Catastrophic Mortality of Stream Fishes Trapped in Shrinking Pools

Abstract

Summer drought caused catastrophic mortality of fish trapped in small stream-bed pools. As they shrank, the pools were closely monitored to determine age and species-specific patterns of mortality. In 2 pools heavy mortality occurred 5 days before the pools disappeared. The deaths were concentrated among species with underslung mouths and/or no air bladders: these species are unable to gulp the oxygenated surface film. In 4 other pools mortality was light until the last day. In 5 of 6 pools, deaths occurred primarily during the day; this was true for all species save Carassius auratus. Pulses of heavy diurnal mortality were generally correlated with failure of percent O2 saturation values to recover from early morning lows. Aside from resistant young fry, there were no obvious age-specific differences in mortality. Terrestrial predators were not a significant cause of mortality since they fed mostly on dead or moribund individuals and were excluded by wire screen. Drought-stress sensitivity was not correlated with migratory vs. sedentary habits, although recolonization from downstream appeared to be more efficient than drought resistance as a means of repopulating the headwaters. By periodically decimating headwater fish faunas, summer droughts may provoke greater reproductive expenditures (r-selection) within fish species proceeding upstream.