Abstract

Total immunoreactive corticosteroid (IRC) levels (adjusted for fish mass; ng/g) were measured in whole-body homogenates of 2- and 8-months old Whitetail Shiners (Cyprinella galactura) and 4-months old federally threatened Spotfin Chubs (Erimonax monachus) exposed for 48 hours to varying suspended sediment concentrations (SSC; 0, 25, 50, 100, and 500 mg/L). Hydrophobic fractions were extracted from individual frozen fish after sonication and centrifugation of tissues. Extracts were resuspended in a buffer compatible with a commercially available enzyme-linked immunosorbent assay (ELISA) kit. Serially diluted concentrations of cortisol and extracts collected from unstressed fish were used to standardize the assay. Two-months old C. galactura had the highest resting level of whole-body IRC at 0 mg/L SSC. They also elicited the greatest response (three- to four-fold increase) when exposed to SSCs greater than 25 mg/L. Resting whole-body IRC levels were lowest in 8-months old C. galactura. For these fish IRC levels at 25, 50, and 100 mg/L SSC were similar to controls. Four-months old E. monachus showed a non-linear response with a possible threshold effect between 50 and 100 mg/L. At SSC greater than 100 mg/L E. monachus demonstrated a three-fold increase in whole-body IRC levels over control fish. Exposure to SSC levels greater than 100 mg/L caused a significant increase in IRC levels above baseline in both species and in all three life stages. This investigation shows that whole-body levels of IRC in young minnows increase dramatically upon exposure to SSCs greater than 25 mg/L. These data suggest that even moderate levels of suspended sediment (i.e., 100 mg/L) can severely stress young-of-year E. monachus. The imperilment of E. monachus may in part be due to stress imposed on young fish by elevated suspended sediment.