Associations between Biotic Integrity and Sport Fish Populations in Upper Midwest, USA Rivers, with Emphasis on Smallmouth Bass.

Indices of biotic integrity (IBIs) are used to assess ecosystem health of streams and rivers. Streams and rivers with high IBI scores should support abundant and healthy populations of recreationally important sport fishes. However, the fundamental assumption that IBI scores and sport fish populations are associated needs to be examined. To verify this assumption, we tested associations between IBI scores and relative abundance of all sport fishes targeted by anglers, with emphasis on relative abundance of four size groups of smallmouth bass Micropterus dolomieu at 54 stream and river reaches in 2012 and 2013. We also tested for associations with smallmouth bass body condition and growth. A total of 13,708 fishes representing 85 species were captured including 11 sport fish species that included 571 smallmouth bass. We found that the maximum potential relative abundance of sport fishes and smallmouth bass size classes, as well as body condition of bass between 180 and 279 mm, could be predicted by IBI scores. We did not observe significant relationships with body condition of other bass size classes or with growth. Whereas abundance patterns were variable at reaches with higher IBI scores, abundance of larger, quality-sized sport fishes were more limited at reaches with IBI scores <30 that were classified as having poor biotic integrity. Maximum potential body condition was predicted to exceed 95, a condition value indicative of healthy fish, at IBI scores exceeding 50, reflective of reaches being classified as either fair, good, or excellent. These results confirm that management activities that enhance or maintain biotic integrity also support high-quality habitat for sport fish. While our findings support using IBIs as an indicator of the fishable goal specified in the United States Clean Water Act, managers should recognize that other factors not necessarily represented by the index can also limit sport fish populations.