Investigating Effects of Rotenone on Amphibians Through Laboratory Experiments and Field Trials

To reverse the impacts of introduced species, fisheries managers are removing non-native and restoring native species to historical habitats.  An efficient method for removal of non-native fish species is the use of approved piscicides, but these chemicals can have measurable, negative effects on amphibian populations.  The focus of this research was to determine the effects of the commonly used piscicide rotenone on amphibians through controlled laboratory experiments and field investigations in southwestern Montana.  Laboratory trials demonstrated that rotenone exposure at 1 mg/L (product) was lethal to tadpoles of two species – Columbia spotted frog Rana luteiventris and boreal toad Anaxyrus boreas – at all three ages tested.  In Columbia spotted frog tadpoles, the probability of mortality decreased as age increased, while age did not affect average mortality in boreal toad tadpoles.  Tadpole species had a significant effect on mortality only at the oldest age group, with Columbia spotted frog tadpoles experiencing lower mortality then boreal toad tadpoles (p<0.001).  Sub-lethal effects on morphology, although statistically different between control and exposed Columbia spotted frog survivors in two instances, were not consistent and were not considered biologically significant.  Our results further indicated that as the duration of rotenone exposure increased, Columbia spotted frog tadpole mortality increased, except at the oldest age.  The results of the field investigations revealed that, in the 24 hours following application, rotenone was lethal to gill-breathing amphibian tadpoles and non-lethal to non-gill breathing metamorphs, juveniles, and adults.  In the year(s) following, tadpole repopulation occurred at all water bodies treated with rotenone product.  The information obtained from these investigations can be used to better inform future fish restoration actions and promote amphibian conservation in fish restoration areas.