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Fish of Montgomery County

Montgomery County is home to more than 60 species of freshwater fish, representing nearly every family of freshwater fish known in Maryland. This includes trout, catfishes, sunfishes and bass, minnows, suckers, sculpins, darters and perch, killifishes, lampreys, American eel, Eastern mudminnow, and Eastern mosquitofish.


Fish are important to Montgomery County stream ecosystems. They are part of our natural history, provide food, recreation and have intrinsic and economic value.  Fish also play a vital ecological role in the overall food web. County biologists study fish communities to provide insight on the health and condition of County streams.


Want to help protect our fish and learn more?


Fish Species

Montgomery County divides fish into three groups based on their ability to survive in polluted waters. Sensitive fish are only able to survive in the county's highest quality streams. Moderately tolerant fish can survive and sometimes thrive in areas that receive minor pollution. Tolerant fish live in most streams in the county, but they are the only fish that can survive in heavily polluted waters.

Each fish has unique requirements for where they live. Check out what species live around you with Maryland Fish Distributions Maps!


Highly Sensitive Fish

Image of a Brown Trout




Brown Trout

Image of Blue Ridge Sculpin




Blue Ridge Sculpin

Image of Northern Hogsucker




Northern Hogsucker


Sensitive Fish

Image of American Eel




American Eel



Greenside Darter

Image of Longnose Dace




Longnose Dace

Image of a Rock Bass




Rock Bass

Image of a Rosyside Dace




Rosyside Dace

Image of a Yellow Bullhead




Yellow Bullhead


Pollution Tolerant Fish

Image of Blacknose Dace




Blacknose Dace

Image of Brown Bullhead




Brown Bullhead

Image of Creek Chub




Creek Chub

Image of Green Sunfish




Green Sunfish

Image of White Sucker




White Sucker



Why are they Important?

Fish are indicators of water quality:

  • Fish live a long time, some over 10 years.

  • Work as chemical, physical, and biological historians of a stream.

  • Can tolerate different levels of pollution and habitat decline. 

  • Utilize a wide variety of habitats.

Fish provide scientists insight into past stream conditions. When a stream is stressed, fish communities respond over time. Streams stressed by excessive sediment have less desirable habitat and lose sensitive fish species like Blueridge Sculpin.

If the water is polluted, then fish may develop lesions or may have physical deformities.

Image of Blueridge Sculpin Image of Deformed Sunfish
Blueridge Sculpin Deformed Green Sunfish



Monitoring and Data


Stream biologists study fish species and any changes in their population size over time, because fish serve as biological indicators of water quality. 

  • Most fish species have long life spans (2 to 10 years or more) and can reflect both long-term and current water resource quality.

  • Fish continually inhabit the receiving water and integrate the chemical, physical, and biological histories of the waters.

  • Since different fish species have varying tolerances to pollution, we can characterize stream water quality based on the presence or absence of pollution-tolerant or pollution-intolerant species.

  • Fish represent a broad spectrum of community tolerances from very sensitive to highly tolerant and respond to chemical, physical, and biological degradation in characteristic response patterns.

  • Fish have large ranges and are less affected by natural microhabitat differences than smaller organisms such as benthic macroinvertebrates. This makes fish extremely useful for assessing regional conditions.

Learn more about the use of freshwater fish as an indicator of water quality.



The Department of Environmental Protection identifies and studies fish in County streams by conducting field testing using electrofishing. Visit our electrofishing page for more information on the process.


What Data Does DEP Collect?

Fish IBI Metrics

Once numbers of individuals are summed for each species present at a stream site, a fish Index of Biotic Integrity (IBI) is calculated based on multiple metrics. The IBI is used to rank the stream in relation to reference stream conditions. To use biological data properly, water resource analysts generally compare the fish data (actually not the raw data but a multi-metric index based on the data) from the stream sites under study, to indices from stream sites in ideal or nearly ideal condition (often called a reference condition). Stream sites are then ranked against the reference condition. This helps DEP set priorities for watershed restoration and improvement.

The fish IBI is averaged with the benthic macroinvertebrate IBI to determine overall stream conditions.

Fish IBI metrics are listed below. A technical, peer-reviewed methodology is used to take raw data and develop them into an acceptable Stream Rating score.

Fish IBI Metrics

Total number of species

Total number of riffle benthic insectivore individuals

Total number of minnow species (cyprinidae)

Total number of intolerant species

Proportion of tolerant individuals

Proportion of individuals as omnivores/generalists

Proportion of individuals as pioneering species

Total number of individuals (excluding tolerant species)

Proportion of individuals with disease/anomalies


Montgomery County has tabular raw fish data and fish narrative summaries from 1994-present for most monitoring sites around the County. Also available are GIS coverages (or maps) showing fish conditions. Maps can be developed to order depending on the request. Submit a request for either raw data or data in maps.

The following tables provide an explanation of the datafields found in our raw tabular data:


Fish Data Table
Field Name Description
STATION The station field is a nine character code that identifies the station name. The stations are a combination of the two letter code for the watershed+the two letter code for the subwatershed+ the single digit stream order code+ the sequential reach number.
SPECIES The official common name of the fish species collected during sampling.
SAMPLE_DATE The date the station was sampled.
PASS1 Number of specimens collected from the first sampling pass.
PASS2 Number of specimens collected from the second sampling pass.
ANOMALIES The total number of anomalies.
ANOMALIES_TYPE The number of anomalies found of a certain type + the two letter code for the type of anomaly found.
Fish Narrative Table
Field Name Description
STATION The station field is a nine character code that identifies the station name. The stations are a combination of the two letter code for the watershed+the two letter code for the subwatershed+ the single digit stream order code+ the sequential reach number.
DATE The date the station was sampled.
SUMMARYSCORE The final IBI summary score (1-5).
NARRATIVE Descriptive word to describe the condition of the stream in relation to reference streams. Narratives are either Excellent (>4.5), Good (3.3-4.5), Fair (2.2-3.2), or Poor (<2.2).














To request data, send an email to with the following information:

  • Name, organization (if applicable), phone number, and/or email address

  • Type of data requested

  • Time frame requested

  • Explanation for use of data (helps to personalize the data request)

  • Preferred method of data retrieval (email, CD by mail, FTP, pick up CD or materials from DEP offices)


Interested in other DEP monitoring data? Request fish, benthic macroinvertebrates, herptofauna and stream habitat data too.