Effectiveness of relative weight (Wr) of bluegill as an indicator of environmental conditions

The relative weight (W_r), or “plumpness” of fish, is often considered to be an indicator of environmental conditions. Of these environmental conditions, prey availability is commonly linked to the W_r of piscivorous and specialist species. However, few evaluations have been made regarding the use of W_r for a generalist species as an indicator of environmental conditions. We compared the W_r of several size classes of bluegill in 30 Illinois lakes to an extensive multi-year data set of environmental conditions, including limnological, population, and prey characteristics. Significant positive correlations were found between Wr of bluegill and conspecific biomass and gizzard shad relative abundance. However, we generally found few significant correlations between W_r and a suite of environmental conditions. Although W_r of bluegill may be linked to productivity and prey availability, these relationships may be obscured by a multitude of complex interactions such as density dependent effects, competition and predation risk, parasitism, and other abiotic factors such as turbidity. Our results suggest that W_r is not a strong direct indicator of environmental conditions for a generalist species, especially in ecologically complex systems.