PHYSIOLOGICAL TOLERANCES OF JUVENILE ROBUST REDHORSE, MOXOSTOMA-ROBUSTUM - CONSERVATION IMPLICATIONS FOR AN IMPERILED SPECIES

The robust redhorse, Moxostoma robustum (Teleostei: Catostomidae), is an imperiled sucker-native to large rivers of the Atlantic slope of th e southeastern United States. Juvenile M. robostum were tested for tol erances to temperature, salinity pH, and hypoxia in order to evaluate basic early life-history requirements. Static (acute) tests resulted i n estimates of mean lower temperature tolerances (5.3-19.4 degrees C) that varied with prior thermal acclimation and indicated no apparent d ifference in tolerance among fish 30, 60, and 90 days old. Fish acclim ated to 20 degrees C and 30 degrees C had significantly different mean critical thermal maxima (34.9 degrees C and 37.2 degrees C, respectiv ely) and exhibited pronounced increased opercular ventilation rates wi th elevated temperatures. Fish exposed to acute and chronic increases in salinity showed unusual patterns of mortality above the isosmotic p oint (9 ppt) that reflected possible differences in body mass and prio r acclimation conditions (i.e., water ionic composition); small fish a nd those held in soft water were the least tolerant of increased salin ity. Abrupt exposure to extreme pH values resulted in greater than 50% mortality at pH values below 4.3 and above 9.5 within a 96-hour perio d. Fish exposed to progressive hypoxja utilized aquatic surface respir ation at a mean oxygen concentration of 0.72-0.80 mg O-2 I-1 (20 degre es C and 30 degrees C acclimated fish, respectively), and lost equilib rium at 0.54-0.57 mg O-2 I-1. Juvenile M. robustum are moderately tole rant of a wide range of ambient physicochemical parameters, but furthe r research is needed to determine how both abiotic and biotic factors have contributed to population decline and extirpation of this species .