Watershed urbanization and changes in fish communities in southeastern Wisconsin streams

We compared watershed land-use and fish community data between the 1970s an d 1990s in 47 small streams in southeastern Wisconsin. Our goal was to quan tify effects of increasing urbanization on stream fishes in what had been a predominantly agricultural region. In the 43 test watersheds, mean surface coverage by agricultural lands decreased from 54 percent to 43 percent and urban lands increased from 24 percent to 31 percent between 1970 and 1990. Agriculture dominated the four reference watersheds, but neither agricultu re (65-59 percent) nor urban (4.4-4.8 percent) land-uses changed significan tly in those watersheds during the study period. From the 1970s to the 1990 s the mean number of fish species for the test stream sites decreased 15 pe rcent, fish density decreased 41 percent, and the index of biotic integrity (IBI) score dropped 32 percent. Fish community attributes at the four refe rence sites did not change significantly during the same period, although d ensity was substantially lower in the 1990s. For both the 1970s and 1990s t est sites, numbers of fish species and IBI scores were positively correlate d with watershed percent agricultural land coverage and negatively correlat ed with watershed urban land uses, as indexed by percent effective connecte d imperviousness. Numbers of fish species per site and IBI scores were high ly variable below 10 percent imperviousness, but consistently low above 10 percent. Sites that had less than 10 percent imperviousness and fewer than 10 fish species in the 1970s suffered the greatest relative increase in imp erviousness and decline in species number over the study period. Our findin gs are consistent with previous studies that have found strong negative eff ects of urban land uses on stream ecosystems and a threshold of environment al dam age at about 10 percent imperviousness. We conclude that although ag ricultural land uses often degrade stream fish communities, agricultural la nd impacts are generally less severe than those from urbanization on a per- unit-area basis.