Comparison of historical and current data are critical in establishing population trends for imperiled taxa. In this paper we revisit the status of the Bayou darter Nothonotus rubrum, an endemic fish restricted to the Bayou Pierre river system, Mississippi, USA. The Bayou Pierre has experienced substantial geomorphic change in the past century, leading to questions about persistence of this imperiled species. We employed historical field notes and museum records to identify collection localities, and we resampled 32 of these localities using methodologies comparable to the original samples. We further sampled an additional 10 sites with similar methodologies to fill in spatial data gaps. Rates of capture and numbers of individuals were similar between historical and contemporary samples; however, collections with multiple individuals in contemporary samples were largely restricted to the upstream periphery of their historical distribution. Qualitative comparisons of numbers of individuals caught over several decades using variable methodology suggest that declines in abundances have happened since the species was first described. Bayou darter occupancy was positively related to stream link magnitude (beta estimate = 3.07), and N-mixture modeling estimated contemporary abundance at 7.36 ± 3.83 individuals per site. Bayou darter counts were negatively related to variance in kilometer-scale stream elevation (beta estimate = -0.60). Our results suggest that this species remains imperiled and is experiencing a declining range, and that continued efforts to study and monitor this species, and to reduce geomorphic change in the system, are merited.

Mechanisms driving patterns of occurrence and co-occurrence among NorthAmerican freshwater fishes are poorly understood. In particular, the influence of biotic interactions on coexistence among stream reaches and their effects on regional species distribution patterns is not well understood for congeneric headwater fishes.2. Occupancy models provide a useful framework for examining patterns of co-occurrence while also accounting for imperfect detection. Occupancy models may be extended to test for evidence that a dominant species influences the occurrence of a subordinate species and thus evaluate support for the hypothesis that species interactions drive patterns of coexistence.3. We examined patterns of occurrence and co-occurrence at the stream-reach scale among three species of darters (Percidae: Etheostomatinae) that occupy headwater streams within a Gulf Coastal Plain drainage in the south-eastern U.S.A. We assessed species occurrences at 97 sites in first-to third-order streams on one occasion each and used data from four sub-reaches sampled with equal effort at each site to estimate species-specific detection probabilities. Following sampling, a suite of habitat variables was collected at three equidistant points along each of the three transects established within a sub-reach. Coarse (stream-segment, catchment, network) scale variables were also incorporated using geospatial data.Single-species and two-species occupancy models were used to examine patterns of occupancy and coexistence. The occupancy of each species was influenced by distinct habitat variables.Goldstripe darters (Etheostoma parvipinne) were constrained by a stream size gradient, groundwater input appeared to influence the occurrence of Yazoo darters (Etheostoma raneyi), and local habitat heterogeneity (e.g. variation in depth and current velocity) appeared to influence the occupancy of redspot darters (Etheostoma artesiae). 5.We found no evidence that the presence of one species influenced the occurrence of another within a stream-reach based on two-species occupancy models.Rather, species co-occurrences were best explained as independent occurrences within a stream-reach according to species-specific habitat associations.

The spatial and hydrological properties of headwaters allow dendritic systems to contribute to patterns of regional diversity. However, such ecological gradients may be disrupted as a result of habitat fragmentation. We tested the hypothesis that coarse-scale anthropogenic disturbances such as upstream land use and proximity to reservoirs can alter ecological gradients, thus influencing instream habitat, headwater fish assemblage composition, and species turnover in the Little Tallahatchie River system in north-central Mississippi. To test this hypothesis, we calculated species turnover coefficients, ordinated samples, and examined the correlations between assemblage composition and environmental and anthropogenic variables. Assemblage composition was strongly correlated with instream habitat and river system connectivity, and instream habitat was strongly associated with land use. Gradients in assemblage composition associated with ecological factors were altered due to land use. Our research highlights the importance of headwaters as distinctive habitat patches driving species turnover and the influence of land use in disrupting the ecological gradients that allow for the formation of these distinctive habitat types.

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The bifurcating configuration of stream networks impacts the connectivity of patches and therefore influences the effects of habitat variables on dispersal and occupancy. In this study, we developed a comparative framework to explore the influence of environmental and anthropogenic variables on the dispersal and occupancy of two headwater darters, the Yazoo darter (Etheostoma raneyi) and goldstripe darter (Etheostoma parvipinne). We conducted our study in two subbasins that are nested within a larger Gulf Coastal Plain drainage in the southeastern United States. We examined patterns of occupancy using detection data collected within standardized stream reaches within the larger subbasin. Genotype by sequencing identified single nucleotide polymorphisms, which we then used to calculate genetic distance among samples obtained from localities distributed across both subbasins. Goldstripe darter and Yazoo darter occupancy were both best modeled by a single environmental variable (Yazoo darter: well depth; goldstripe darter: drainage area). Similarly, model rankings suggested that well depth explained a substantial amount of the variability in Yazoo darter genetic distance in this subbasin. However, we found no evidence that drainage area strongly affected goldstripe darter genetic distance within the larger subbasin. Our findings also suggested that the best predictors of Yazoo darter and goldstripe darter genetic distance differed between the two subbasins, and that variation in goldstripe darter genetic distance in the smaller subbasin was largely a consequence of anthropogenic variables. The comparative approach we used in this study suggests that the habitat variables regulating the dispersal and occupancy of some headwater fishes may differ, thus the use of site‐occupancy models to predict the dispersal habitat of these fishes may be inappropriate. Therefore, we suggest that understanding the effects of habitat variables regulating dispersal and occupancy may be crucial to understanding the metapopulation dynamics of headwater fishes.