The type of sample collected to describe fish community structure for NAWQA is a representative sample (Hocutt and others, 1974; Hocutt, 1981). A representative sample provides information on the presence and relative abundance of species. The purpose of a representative sample is to provide a realistic sample of the fish community that represents the fish community inhabiting the sampled stream. The suitability of this type of sample for water-quality assessments has been documented, and a representative sample of fish is the type of sample recommended for assessing water quality (Ohio Environmental Protection Agency, 1987; Plafkin and others, 1989).
Collection of a representative sample requires that the geomorphic channel units (pools, riffles, and runs) of the sampled stream section are representative of the geomorphology of the stream and that multiple sampling methods are used. Of the numerous methods for sampling fish (Nielsen and Johnson, 1983), each has limitations in a particular environment (Backiel and Welcomme, 1980). Nearly all methods designed to collect fish are selective for some component of the fish community and vary in their sampling efficiency. Thus, the combined methods used in collecting a representative sample complement each other, taking advantage of differences in selectivity and efficiency among methods to achieve a more precise representation of the fish community structure. Such a multigear approach has been strongly recommended for collecting a sample of the fish community (Lundberg and McDade, 1990).
The sampling reach is a section of stream designated as the sampling unit for describing fish community structure. The length of the sampling reach is determined by a combination of factors, including stream geomorphology, meander wavelength, and a minimum-maximum length criterion. The primary determinant of sampling reach length is geomorphology. The sampling reach should include at least two examples each of two different types of geomorphic channel units. However, where this is not possible (for example, a stream that is a continuous run), the length of the sampling reach should include one meander wavelength, based on 20 times the distance of the channel width (Leopold and others, 1964). These criteria have been recommended for determining the length of sampling reach for sampling fish community structure (Lyons, 1992) because fish species richness is a function of the number of geomorphic channel units sampled (Gorman and Karr, 1978; Angermeier and Schlosser, 1989), and the size and spacing of these units are functions of stream size (Leopold and others, 1964). In addition, a minimum-maximum length criterion is used to provide a minimum sampling reach length necessary to ensure the collection of a representative sample of the fish community and limit sampling reach length to a distance that prevents unnecessary sampling and minimizes crew fatigue (and associated reduction of sampling efficiency).
There is little information concerning the minimum sampling reach length required to obtain a representative sample of the fish community. Studies that examined the relation between the length of stream sampled and fish species richness revealed that the number of species captured increased rapidly over the initial sampling distance (Angermeier and Karr,