Positive Effects of Agricultural Land Use Changes on Coldwater Fish Communities in Southwest Wisconsin Streams

Abstract

AbstractThe Conservation Reserve Program (CRP) is a federal program that encourages the planting of cool‐ or warm‐season grass cover on highly erodible croplands and along stream corridors. We sought to determine whether fish community structure in coldwater streams was associated with CRP and other agricultural land use changes in southwestern Wisconsin. We compared coldwater fish index of biotic integrity (IBI) scores and species richness in streams located in areas of relatively high (21.3% of land area; high‐CRP area) versus relatively low (12.1% of land area; low‐CRP area) CRP participation. All of the streams were sampled in the 1970s before implementation of the CRP and again at the same locations after implementation, from 2000 to 2005. Pre‐CRP fish communities were characterized by a relatively high diversity of eurythermal species and low coldwater IBI scores. We found significant increases in coldwater IBI scores over time in streams within the high‐CRP area relative to streams within the low‐CRP area. Fish populations in streams within the high‐CRP area shifted from eurythermal and tolerant species before CRP implementation to stenothermal, cool‐ and coldwater species after implementation. Ecological responses within the high‐CRP streams also included a reduction in species richness. Without intensive monitoring of watershed nutrients, the fish community changes cannot be mechanistically linked to specific land use practices. However, we demonstrate that IBI scores and species richness were correlated with phosphorus loading estimates and that predicted phosphorus reductions were greater within the high‐CRP grassland area. The estimated phosphorus loading declines reflected reduced cropland areas and reduced density of dairy farms. The estimates did not capture all environmental factors, such as trends in production of hogs and cattle or streamflow regime changes associated with conservation practices. We conclude that the combination of extensive grassland management, livestock reductions, and other long‐term agricultural land use changes benefited the coldwater fish communities within the high‐CRP area.