The DNR’s Lake Biological Monitoring and Assessment (LBMA) program tracks the health of lakes across Minnesota by studying their fish communities and associated habitat. One way scientists measure lake health is with an index of biological integrity (IBI), which scores lakes based on the types and numbers of fish observed compared to what is expected in a healthy lake.
Certain species cannot survive without clean water and a healthy habitat while others are tolerant of degraded conditions. Species in either category serve as "indicators" of a lake’s condition. The biological communities present in a lake reflect the combined effects of natural factors and human activities across the entire watershed—the land and water that flows into the lake. These communities change in predictable ways in response to stressors such as poor water quality or loss of shoreline habitat, allowing scientists to assess the overall biological health of a lake.
Fish-based index of biological integrity
The DNR developed fish-based IBIs by sampling a wide range of lakes, from high-quality lakes to those with significantly degraded water quality or shoreline habitat. A statistical analysis found a relationship between the fish communities and water quality and physical habitat characteristics. Additional details about this process can be found in the development of fish-based IBIs for Minnesota lakes journal article.
The Minnesota Pollution Control Agency (MPCA) has developed similar IBIs to assess biological communities in streams and rivers in Minnesota. For more information on these IBIs, go to the MPCA index of biological integrity and river and stream biological monitoring webpages.
Different IBIs for different lake types
Four fish-based IBIs were developed to accurately evaluate different types of lakes and expectations for their associated fish communities. For example, healthy shallower lakes have different indicator species than deeper lakes and healthy lakes with more complex shaped shorelines have more fish species than lakes with rounder shaped shorelines.
Deep, complex
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Big Cormorant Lake is an example of a deep lake with a complex shaped shoreline.Lake group:
Very deep and large with complex shoreline, primarily located in central and northern MinnesotaFish species diversity in a "healthy" lake:
High - average of 24 native speciesFish commonly sampled that are sensitive to water quality and habitat changes:
- Bluegill, Pumpkinseed and Rock Bass
- Cisco
- Smallmouth and Largemouth Bass, Northern Pike, Muskellunge and Walleye
- Several darter species (e.g., Johnny, Iowa, and Least)
- Several shiner species (e.g., Mimic, Spottail, Blacknose, and Blackchin)
Fish commonly sampled that are tolerant of water quality and habitat changes:- Common Carp
- Black Bullhead
- Green Sunfish
- Deep, simple
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Big Spunk Lake is an example of a moderately deep lake with a rounder shoreline.Lake group:
Moderately deep with a rounder shoreline, primarily located in central and northern MinnesotaFish species diversity in a "healthy" lake:
Intermediate - average of 19 native speciesFish commonly sampled that are sensitive to water quality and habitat changes:
- Bluegill, Pumpkinseed and Rock Bass
- Cisco
- Smallmouth and Largemouth Bass, Northern Pike, Muskellunge and Walleye
- Several darter species (e.g., Iowa, Johnny, and Least)
- Several shiner species (e.g., Mimic, Blacknose, and Blackchin)
- Common Carp
- Black Bullhead
- Green Sunfish
- Moderately shallow, vegetated
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Spitzer Lake is an example of a moderately shallow, heavily vegetated lake.Lake group:
Moderately shallow, heavily vegetated, primarily located in central and northern Minnesota.Fish species diversity in a "healthy" lake:
Low - average of 17 native speciesFish commonly sampled that are sensitive to water quality and habitat changes:
- Bluegill and Pumpkinseed
- Largemouth Bass, Northern Pike, and Walleye
- One or more shiner species (e.g., Spottail or Golden)
- Common Carp
- Black Bullhead
- Green Sunfish
Shallow, productive
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Lake Minnewawa is an example of a shallow lake.Lake group:
Shallow (80-100% of the lake area less than 15 feet deep), naturally productive, primarily located in southern and central MinnesotaFish species diversity in a "healthy" lake:
Intermediate - average of 19 native speciesFish commonly sampled that are sensitive to water quality and habitat changes:
- Bluegill
- Northern Pike
- One or more darter species (e.g., Johnny or Iowa)
- Common Carp
- Black Bullhead
- Green or Orangespotted Sunfish
- Fathead Minnow
Fish sampling and IBI score calculation
There are many different methods to sample fish in a lake, and each method targets different kinds of fish. The DNR uses gill netting, trap netting, seining and backpack electrofishing (pictured at the top of this page) to collect the fish community information needed to calculate an IBI score for a lake.
Once the fish have been collected, they are identified to species, counted, weighed, measured and released back into the lake when possible. Across Minnesota lakes, surveys may document up to 89 different fish species, although only a subset of those species occurs in any individual lake. Information on the number of species and quantity of individuals collected are used to calculate the IBI score. Fish IBI survey data and reports for individual lakes can be found on the MNDNR LakeFinder webpage.
The fish-based IBI incorporates multiple measurements of the fish community. When the measurements are added together, they produce a score that reflects the lake's biological health.
Each IBI score is generated using between eight and fifteen calculated measurements, depending on lake group. Examples include either raw counts or proportions of species that are classified as: native, intolerant, tolerant, vegetation-dwelling, small benthic-dwelling, insectivorous, carnivorous, omnivorous, or belonging to the leuciscid (i.e., minnows and shiners) family. More information about these measurements and the species included in each can be found on the classification of fish species in Minnesota lakes webpage. IBI scores can be found in the MN Geospatial Commons or on the Watershed Health Assessment Framework: Lakes webpage.
These measurements are known to correspond with varying levels of human-caused stress from activities such as land use alteration within a lake’s watershed and physical habitat alteration along a lake’s shoreline.
Using IBI data in lake aquatic life use assessment
The fish-based IBI is one important component that is considered during the MPCA watershed assessment process. Specifically, it is one indicator used to assess whether a lake fully supports aquatic life use. Similarly, the MPCA lake water quality monitoring program uses physical measurements of water quality such uses physical measurements of water quality such as water clarity and amount of phosphorus to assess whether a lake fully supports aquatic recreation. These measurements represent a snapshot in time of lake conditions whereas the biological community as measured by the IBI reflects changes to the lake over several years or more. Together, these assessments provide important information to guide Clean Water planning and restoration.
Use designations
Fish-based IBI scores are used by DNR staff to classify lakes into one of two tiered aquatic life use (TALU) categories based on their potential to support aquatic life: general use and exceptional use. These TALU categories move away from a “one-size-fits-all” standard, meaning that high-quality or exceptional use waters receive extra protection to help maintain their healthy habitats for aquatic life. More information about TALU can be found on the MPCA aquatic life and recreation beneficial uses webpage.
General use
- General use lakes represent the standard aquatic life goal for most lakes. They provide or should provide habitat capable of supporting balanced fish communities. IBI scores are used to indicate whether these lakes are fully supporting healthy communities or experiencing impairment, and this information guides management, restoration, and protection efforts.
Exceptional
Exceptional use lakes represent the highest level of aquatic life designation based on documented attainment of exceptional biological condition. These lakes have IBI scores showing they met the exceptional use threshold at one or more points in the past, reflecting the capacity to support high-quality habitat and highly diverse, balanced fish communities. Because of their recognized ecological value, these lakes are a top priority for protection.
Assessment categories
Fish-based IBI scores are used by DNR staff to place lakes into one of four parameter-level categories: meets, exceeds (impaired), inconclusive information, or insufficient information. Some lakes are also considered vulnerable if their IBI scores are close to an impairment threshold. Often, multiple scores are considered when making a parameter-level judgement on an individual lake. These judgements contribute to an overall lake aquatic life use assessment and help guide lake management, restoration, and protection decisions for agencies, local governments, conservation groups, lake associations, and homeowners.
Meets
- Healthy lakes that support diverse, balanced fish communities. These lakes have IBI scores above the impairment threshold. They are a priority for protection, and some restoration may also be recommended to maintain their condition.
Exceeds (impaired)
Lakes that are degraded and where sensitive fish species typically no longer live. These lakes have IBI scores below the impairment threshold. They are a top priority for restoration. These lakes are added to Minnesota’s Impaired Waters List.
Inconclusive information
Sampling produced enough data, but results were conflicting or very close to the impairment threshold, which prevented the ability to make a clear determination about the health of the lake.
Insufficient information
Sampling did not produce enough data to make a clear determination about the health of the lake.
Maps
Maps of lake groups, parameter-level judgements, and exceptional and vulnerable lakes, based on the fish-based IBI. Click each individual map to enlarge.