The susceptibility of the Blacknose Dace, Rhinichthys Atratulus, to acidification in Shenandoah National Park

Dennis, Todd

Due to its relatively high degree of sensitivity, the blacknose dace (Rhinichthys atratulus) was utilized as an indicator species to assess the susceptibility of the ichthyofaunal community of Shenandoah National Park to acidification. Water chemistry measurements and bioassays were conducted in three streams, representative of the Park's three major bedrock types (silici-clastic, granitic, and basaltic) and differing in acid neutralizing capacity (ANC). pH, ANC, total monomeric aluminum, and calcium were measured in water samples taken during both base flow conditions and storm events. Survival bioassays of adult blacknose dace placed in cages in each stream were conducted over a fourteen month period to determine the extent that mortality rates were associated with differences in stream water chemistry. Fish survived equally well in all streams (except during two nonacidification related instances in the stream with highest ANC) during the bioassay period. Body sodium concentrations were measured from fish placed in cages in each stream during a nine week period from late spring to mid-summer 1993 in order to evaluate stream differences in chronic and storm-event sublethal stress. No significant differences in body sodium concentrations were observed during base flow conditions. The absence of major storm events prohibited the determination of episodic streamwide differences in sublethal stress. Repeated surveys over a six month period indicated that the condition of fish (measured as the ponderal index or condition factor) from the low ANC stream was significantly poorer on all sampling occasions. Comparison of results to related studies indicates that the blacknose dace population of the low ANC stream presently experiences pH and aluminum concentrations associated with deleterious effects in Ontario and the Adirondack mountains. If current atmospheric deposition rates of sulfur remain constant, blacknose dace (and other fish species) populations in SNP may in the future be extirpated from low ANC streams.