Wisconsin Fly Fishing for Trout

Measuring the Health of a Trout Stream
using the Hilsenhoff Biotic Index

seining aquatic insectsBiological trout stream monitoring is useful in helping to determine not only the health of an aquatic ecosystem but also the overall health of a stream. Many factors, including climate, floods, droughts, water quality, and human disturbances (such as agricultural run-off, cattle grazing along a river, homeowners altering the shoreline, and chemical contaminants) can influence the benthic macroinvertebrates that inhabit the ecosystem. All these factors can cause a decline in benthic macroinvertebrate taxa richness and eventually the stream itself. By periodically sampling the density and diversity of benthic macroinvertebrates within our streams we can then more accurately monitor the overall health of our trout streams, rather than counting the trout within the streams.

William L. Hilsenhoff (UW Madison) analyzed data from 1000+ streams that were sampled in Wisconsin over a several-year period by the WI DNR. Using the stream's benthic macroinvertebrate data and his expert knowledge, he eventually came up with a method to estimate the overall health of a stream. This method was eventually called the Hilsenhoff Biotic Index (HBI). He began by assigning a tolerance value to organic pollution (0-10) to each macroinvertebrate genus or species (taxon), with 0 representing highly sensitive to pollution and 10 being very low sensitivity to pollution.

Hilsenhoff then created a mathematical formula by multiplying the tolerance value of each macroinvertebrate taxon by the number of specimens of that taxon in the sample, and then summing all the products. The final sum total was then divided by the sum of all individuals in the sample. By applying these values in his formula, he compared the results to a scale. The water quality rating scale goes from "excellent" to "very poor".

Biotic Index

Water Quality






Very Good









Fairly poor






Very poor


Acknowledgement: I would like to thank Dr. Kurt L. Schmude, Professor and Senior Scientist at UW Superior for his aquatic entomology expertise. Also, Tom H. Klubertanz, Professor of Biology at UW-Whitewater at Rock County and Jeffrey Dimick, Laboratory Manager at UW Stevens Point, WI. for their assistance and aquatic entomology expertise.

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