Relation of Antimycin A Exposure to Mortality in Rainbow Trout

Alpine lakes in North Cascades National Park were fishless prior to stocking, but self-sustaining populations now exist causing detrimental effects in some of these ecologically sensitive areas. For more vulnerable ecosystems, eradication is one proposed solution. To minimize effects to other biota antimycin A was identified as the piscicide of choice. Initial efforts to remove fish underscored the need for a better understanding of the dynamics of concentration and exposure time on mortality with the goal of eradicating fish while minimizing toxicant application. Our principal objective was to determine the duration of exposure to antimycin A required to ensure complete mortality of trout based on a typical target concentration of 4 µg/L. Commercial production was on hiatus, so antimycin A was obtained from two separate storehouses. Toxin mixtures were injected into 55-liter aquaria to expose bioassay-certified fingerling rainbow trout (Oncorhynchus mykiss) to either 4 or 8 µg/L. In the first experiment we exposed duplicate tanks of twenty fish to 0 or 4 µg/L antimycin A.  Five fish per tank were removed at 1-hr intervals for 4 hours and placed into fresh water.Observations of physical and behavioral responses were made for 72 hrs.  We extended the duration and included a 8 µg/L concentration in a second experiment. Thirty fish were added to each aquarium with triplicates at each concentration.  Fish were moved as previously into fresh water and observed for 48 hrs. In the first experiment no fish exposed for 2 hours or less died, some fish did not change appearance or behavior prior to death, and no fish died within 24 hrs of removal from the toxin.  In the second experiment no fish exposed to 4 µg/L or 8 µg/L antimycin A for 4 or 8 hours recovered, and none of these fish survived longer than 12 hours after removal from the toxicant.  All fish exposed for more than 4 hours demonstrated physical and behavioral signs of stress, and time to death was positively related to fish weight. Variability encountered in mortality within treatments may be attributable to aeration-induced destruction of the antimycin A molecule and to a non-uniform distribution of the toxicant in a tank.  Variability between experiments may reflect different storage conditions of concentrate. In conclusion, the typical working concentration of 4 µg/L for a minimum exposure time of 4 hours ensures mortality, if uniform exposure can be achieved. Even fish without visible signs of stress succumb.