SURVIVAL OF BROOK TROUT (SALVELINUS-FONTINALIS) EMBRYOS AND FRY IN STREAMS OF DIFFERENT ACID SENSITIVITY IN SHENANDOAH NATIONAL-PARK, USA

Shenandoah National Park receives more atmospheric sulfate loading tha n any other USA national park. pH has been gradually declining in low- ANC streams for more than 10 years. We have completed four 1-to-3 mont h-long field bioassays in three streams differing in acid neutralizing capacity (ANC), using a total of 18,000 hatchery brook trout eyed-egg s through fry. In three of the four bioassays, embryos/fry showed poor er survivorship in the low-ANC stream, compared to the high-ANC stream . Substantial mortalities occurred under different hydrological condit ions, including steady rain plus significant rain events (fall 1992), low rainfall followed by a significant rain event (spring 1993), and s teady light rain and snowmelt with no large rain events (spring 1994). In a fourth bioassay (fall 1993), poor survivorship occurred in all t hree streams due to drought conditions. Trout placed in the intermedia te-ANC stream showed variable survivorship, in two bioassays as high a s in the high-ANC stream, and in one bioassay as poor as in the low-AN C stream. Baseflow ANC in the intermediate-ANC stream is 40-100 mu eq/ L, and pH never falls below 6.0. However, during episodes, pH in this stream sometimes fluctuates rapidly in the range of 6.0 to 7.0, and th is fluctuation itself may be a source of physiological stress.