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Biological Monitoring Program

Watershed Management and Monitoring
Phone: 410-887-5683
Fax: 410-887-4804

Aquactic insects are good indicators of a stream's health.
Nets are used to block off a stream
reach before it is sampled.

Streams, even the one meandering through your backyard, are habitats for fish, aquatic insects and other life. If nothing is living in the stream, then chances are there is a problem. Even polluted streams are home for pollution-tolerant species like the blacknose dace, mummichog or black fly larvae.

Biological monitoring, watching and tracking the living things in our waters is one of the ways to determine that waterway’s health. By combining information on species living in streams with data from chemical analysis and the physical condition of the stream channel scientists determine stream health.

Fish Sampling

In the summer, teams of scientists and volunteers don their waders and head to the streams to count, weigh and categorize the fish. Pumpkinseed, stoneroller, white suckers, rosyside dace, greenside darters, brown trout and about 100 other species are catalogued during the survey.

Electro-shocking allows fish to be sampled within a stream reach.
Fish are collected and then
identified and weighed before
they are returned to the stream.

Some fish species can tolerate a great deal of pollution and can live in conditions that other fish would not survive. During the survey, scientists record the number of different species, the types of species and the number of fish. They also weigh and measure the fish. The information that they collect is entered into a variety of mathematical formulas which provide an index, called the Index of Biological Integrity (IBI). This index allows scientists to quantify and compare for streams around the county and state.

Similar mathematical formulas are used to evaluate the insects in streams. The insects, like the fish, vary in their tolerance for pollutants. If particular species are found in a waterway, you can be relatively confident that the waterway is healthy.

Aquatic Insects

A "Dnet" is used to collect aquatic insects.
A "Dnet" is used to
collect aquatic insects.
These are used as
indicator of a
stream's health.

Every spring insects are collected from rocky, fast flowing, shallow sections of streams. These spots are called riffles and are the part of the stream referred to when we speak of babbling brooks. The insects we are interested in live between and beneath the rocks on the bottom of the stream and are called benthic macroinvertebrates. Benthic, because they live on the bottom, and macroinvertebrate because they are large invertebrates (without backbones.) Large is a relative term: the largest of these insects is a couple of inches long and the smallest, quite small, but still visible to the naked eye. Scientists sometimes refer to these pollution-sensitive insects as EPT. This is an acronym for the scientific orders Ephemeroptera, Plecoptera, and Trichoptera. Using macroinvertebrates as a measure for stream health is an easy and effective way to track changes in water quality.

When the information on insect species is combined with a fish index and other monitoring efforts, the county gains a great deal of understanding about stream health. The IBI for insects is often referred to as the BIBI which stands for the Benthic Index of Biotic Integrity. Using these statistics, we are able to assign values to the biological health of a stream and determine if the health of the stream is improving or declining.

See for yourself how our natural resource specialists collect macroinvertebrate samples, examine them under the microscope and assess  the health of our streams by watching the short video, "Watershed Moments—Keepers of the Stream," featuring County Executive Johnny Olszewski.

We look to the life in the streams to determine whether the streams are healthy, but don’t forget that it is that beauty and diversity of life that we’re working to protect. You can join our science geeks looking for insects and counting fish. Check out the Maryland Stream Waders program and get your waders on.

Tree Limbs and Branches

Wood is good for streams for several reasons. As water is forced around the wood different velocities result in the stream creating a variety of habitats. Microbes feed on the wood and in turn macroinvertebrates, aquatic insects, feed on the microbes. Wood provides overhead cover to protect fish. Wood is only a problem if it has the potential to move downstream and damage infrastructure such as bridges and culverts.

Revised May 4, 2021         


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