Two‐pass backpack electrofishing data collected as part of the U.S. Geological Survey's National Water‐Quality Assessment Program were analyzed to assess the efficacy of single‐pass backpack electrofishing. A two‐capture removal model was used to estimate, within 10 river basins across the United States, proportional fish species richness from one‐pass electrofishing and probabilities of detection for individual fish species. Mean estimated species richness from first‐pass sampling ([pcirc],s1) ranged from 80.7% to 100% of estimated total species richness for each river basin, based on at least seven samples per basin. However, [pcirc],s1 values for individual sites ranged from 40% to 100% of estimated total species richness. Additional species unique to the second pass were collected in 50.3% of the samples. Of these, cyprinids and centrarchids were collected most frequently. Proportional fish species richness estimated for the first pass increased significantly with decreasing stream width for 1 of the 10 river basins. When used to calculate probabilities of detection of individual fish species, the removal model failed 48% of the time because the number of individuals of a species was greater in the second pass than in the first pass. Single‐pass backpack electrofishing data alone may make it difficult to determine whether characterized fish community structure data are real or spurious. The two‐pass removal model can be used to assess the effectiveness of sampling species richness with a single electrofishing pass. However, the two‐pass removal model may have limited utility to determine probabilities of detection of individual species and, thus, limit the ability to assess the effectiveness of single‐pass sampling to characterize species relative abundances. Multiple‐pass (at least three passes) backpack electrofishing at a large number of sites may not be cost‐effective as part of a standardized sampling protocol for large‐geographic‐scale studies. However, multiple‐pass electrofishing at some sites may be necessary to better evaluate the adequacy of single‐pass electrofishing and to help make meaningful interpretations of fish community structure.

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Freeze tolerance and freeze avoidance are typically described as mutually exclusive strategies for overwintering in animals. Here we show an insect species that combines both strategies. Individual fungus gnats, collected in Fairbanks, Alaska, display two freezing events when experimentally cooled and different rates of survival after each event (mean +/- SEM: -31.5 +/- 0.2 degrees C, 70% survival and -50.7 +/- 0.4 degrees C, 0% survival). To determine which body compartments froze at each event, we dissected the abdomen from the head/thorax and cooled each part separately. There was a significant difference between temperature levels of abdominal freezing (-30.1 +/- 1.1 degrees C) and head/thorax freezing (-48.7 +/- 1.3 degrees C). We suggest that freezing is initially restricted to one body compartment by regional dehydration in the head/thorax that prevents inoculative freezing between the freeze-tolerant abdomen (71.0 +/- 0.8% water) and the supercooled, freeze-sensitive head/thorax (46.6 +/- 0.8% water).

Density estimates based on point processes are often restrained to regions with irregular boundaries or holes. We propose a density estimator, the lattice-based density estimator, which produces reasonable density estimates under these circumstances. The estimation process starts with overlaying the region with nodes, linking these together in a lattice and then computing the density of random walks of length k on the lattice. We use an approximation to the unbiased crossvalidation criterion to find the optimal walk length k. The technique is illustrated using walleye (Sander vitreus) radiotelemetry relocations in Lake Monroe, Indiana. We also use simulation to compare the technique to the traditional kernel density estimate in the situation where there are no significant boundary effects.

Abstract.-Fish community data collected from 24 major river basins between 1993 and 1998 as part of the U.S. Geological Survey's National Water-Quality Assessment Program were analyzed to assess multiple-reach (three consecutive reaches) and multiple-year (three consecutive years) variability in samples collected at a site. Variability was assessed using the coefficient of variation (CV; SD/mean) of species richness, the Jaccard index (JI), and the percent similarity index (PSI). Data were categorized by three electrofishing sample collection methods: backpack, towed barge, and boat. Overall, multiple-reach CV values were significantly lower than those for multiple years, whereas multiple-reach JI and PSI values were significantly greater than those for multiple years. Multiple-reach and multiple-year CV values did not vary significantly among electrofishing methods, although JI and PSI values were significantly greatest for backpack electrofishing across multiple reaches and multiple years. The absolute difference between mean species richness for multiple-reach samples and mean species richness for multiple-year samples was 0.8 species (9.5% of total species richness) for backpack samples, 1.7 species (10.1%) for towed-barge samples, and 4.5 species (24.4%) for boat-collected samples. Review of boat-collected fish samples indicated that representatives of four taxonomic families-Catostomidae, Centrarchidae, Cyprinidae, and Ictaluridae-were collected at all sites. Of these, catostomids exhibited greater interannual variability than centrarchids, cyprinids, or ictalurids. Caution should be exercised when combining boat-collected fish community data from different years because of relatively high interannual variability, which is primarily due to certain relatively mobile species. Such variability may obscure longer-term trends.