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determined as sampling is conducted. Sampling would be discontinued when little or no increase in the cumulative number of species is observed. The minimum sampling reach length would then be calculated at every sampling reach as a result of sampling. However, such a procedure is likely to be more time consuming and difficult than sampling a specified distance. It would also require that fish sampling always be conducted prior to other NAWQA sampling efforts, and this may not be feasible.

Fish community sampling as part of NAWQA is based on a minimum sampling reach length established prior to fish sampling and not as a result of fish sampling. Matthews (1990) recommended a minimum sampling reach length of greater than 100 m for sampling fish communities in wadeable (less than or equal to about 1 m deep) warm-water midwestern streams. Lyons (1992), in a study of Wisconsin warmwater streams, calculated a median asymptotic sampling distance of 222 m. However, the author noted that at several sites, substantially shorter sampling distances yielded similar estimates of fish species richness. Based on these studies, a minimum sampling reach length can be estimated at more than 100 m but less than about 200 m. Therefore, the minimum sampling reach length for NAWQA fish sampling in wadeable streams is designated as 150 m, which is the same as that recommended by the Ohio Environmental Protection Agency (1987).

Criteria for determining minimum-maximum sampling reach lengths for nonwadeable streams (those requiring a boat to sample) are less clear than those for wadeable streams. Lyons (1992) suggested that a positive correlation is likely to exist between asymptotic sampling distance and stream size (width), and that larger streams may require greater sampling distances than do small streams. This suggestion is supported by other water-quality monitoring protocols that describe procedures for collecting fish samples to characterize fish community structure in nonwadeable streams (Ohio Environmental Protection Agency, 1987; Plafkin and others, 1989). These sources suggest that a minimum sampling reach length of 500 m is necessary to collect a representative sample of fish in nonwadeable streams. Therefore, the minimum sampling reach length for NAWQA fish community assessment at nonwadeable sites is designated as 500 m.

A maximum sampling reach length criterion is required to prevent unnecessary sampling and minimize any reduction of sampling efficiency as a result of crew fatigue. In some cases, determination of the sampling reach length based on geomorphic channel units or meander wavelength results in a sampling reach length that exceeds the minimum length required to collect a representative sample of fish. For example, some streams are relatively wide (greater than 30 m), yet too shallow to accommodate a boat. Sampling of such streams could result in sampling reach lengths greater than 600 m, a sampling distance that greatly exceeds the distance required to obtain a representative sample and one that could result in reduced sampling efficiency as a result of crew fatigue.

No published studies have addressed maximum sampling reach length with respect to fish sampling. A maximum sampling reach length, as with a minimum sampling reach length, is stream dependent. However, for reasons discussed in determining a minimum sampling reach length, an estimated maximum sampling reach length must be established prior to sampling. The Ohio Environmental Protection Agency (1987) and Plafkin and others