream sites. It appears that proximity to effluent from WWTPs may influence the reproductive health of bass in the Potomac watershed, but inputs from other sources likely contribute to the widespread, high incidence of testicular oocytes.

The seasonal occurrence of organic contaminants, many of which are potential endocrine disruptors, entering the Potomac River, USA, watershed was investigated using a two-pronged approach during the fall of 2005 and spring of 2006. Passive samplers (semipermeable membrane device and polar organic chemical integrative sampler [POCIS]) were deployed in tandem at sites above and below wastewater treatment plant discharges within the watershed. Analysis of the samplers resulted in detection of 84 of 138 targeted chemicals. The agricultural pesticides atrazine and metolachlor had the greatest seasonal changes in water concentrations, with a 3.1- to 91-fold increase in the spring compared with the level in the previous fall. Coinciding with the elevated concentrations of atrazine in the spring were increasing concentrations of the atrazine degradation products desethylatrazine and desisopropylatrazine in the fall following spring and summer application of the parent compound. Other targeted chemicals (organochlorine pesticides, polycyclic aromatic hydrocarbons, and organic wastewater chemicals) did not indicate seasonal changes in occurrence or concentration; however, the overall concentrations and number of chemicals present were greater at the sites downstream of wastewater treatment plant discharges. Several fragrances and flame retardants were identified in these downstream sites, which are characteristic of wastewater effluent and human activities. The bioluminescent yeast estrogen screen in vitro assay of the POCIS extracts indicated the presence of chemicals that were capable of producing an estrogenic response at all sampling sites.