Relationships among varying sampling distance and the IBI in warmwater, headwater streams of the Eastern Corn Belt...

Single-pass electrofishing was used to define the most efficient sampling distance to assess stream condition using the index of biotic integrity (IBI) methodology in headwater (<36 km(2) drainage area), warmwater streams in the Eastern Corn Belt Plain ecoregion. Based on wetted widths (1-3.3 m) of sampled reaches, we defined effort based on increased area (range 50-555 m(2)). Sampled area necessary to capture a representative fish assemblage increased until 167-m(2) distance, which is equivalent to a minimum sampling distance of one habitat cycle. No significant difference in metric actual observed value response was found with increasing habitat cycle. Increased effort is required in smaller streams widths (≤1 m) to achieve the recommended sample area. The effect of rare fish on the IBI was tested using a modified Walford method. A significant decrease in IBI score was observed when 10% of the rare data were removed. The presence of rare fish did not influence individual IBI metrics or scores for either the increased effort or reduced effort calibrations until greater than 3% of the data was removed for number of species, 15% removal of data for number of minnow species, and 5% removal of data for catch per unit effort (CPUE). Increased effort did not affect any metric or IBI score, while reduced effort influenced the number of darter, madtom, and sculpin species and catch per unit effort metric scores but did not affect IBI score.