Lake Index of Biological Integrity

Fisheries staff using a 50 foot seine to capture fish in nearshore habitat

 

One measure of a lake's health is the community of fish, plants and aquatic life it sustains. Certain species can't survive without clean water and a healthy habitat while other species are tolerant of degraded conditions. These species are considered "indicators" of the health of a lake. An index of biological integrity (IBI) is a score that compares the types and numbers of fish or plants observed in a lake to what is expected for a healthy lake.

The biological communities of a lake are a reflection of the cumulative effects of natural and human-caused influences on the lake. These communities change in predictable ways in response to degrading water quality or loss of shoreline habitat. Scientists are able to judge the overall health of a lake based on measurements of these biological communities.

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 biological populations and water quality and habitat characteristics. Separate analyses were done for different categories of lakes because healthy shallower lakes have different indicator species than deeper lakes. Lakes with more complex shaped shorelines have more fish species than lakes with a rounder shaped shoreline.

Lakes used for fish-based IBI

DNR researchers developed four fish-based IBIs based on different types of lakes.

Deep lakes with complex shaped shorelines

Lake Type:
Very deep and large with complex shoreline, located in central and northern Minnesota

Species Diversity in a "Healthy" Lake:
High - average of 28 native species

Fish 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
  • Several Shiner species

Fish Tolerant of Water Quality and Habitat Changes:

  • Common Carp
  • Black Bullhead
  • Green Sunfish
Moderately deep lakes with rounder shorelines

Lake Type:
Moderately deep with a rounder shoreline, located in central and northern Minnesota

Species Diversity in a "Healthy" Lake:
Intermediate - average of 21 native species

Fish Sensitive to Water Quality and Habitat Changes:

  • Bluegill, Pumpkinseed and Rock Bass
  • Cisco
  • Largemouth Bass, Northern Pike, Muskellunge and Walleye
  • One or more Darter species
  • One or more Shiner species

Fish Tolerant of Water Quality and Habitat Changes:

  • Common Carp
  • Black Bullhead
  • Green Sunfish
Moderately shallow, heavily vegetated lakes

Lake Type:
Moderately shallow, heavily vegetated, naturally productive, located in central and northern Minnesota.

Species Diversity in a "Healthy" Lake:
Low - average of 18 native species

Fish Sensitive to Water Quality and Habitat Changes:

  • Bluegill and Pumpkinseed
  • Largemouth Bass, Northern Pike, and Walleye
  • One or more Shiner species

Fish Tolerant of Water Quality and Habitat Changes:

  • Common Carp
  • Black Bullhead
  • Green Sunfish
Shallow lakes

Lake Type:
Shallow (80-100% of the lake area less than 15 feet deep), naturally productive, located in southern and central Minnesota

Species Diversity in a "Healthy" Lake:
Intermediate - average of 21 native species

Fish Sensitive to Water Quality and Habitat Changes:

  • Bluegill
  • Northern Pike
  • One or more Darter species

Fish Tolerant of Water Quality and Habitat Changes:

  • Common Carp
  • Black Bullhead
  • Green or Orangespotted Sunfish
  • Fathead Minnow

Fish sampling

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 to develop a full picture of the fish community living in a lake.

Once the fish have been collected, they are identified to species, counted, weighed, measured and released back into the lake when possible. Information on the number of species and quantity of individuals collected are used to calculate the IBI score.

The fish-based IBI scores incorporate several measurements of the fish community. When the measurements are analyzed together, they are given a score that reflects the lake's health.

Each IBI score is generated using between eight and 15 calculated measurements. Examples include: the number of intolerant species sampled, the number of tolerant species sampled, the number of vegetation-dwelling species sampled, the percentage of small benthic individuals captured in seining and electrofishing sampling gears, and the percentage of insect-eating species sampled in trap nets.

These measurements are known to correspond with human-induced stress, such as watershed land use patterns and quality of aquatic habitat.

Lake designations

The fish-based IBI score places lakes into five designations: exceptional, fully supporting, vulnerable, not supporting, and insufficient information. The designations can be used by the Minnesota Pollution Control Agency (MPCA), local governments and conservation groups, lake associations and homeowners to guide future lake management actions.

Exceptional Lakes
Minnesota's highest quality lakes. These lakes are a top priority for protection, especially those at risk of land use change or shoreline development.

Full Supporting Lakes
Healthy lakes that support diverse fish populations. Protection needed and some restoration may also be required.

Vulnerable Lakes
Lakes that are somewhat degraded but still support ecosystem functions. Lakes are a top priority for restoration and protection of remaining quality habitat needed

Not Supporting Lakes
Lakes that are degraded and where sensitive species typically no longer live. Restoration, sometimes extensive, will be needed. These lakes will be added to the impaired waters list.

Insufficient Information Lakes
Sampling did not yield enough data to make a clear determination about the health of the lake. Additional data collection is needed.

Maps

Maps of lakes sampled and watersheds assessed based on the fish-based IBI. Click the map to enlarge.

 

Assessing Minnesota lakes

The fish-based IBI score is an important component of the MPCA's watershed assessment process.

The MPCA uses physical measurements of water quality such as water clarity and amount of phosphorus to determine the type of recreation a lake supports. These measurements represent a snapshot in time of lake conditions whereas the biological community reflects changes to the lake over several years or more. The IBI is used as a biological measurement to determine if the lake supports the expected aquatic life. These biological assessments provide important information to guide Clean Water planning and restoration.

The DNR supports the MPCA's lake assessments with expertise and data on fish and plant communities in Minnesota lakes to provide this important component to the assessment process.

In addition to assessing lakes, DNR staff study stressors impacting vulnerable and impaired lakes. Numerous stressors are considered, but most often, the focus is on the impacts of water quality and the shoreline habitat on the fish community.