MINNOWS CAN ACCLIMATE TO TOTAL RESIDUAL CHLORINE

Observations in a chlorine-contaminated stream in eastern Tennessee re vealed several species of fish in areas where total residual chlorine (TRC) concentrations were often great enough to be lethal, based on th e results of other studies. We used in situ and laboratory experiments to test the hypothesis that minnows could acclimate to TRC. Acclimati on was assessed by time-to-death for fish exposed to a single lethal r egime of TRC. In in situ experiments, striped shiner and central stone roller minnows collected from East Fork Poplar Creek at a TRC-contamin ated site and a site farther downstream, where TRC was below detection , were caged at sites in the stream where TRC concentrations were eith er nil or great enough to be lethal. At the cage with high TRC, minnow s collected from the ''TRC-contaminated'' site survived two to four ti mes longer (15.8 h vs. 6.1 h) than the same species from the ''TRC-fre e'' site. In laboratory experiments, golden shiner minnows were expose d to TRC in a stepwise increasing concentration regime (0.04 mg/L duri ng week 1, 0.08 mg/L during week 2, and 0.12 mg/L during week 3) befor e being challenge-tested with a lethal concentration of TRC. Minnows t hat had been previously exposed to TRC survived longer than controls i n each test, and the magnitude of the mean time to death increased as the TRC concentration and duration of exposure increased (1.2-, 2.5-, and 3.9-fold increases for weeks 1, 2, and 3, respectively). Another e xperiment with golden shiner minnows showed that a 21-d exposure to TR C at a low concentration (0.04 mg/L) only slightly increased their tol erance to TRC. Thus, several minnow species apparently can acclimate, fairly rapidly, to TRC. This study may help explain why fish kills are less common than expected in TRC-contaminated streams.