We sampled fishes and measured microhabitat in series of contiguous habitat units (riffles, runs, pools) in three Virginia streams. We used Monte Carlo simulations to construct hypothetical series of habitat units, then examined how number of species, similarity in relative abundances, and number of microhabitats accumulated with increasing number of habitat units (i.e., sampling effort). Proportions of all species and microhabitats represented were relatively low and variable at low sampling effort, but increased asymptotically and became less variable with greater sampling effort. To facilitate comparisons among streams, we fitted simulation results to negative exponential curves. The curves indicated that 90% of the species present were usually found by sampling 5 to 24 habitat units (stream length of 22-67 stream widths). Estimates of species relative abundances required less sampling effort for a given accuracy than estimates of number of species. Rates of species accumulation (with effort) varied among streams and reflected discontinuity in species distributions among habitat units. Most discontinuity seemed to be due to low population density rather than to habitat selectivity. Results from an Illinois stream corroborated our findings from Virginia, and suggested that greater sampling effort is needed to characterize fish community structure in more homogeneous stream reaches. RCsumC: Nous avons CchantillonnC les poissons et mesurC le microhabitat dans une sCrie d'unit6s contigues (rapides, seuils et fosses) dans trois cours d'eau de la Virginie. A 19aide de sirnullations fondCes sur la rnCthode de Monte Carlo, nous avons form6 des series hypothktiques d9unitCs d'habitat et nous avons analyst5 B a fagon dont le nombre d9esp&ces9 la similitude dans les abondances relatives et le nombre de microhabitats se sont accrus avec l'augmentation du nombre d9unitCs (c9est-B-dire avec Ikffort d9Cchantillonnage). Les proportions des espkces et des microhabitats reprCsentCs Ctaient relativement basses et variables, B un effort d'CcIaantilHonnage faible, mais s'accroissaient de manikre asymptomatique et perdaient de la variabilitk mesure que l'effort d7CchantilIonnage augmentait. Afin de faciliter les comparaisons entre les cours d'eau, nous avsns ajust6 les rCsultats de la simulation 2i des courbes exponentielles negatives. Ces courbes indiquaient que 90% des esp&ces prCsentes poamvaient habituellement Ztre trouvies par 19ichantillonnage de 5 B 14 unit& (longueur du cours d'eau 22 sur 67 largeurs). Les estimations des abondances relatives des espkces ont nCcessitC un effort d9Cchantillonnage moindre, pour une exactitude donnCe, que les estimations du nombre d'espkces. Les taux d9accumulation des espkces (en f'onction de I'effort) variaient selon les cours d'eau et traduisaient la discontinuit6 de la distribution des espkces entre les diffkrentes unit&. La majeure gartie de la discontinuit6 semblait attribuable B la faible densite des populations et non pas B B a sClectivitC de 17habitat. Les ~Csamltats obtenus dans u...

We examined distribution and abundance of small, medium, and large American eels Anguilla rostrata in Virginia streams by comparing observed with expected null-model patterns. At large spatial scales (across drainages or physiographies), densities of small and medium American eels decreased with distance from the ocean, and densities were not strongly or consistently related to local habitat features (e.g., habitat type, abundance of cover, water temperature, density of predators). A mathematical function, analogous to diffusion of particles from a concentrated source, accounted for up to 85% of the variance in densities of small-or medium-sized eels at statewide, physiographic, or river-drainage, scale?; At smaller spatial scales (e.g.. within a drainage in a particular physiography), we found few relations between American eel density and habitat features. Exceptions to this pattern occurred at Coastal Plain sites of the James River drainage, where American eel densities were exceptionally high. Such small-scale, nonrandom patterns may reflect local density-dependent processes that only rarely affect American eel distribution and abundance more than do ubiquitous, large-scale random processes. Parsimonious, large-scale models (e.g., diffusion models) may predict the distribution and abundance of American eels (and other widespread or generalist stream fishes) more reliably than do more typical, small-scale habitat models (e.g., habitat suitability models).

Regionally stratified biological criteria are being used increasingly to assess stream quality. We used multivariate analysis of variance and canonical analysis to examine the utility of two regional frameworks (basins and ecoregions) and 14 candidate metrics of local fish assemblages for assessing the biotic integrity of streams in the mid‐Atlantic highlands (montane areas from Pennsylvania to Virginia). In particular, we determined (1) how metrics varied naturally among basins and ecoregions and (2) which metrics varied most consistently with site quality. We also examined the ability of preliminary multimetric indices (MMIs) to distinguish site quality. Metrics varied meaningfully among both basins and ecoregions, but most metrics differed more among basins. The basin effect was especially strong for taxonomic metrics (e.g., number of species [TOTSP]), which reflected the influence of zoogeography on fish community composition. Few metrics differed strongly among both basins and ecoregions. Collectively, metrics distinguished among high‐, medium‐, and low‐quality sites within most regions, but the discriminative ability of individual metrics differed by region. The number of darter or sculpin species (DOSSP) was the only metric related to site quality both in most basins and most ecoregions. Metric differences among site‐quality classes were more consistent with a priori expectations within basins than within ecoregions. In each of five regions, we built an MMI from the most discriminative metrics. Only DOSSP and the proportional abundance of tolerants were included in all five MMIs. All MMIs included taxonomic and reproductive (e.g., proportional abundance of simple lithophils, excluding tolerants) metrics, but not all included trophic metrics (e.g., proportional abundance of invertivores). Multimetric indices distinguished between high‐ and low‐quality sites in each region tested, but they usually did not do so to a greater degree than did taxonomic metrics alone. Among the metrics included in MMIs, TOTSP was most consistently related to site quality. Our findings indicate that both basins and ecoregions provide useful frameworks for regionalizing biotic assessments based on fishes and that metric utility may vary considerably among regions even when regions are environmentally similar. To enhance MMI performance, we encourage an increased reliance on region‐specific empirical relations in the development of metrics and scoring criteria.

The utility of an index of biotic integrity (IBI) depends on its ability to distinguish anthropogenic effects on biota amid natural biological variability. To enhance this ability, we examined fish assemblage data from least‐disturbed stream sites in Virginia to determine the best way to regionally stratify natural variation in candidate IBI metrics and their scoring criteria. Specifically, we examined metric variation among physiographic regions, U.S. Environmental Protection Agency ecoregions, and drainage basins to judge their utility as regions in which to develop and use distinct versions of the IBI for Virginia warmwater streams. Statewide, metrics differed most among physiographic regions; thus, we recommend their use as IBI regions. Largest differences were found for taxonomic metrics between coastal plain and mountain sites, particularly in numbers of native minnow (Cyprinidae), sunfish (Centrarchidae), and darter (Percidae) species. Trophic and reproductive metrics also differed between coastal plain and more‐upland streams, presumably reflecting differences in functional adaptations of fishes to upland versus lowland stream habitats. We suggest three preliminary regional IBIs for Virginia, each having a distinctive set of taxonomic, trophic, and reproductive metrics and corresponding scoring criteria.

We describe the design and use of an electric seine for collecting fish that is reliable, efficient, and broadly applicable in small to medium‐size streams. Modifications of previous designs include (1) addition of a rheostat for regulating circuit amperage, (2) use of fiberglass tubing in the brail infrastructure, (3) readily engaged or disengaged connections between the brails and drop‐electrode array, and (4) use of automobile brake or speedometer cables as drop‐electrodes with adjustable lengths. To estimate capture efficiency of the electric seine, we repeatedly electrofished a pool and riffle in each of two Virginia streams. The first two passes with the electric seine captured between 53 and 79% of fish numbers and between 60 and 88% of fish biomass collected by 10 passes. Capture efficiency was relatively high for suckers, low for darters, and variable for minnows and sunfishes. Capture efficiency of the electric seine was similar or superior to that of other electrofishing gear used in wadeable streams. The electric seine is most appropriate for quantitative estimates of species composition or population densities of entire fish assemblages.