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Management actions intended to benefit fish in large rivers can directly or indirectly affect multiple ecosystem components. Without consideration of the effects of management on non-target ecosystem components, unintended consequences may limit management efficacy. Monitoring can help clarify the effects of management actions, including on non-target ecosystem components, but only if data are collected to characterize key ecosystem processes that could affect the outcome. Scientists from across the U.S. convened to develop a conceptual model that would help identify monitoring information needed to better understand how natural and anthropogenic factors affect large river fishes. We applied the conceptual model to case studies in four large U.S. rivers. The application of the conceptual model indicates the model is flexible and relevant to large rivers in different geographic settings and with different management challenges. By visualizing how natural and anthropogenic drivers directly or indirectly affect cascading ecosystem tiers, our model identified critical information gaps and uncertainties that, if resolved, could inform how to best meet management objectives. Despite large differences in the physical and ecological contexts of the river systems, the case studies also demonstrated substantial commonalities in the data needed to better understand how stressors affect fish in these systems. For example, in most systems information on river discharge and water temperature were needed and available. Conversely, information regarding trophic relationships and the habitat requirements of larval fishes were generally lacking. This result suggests that there is a need to better understand a set of common factors across large-river systems. We provide a stepwise procedure to facilitate the application of our conceptual model to other river systems and management goals. [ABSTRACT FROM AUTHOR]

We assessed the effects of hydrology on growth and hatching success of age‐0 Channel Catfish Ictalurus punctatus in regulated and unregulated reaches of the Tallapoosa River basin, Alabama. Age‐0 Channel Catfish (N = 91) were collected from sites in both the Coastal Plain and Piedmont regions in fall 2003 and fall 2005. Lapillus otoliths were used to estimate the daily ages of age‐0 Channel Catfish, for which hatch dates were back‐calculated. We performed growth analysis to determine growth histories of each fish at 20‐d increments from hatch. Across the 2 years of sampling, Channel Catfish hatches were documented from June 7 to September 15. Ages and growth rates of age‐0 Channel Catfish ranged from 20 to 126 d and 0.60 to 1.5 mm/d, respectively. In general, growth was highest among age‐0 Channel Catfish from unregulated sites in the lower Coastal Plain, lowest among fish from unregulated sites in the Piedmont, and intermediate from regulated sites in the Piedmont. Successful hatching typically occurred during periods when mean discharges were in the upper two quartiles of flows for each site but not during exceptionally high peaks in flow. Physiographic province, the frequency of high pulses, and the number of flow reversals were the most important factors influencing the growth of recently hatched Channel Catfish. Results suggest that a low to moderate frequency of high pulses (25–150 pulses per 20‐d increment) and a moderate number of flow reversals (~100–175 reversals per 20‐d increment) enhances early growth of Channel Catfish in the Tallapoosa River system. Managing flow, when possible, to minimize large releases of water that result in exceptionally high pulses and providing minimal hydropeaking may improve hatching success during the Channel Catfish spawning season. [ABSTRACT FROM AUTHOR]

Understanding trends in the diverse resources provided by large rivers will help balance tradeoffs among stakeholders and inform strategies to mitigate the effects of landscape scale stressors such as climate change and invasive species. Absent a cohesive coordinated effort to assess trends in important large river resources, a logical starting point is to assess our ability to draw inferences from existing efforts. In this paper, we use a common analytical framework to analyze data from five disparate fish monitoring programs to better understand the nature of spatial and temporal trends in large river fish assemblages. We evaluated data from programs that monitor fishes in the Colorado, Columbia, Illinois, Mississippi, and Tallapoosa rivers using non-metric dimensional scaling ordinations and associated tests to evaluate trends in fish assemblage structure and native fish biodiversity. Our results indicate that fish assemblages exhibited significant spatial and temporal trends in all five of the rivers. We also document native species diversity trends that were variable within and between rivers and generally more evident in rivers with higher species richness and programs of longer duration. We discuss shared and basin-specific landscape level stressors. Having a basic understanding of the nature and extent of trends in fish assemblages is a necessary first step towards understanding factors affecting biodiversity and fisheries in large rivers. [ABSTRACT FROM AUTHOR]

We used an information-theoretic approach to examine the variation in detection probabilities for 87 Piedmont and Coastal Plain fishes in relation to instream gravel mining in four Alabama streams of the Mobile River drainage. Biotic and abiotic variables were also included in candidate models. Detection probabilities were heterogeneous across species and varied with habitat type, stream, season, and water quality. Instream gravel mining influenced the variation in detection probabilities for 38% of the species collected, probably because it led to habitat loss and increased sedimentation. Higher detection probabilities were apparent at unmined sites than at mined sites for 78% of the species for which gravel mining was shown to influence detection probabilities, indicating potential negative impacts to these species. Physical and chemical attributes also explained the variation in detection probabilities for many species. These results indicate that anthropogenic impacts can affect detection probabilities for fishes, and such variation should be considered when developing monitoring programs or routine sampling protocols. [ABSTRACT FROM AUTHOR]

We developed and validated methods for estimating the daily age of age-0 channel catfish Ictalurus punctatus. Two clutches of channel catfish eggs were hatched in the laboratory; subsequently, one was stocked in a 186-m2 earthen nursery pond and the other in a 757-L outdoor circular tank. Before stocking, subsamples of fish were collected at swim-up and 3 d after swim-up to evaluate early ring formation. Fish were sampled from the pond and tank on eight occasions ranging from 30 to 119 d posthatch. Distinct differences in early ring formation were found between yolk sac and free-swimming larval stages. Mean ring count and known age were closely related for tank- and pond-raised fish, indicating that daily ring deposition occurred in the otoliths of age-0 channel catfish up to 119 d posthatch. The accuracy of daily age estimation was similar between tank and pond samples, and daily ring counts were considerably accurate up to 60 d posthatch. Pond-raised fish were more difficult to age than tank-raised fish, which we attributed to ring compression resulting from slower growth among pond-raised fish after 30 d. The total length of tank- and pond-raised fish was positively related to otolith size; however, the slopes of the relationships between fish length and otolith radius were different between treatments. Therefore, we could not confirm that the relationship between fish length and otolith size was directly proportional for age-0 channel catfish. We encourage researchers to use this aging technique to determine how abiotic and biotic factors influence early life history characteristics and ultimately the population dynamics of catfishes (Ictaluridae). [ABSTRACT FROM AUTHOR]

We compared growth of flathead catfish Pylodictis olivaris from two native populations in Alabama (Coosa and Tallapoosa rivers) and two introduced populations in Georgia (Ocmulgee and Satilla rivers). We also compared mortality rates and potential outcomes of various management regimes (minimum length limits [MLLs]) among the populations. Total length-logl0(age) regression slopes for introduced fish were higher than those for native fish, and von Bertalanffy growth coefficients (K) were greater for introduced fish (Ocmulgee: 0.195; Satilla: 0.201) than for native individuals (Coosa: 0.057; Tallapoosa: 0.059). Therefore, introduced flathead catfish grew more rapidly than those in their native range. Mortality (instantaneous mortality rate, Z) was higher in the Satilla River population (Z =-0.602) than in the Ocmulgee River (Z =-0.227) and Coosa River (Z =-0.156) populations. However, fish in the Satilla River population had been introduced for only 10 years and presumably did not reach their theoretical maximum age, potentially biasing the mortality estimate for that population. Simulation of management regimes in Fishery Analyses and Simulation Tools software predicted that maximum biomass of flathead catfish in the Ocmulgee (1,668 kg) and Satilla (1,137 kg) rivers was substantially larger than that in the Coosa (873 kg) and Tallapoosa (768 kg) populations. However, increased exploitation rates in the Ocmulgee and Satilla River populations resulted in dramatic declines in overall biomass, especially at lower MLLs (254 and 356 mm, respectively). Therefore, in systems where introduced flathead catfish represent an important recreational fishery but have dramatically reduced the abundance of native fishes through predation, minimal protection is recommended. We contend that rapid growth of introduced flathead catfish has major implications for their management and the conservation of native fishes. [ABSTRACT FROM AUTHOR]

The toxicity of an acid mine drainage (AMD) mixing zone was investigated by placing bluegill (Lepomis macrochirus) at the confluence of a stream contaminated by AMD and a stream having neutral pH. A mixing channel receiving water from both streams was assembled in the field, during July and October 1996, to determine the toxicity of freshly mixed and aged water (2.9-7.5 min). The AMD stream had elevated concentrations of Al and Fe, which precipitated upon mixing, and of Mn, which did not precipitate in the mixing zone. Fish exposed to freshly mixed water had higher mortality than fish exposed to water after aging. Precipitating Al, but not Fe, accumulated on the gills of bluegill, and accumulation was more rapid early during the mixing process than after aging. Fish exposed for 3.5 h to freshly mixed water had hypertrophy and hyperplasia of gill filament and lamellar epithelial cells. Similar lesions were observed after 6.0 h in fish exposed to water aged after mixing. Results demonstrated that Al was the predominant metal accumulating on the gills of fish in this AMD mixing zone, and that mixing zones can be more toxic than AMD streams in equilibrium. [ABSTRACT FROM AUTHOR]

The toxicity of an acid mixing zone produced at the confluence of a stream that was contaminated by acid mine drainage (AMD) and a pH-neutral stream was investigated in toxicity tests with juvenile bluegill Lepomis macrochirus and largemouth bass Micropterus salmoides. Fish mortalities in instream cages located in the mixing zone, below the mixing zone, and upstream in both tributaries were compared to determine relative toxicity at each site. In all tests and for both species, significantly higher mortality was observed in the mixing zone than at any other location, including the acid stream, which had lower pH (2.9-4.3). The mixing zone was defined chemically by rapid precipitation of dissolved aluminum and iron, which arrived from the low-pH stream, and by the presence of white precipitates, which were attached to the substratum and which extended below the confluence. Possible seasonal changes in mixing zone toxicity were investigated by conducting field tests with bluegill in June, July, and August 1996 and in January 1997 and by conducting field tests with largemouth bass in April and May 1997. Toxicity was not significantly different at the extremes of temperature, pH, and metal concentration that occurred in June and July, as compared with January. Toxicity was significantly lower in August; however, elevated stream discharge during the August test may have disturbed mixing zone characteristics. High toxicity in AMD mixing zones may lower the survival of fishes in streams, reduce available habitat, and impede movements of migratory fish. [ABSTRACT FROM AUTHOR]

• Nitrogen removal service provided by oyster reefs in Mobile Bay, Alabama was estimated. • 34,911 ± 5,032 kg N yr−1 was removed through denitrification, sediment burial, and oyster harvest. • A replacement cost method was used to estimate the nitrogen benefit of $76,455 ± 11,020 yr−1. • The approach could be used elsewhere to estimate nitrogen removal service provided by oysters. Eastern oysters have been acknowledged for their important contribution to human well-being by providing goods and services including nitrogen removal from water bodies. In this study, we integrated daily environmental data (2008–2016) and filtration rate model parameter uncertainty to estimate nitrogen removal from denitrification and nitrogen burial services provided by the current extent of oyster (Crassostrea virginica) reefs in Mobile Bay, Alabama. Oyster landing data (2008–2016) in the Bay were also used to estimate nitrogen removal through oyster harvest. A replacement cost method using an engineering solution from wastewater treatment plants was implemented to quantify the economic benefit of the nitrogen removal. The estimated total nitrogen removal services provided by oyster reefs in Mobile Bay was 34,911 ± 5,032 kg N yr−1 (mean ± 1sd), in which 22,095 ± 3,305 kg N yr−1 from denitrification, 11,047 ± 1,652 kg N yr−1 from burial of nitrogen into sediments and 1,769 ± 876 kg N yr−1 by oyster harvest. The mean economic benefit was $76,455 ± 11,020 yr−1 which was estimated as $73.2 ± 11.5 ha−1 yr−1. This method could be used for any time period to estimate the nitrogen removal service in Mobile Bay. With proper modification of model parameters, this method could also be used elsewhere to estimate nitrogen removal services provided by oysters. [ABSTRACT FROM AUTHOR]

An electronic bacterial colony counter was used to count eggs of two clupeid species. Accuracy was evaluated by using regression techniques to model the relation between actual number of eggs and average machine count. When the data were log10 transformed, both models were significant (P < 0.001; r 2 ≥ 0.94). The method is accurate and fast. [ABSTRACT FROM PUBLISHER]