CONDITION FACTOR AND WHOLE-BODY SODIUM CONCENTRATIONS IN A FRESH-WATER FISH - EVIDENCE FOR ACIDIFICATION STRESS AND POSSIBLE IONOREGULATORYOVER-COMPENSATION

Condition factor, ''k'', was measured for 1202 blacknose dace (Rhinich thys atratulus) from three streams in Shenandoah National Park (USA) o f different acid neutralizing capacities (ANCs). ''K'' is a ratio of w eight standardized to length; it is an indication of the health of the individuals in a population. R. atratulus condition factor in the low -ANC stream was found to be significantly lower (Ii %) than that of da ce measured for fish from the intermediate and high-ANC streams. This difference, according to the results of related investigations, is lik ely to be biologically significant. Whole-body sodium concentrations w ere measured as an additional test of sublethal stress in these stream s. During summer base now conditions, mean whole-body sodium concentra tions of adult R. atratulus maintained in cages were found to be highe st in the low-ANC stream and lowest in the high-ANC stream The lower c ondition factor of dace in the low-ANC stream may be related to whole- body sodium concentration and ion regulation Ion regulation in the low -ANC stream may be more metabolically costly because of chronic sublet hal pH stress. R atratulus may maintain high body Na+ concentrations i n low ANC- and ionic strength waters in order to provide a buffer agai nst large episodic pH depressions. The metabolic cost of this ionoregu latory over-compensation may necessitate the diversion of energy from somatic growth and explain the poorer condition of fish from such wate rs.