To evaluate the efficiency and selectivity of gill netting for assessing fish biodiversity in the upper Ohio River system, we compared the efficiency of five gill-net types for sampling large-bodied fishes (adult total length greater than 250 mm) during fall 2001 and spring and fall 2002. Mesh sizes ranged from 3.8 cm to 14 cm (bar measure). We set the gill nets in selected pools of the Allegheny, Monongahela, and Ohio rivers 186 times over three seasons for a total of 1,644 nethours. Nets were attached to a variety of structures, including trees and rootwads, bridge pylons, lock and dam chambers, and channel marker buoys. Nets were fished from late evening to first light, and all fish captured were identified, enumerated, and released. A total of 823 individuals representing 30 species or hybrids were captured. All net types captured common carp Cyprinus carpio and flathead catfish Pylodictis olivaris, but we captured a significantly greater diversity of fishes in graded-mesh gill nets with small mesh (38 m in length with variable-bar mesh and 15.2 m in length with 3.8cm-bar mesh). When adjusted for length, smaller-bar mesh nets (3.8-cm-bar mesh) were more efficient for capturing target species than graded-mesh nets. To maximize species richness, 200-225 h of effort were necessary to characterize target fish communities of largebodied riverine species.
We assessed the impacts of a specific conductance gradient attributable to treated coalmining discharges on the fish communities of a southwestern Pennsylvania stream. Total dissolved solids concentrations were determined from specific conductance values. A total of 10,940 fish representing seven families and 42 species/hybrids were collected from 17 stations over the entire survey. Species richness, density, and the coefficient of community loss (I) showed marked impairment at the two stations directly below the discharges and the downstream recovery was interrupted at one station by untreated discharges from a mine refuse pile. Species richness declined from 28 at the reference site to 7 at the station directly below the treated effluents. This study suggests that the threshold for in-stream conductivity impairment to fish communities in this region is in the range of 3,000-3,500 µS/cm and 2,000-2,300 mg/l of total dissolved solids, respectively.
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