Fish Populations assesses the condition of river and stream fish communities. Fish play important roles in these ecosystems, are diverse, relatively long-lived, and can respond quickly to changes in environmental conditions. Combined with the fact that species vary in their tolerance to environmental disturbances, these characteristics mean that the presence and relative abundance of different species can help us understand the overall condition of a river or stream.
How is it scored?
Stream sites were scored using a subset of Michigan’s P51 scoring metrics (Michigan Biological Assessments): % tolerant species, % simple lithophilic species, % omnivorous species, % insectivorous species, and % piscivorous species. We were unable to use the full set of 10 metrics because the thresholds for some metrics require information about stream widths, which were missing for the majority of sites. Thresholds are ecoregion-specific and overall scores were converted to the report card scale using threshold-specific equations (Tabe 1). If sites were sampled multiple times, the average score was used and those scores were averaged across a watershed to calculate an overall watershed score. The area-weighted watershed score was used to calculate the overall region score. R code for these calculations was modified from code generously provided by Alofs Lab at the University of Michigan School for Environment and Sustainability.
We also considered using Simpson’s Diversity Index to score this indicator, but ultimately chose the P51 method because it is widely used and understood within Michigan and also provides a more detailed assessment of ecological condition than Simpson’s Diversity Index.
Table 1. Original narrative and scoring thresholds from Michigan’s P51 and the equations used to convert those scores to the report card scale. Note that we could only include 5 of the 10 metrics used in the Michigan P51, so our scores range from -5 to 5 (Reduced P51 Score) compared to -10 to 10 normally used (Full P51 Score).
P51 Narrative | Full P51 Score | Reduced P51 Score | Report Card Score | Equation |
Excellent | 10 to 5 | 5 to 2.5 | 80–100 | y = 8x + 60 |
Acceptable | 5 to -5 | 2.5 to -2.5 | 79–20 | y = 12x + 50 |
Poor | -5 to -10 | -2.5 to -5 | 190 | y = 8x + 40 |
Data source: Michigan Department of Natural Resources.