Populations Appear to Thrive in Impaired Streams: Factors Affecting Relative Weight

The range of bluegill (Lepomis macrochirus) is reported to be increasing as land-use practices impair streams.  We examined bluegill relative weight (W_r) at 229 sites across the eastern U.S. to better understand mechanisms that may explain changes in bluegill occurrence in streams.  Individual fish were categorized into length groups and a mean W_r value was determined for each length group at each site.  Relative weight was compared between streams classified as impaired and not impaired.  Random forest classification analysis was conducted to assess the relative importance of selected biotic and abiotic factors in predicting bluegill populations classified as “high” (mean W_r >100) and “low” (mean W_r < 90) relative weight.  Small-size bluegills (80 to 150 mm) were most abundant and W_r analyses were conducted on small-size bluegill only.  Mean W_r values for bluegill populations at impaired sites were significantly greater than those at non-impaired sites.  Nine factors had values that were significantly greater at sites with bluegill populations classified as having high relative weight compared to those at sites with bluegill populations classified as having low relative weight.  These factors, in order of relative importance, were specific conductance, stream base flow, canopy cover, bluegill prey availability, native fish species loss, pool/riffle ratio, stream flow stability, percent urban land use, and the presence of canals in the basin.  Of these, bluegill prey availability, native fish species loss, stream flow stability, and percent urban land use at impaired sites were significantly greater than at non-impaired sites.  Impairment of streams, particularly in urban environments, may result in increased preferred food resources, reduced competition, and altered stream flow conditions.  These changes appear to be beneficial to bluegill populations, as evidenced by increased relative weight.