The pH preferred and avoided by wild, adult brook trout Salvelinus fontinalis and brown trout Salmo trutta was examined in a series a laboratory tests using gradual and steep-gradient flow-through aquaria. The results were compared with those published for the observed segregation patterns of juvenile S. fontinalis and S. trutta in Pennsylvania streams. The adult S. trutta tested showed a preference for pH 4·0 while adult S. fontinalis did not prefer any pH within the range tested. Salmo trutta are not found in Pennsylvania streams with a base-flow pH < 5·8 which suggests that S. trutta prefer pH well above 4·0. Adult S. trutta displayed a lack of avoidance at pH below 5·0, as also reported earlier for juveniles. The avoidance pH of wild, adult S. fontinalis (between pH 5·5 and 6·0) and S. trutta (between pH 6·5 and 7·0) did not differ appreciably from earlier study results for the avoidance pH of juvenile S. fontinalis and S. trutta. A comparison of c.i. around these avoidance estimates indicates that avoidance pH is similar among adult S. fontinalis and S. trutta in this study. The limited overlap of c.i. for avoidance pH values for the two species, however, suggests that some S. trutta will display avoidance at a higher pH when S. fontinalis will not. The results of this study indicate that segregation patterns of adult S. fontinalis and S. trutta in Pennsylvania streams could be related to pH and that competition with S. trutta could be mediating the occurrence of S. fontinalis at some pH levels.

Early life stage (ELS) fishes provide a valuable metric for species, population, and ecosystem monitoring. Industrial, manufacturing, and power generating facilities in the United States can be required to monitor ELS fishes to assess impacts of facility intake waters.Traditional methods for collecting, identifying, and enumerating ELS fishes include ichthyoplankton netting and pumping from intake and discharge waters. However, sampling at these sites can prove challenging from logistical and safety standpoints, with added challenges of identifying and post-processing of ELS fishes to quantify facility impacts on waterbodies for regulatory purposes. Methods utilizing environmental DNA (eDNA) may offer improvements in these areas. This study assessed the utility of novel, species-specific qPCR assays to detect eDNA from three differentially abundant fish species (alewife (Alosa pseudoharengus; Wilson, 1811), gizzard shad (Dorosoma cepedianum; Lesueur, 1818), and yellow perch (Perca flavescens; Mitchill, 1814)) at the outflow of an industrial site. eDNA concentrations were compared with abundance estimates derived from two conventional collection methods to explore the potential utility of eDNA-based methods in future monitoring. The likelihood of detecting gizzard shad, yellow perch, and alewife in the discharge waters of the power generation station was not significantly different among eDNA, plankton netting, or pump sampling. Results suggest successful detections differ by species and time of year for each method.Gizzard shad eDNA relative abundance correlated strongly with total larval abundances captured by the pumping but not the netting methods, whereas yellow perch eDNA abundance was found to correlate with both conventional methods. Alewife was not detected by any method, consistent with documentation of the decline in this species within the lake. Overall, our study found a positive relationship between eDNA abundance and larval fish abundance in both daily and seasonal sampling, suggesting that fluctuations in eDNA concentration may be linked to larval abundance.

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Larval fishes provide a valuable metric for assessing and monitoring species, populations, and ecosystem trends and condition. However, taxonomic resolution for this life stage is inherently problematic because of their individual sizes, limited morphological characteristics and high tissue degradation rates. There is little research on methods that rapidly preserve larval tissues for later morphological and molecular identification. The goal of this study was to test methods of rapidly killing fish embryos that maintain both morphological and molecular integrity. Rapid cooling with dry ice successfully maintained morphological and molecular integrity and may offer a simple and cost‐effective approach for larval fish identification.