This index is designed to measure the aquatic vertebrate community and the surrounding conditions by using fish species as indicators. Overall, there are 12 fish community variables that can be broken down into three main categories: species richness and composition, trophic composition, and fish abundance and condition. By assessing the variables within these parameters, scientists can compare a sampled site with a relatively undisturbed site with similar geographical and climatic conditions. With this rationale, the only variable would be stressors resulting from human development and disturbance. The following table lists the 12 variables measured in the IBI and their applicability depending on particular sites.
| Variable Measured | Type of Site | |
|---|---|---|
| 1. | Total Number of Species | H W B |
| 2. | Number of Darter Species | H W |
| Percent Round-bodied Suckers | B | |
| 3. | Number of Sunfish Species | W B |
| Number of Headwater Species | H | |
| 4. | Number of Sucker Species | W B |
| Number of Minnow Species | H | |
| 5. | Number of Intolerant Species | W B |
| Number of Sensitive Species | H | |
| 6. | Percent of Tolerant Species | H W B |
| 7. | Percent of Omnivorous Species | H W B |
| 8. | Percent of Insectivorous Species | H W B |
| 9. | Percent of Top Carnivores | W B |
| Percent of Pioneering Species | H | |
| 10. | Number of Individuals | H W B |
| 11. | Percent of Hybrids | W B |
| Number of Simple Lithophilic Species | ||
| 12. | Percent of DELT Anomalies | H W B |
Type of Site: H-Headwater, W-Wading, B-Boat
IBI scores can range from 12-60 depending on the amount of disturbance that has taken place at and around the sampling site. Possible scores range from:
DELT-Deformities, eroded fins, lesions, and tumors
IBI criteria as taken from Ohio EPA 1987aMaximum Score of 12(variables) * 5(highest score) = 60
Minimum score of 12(variables) * 1(lowest score) = 12