LETHAL LIMITS AND SUBLETHAL EFFECTS OF HYPOXIA ON THE AMPHIPOD GAMMARUS-PSEUDOLIMNAEUS

The amphipod Gammarus pseudolimnaeus is an important prey species for trout in certain tailwater fisheries below hydropower dams. The effect s of low dissolved oxygen (DO) on this species were investigated in la boratory experiments. The duration of survival in anoxia and in lethal ly low DO concentrations differed among adult female, adult male, and juvenile individuals. Adult females were the group most sensitive to b oth anoxia and hypoxia; for example, LC(50) in hypoxia was 2.00, 1.28, and 1.05 mg/L, respectively, for females,males and juveniles (48 h, 1 5 degrees C). Juveniles were generally less sensitive to hypoxia but m ore sensitive to anoxia than were adult males. LC(50) increased with i ncreasing temperature but changed little with duration of exposure aft er 24 h. The rate of oxygen consumption was a hyperbolic function of D O without a distinct critical level; oxygen consumption was depressed 10% at 5.7 mg/L and 50% at 1.3 mg/L (15 degrees C). Hypoxia induced th e separation ration of amplexing pairs at 2 ppm and inhibited the reun ion of separated pairs at 5 ppm (15 degrees C). Current US government (US Environmental Protection Agency) criteria for DO appear to be suff icient for protection of this species, but these criteria may often no t be met below hypolimnetic-release hydropower dams.