Physiological Stress in Native Brook Trout (\u3cem\u3eSalvelinus Fontinalis\u3c/em\u3e) During Episodic Acidification of Streams in the Great Smoky Mountains National Park

Episodes of stream acidification are suspected to be the primary cause of the extirpation of native southern brook trout (Salvelinus fontinalis) from six headwater streams in the Great Smoky Mountains National Park (GRSM). During periods of increased flow from storm events, stream pH can drop below 5.0 (minimum of 4.0) for 2-days or longer. To provide evidence that native brook trout are impacted by stream acidification, in situ bioassay experiments were conducted. Changes in stream water chemistry and brook trout physiology were determined during a 36-hour acidic episode at three remote headwater stream sites in the Middle Prong of the Little Pigeon River watershed. Conductivity, pH, turbidity, stage height and temperature were monitored continuously; and water samples were collected for laboratory analyses (metals, cations, anions, ANC). Native brook trout were put in cages at the three sites and fish were sampled before and after the acid storm event. Physiological stress in brook trout was assessed by measuring whole-body sodium in individual fish sampled before and after the stormflow, and evaluating whole-body sodium loss as a response to acid conditions. The pH decreased at all three sites during the acidic episode. Stream pH dropped to approximately 5.0 at two sites and 4.66 at the third site. Prior to the storm, there was no difference in the whole-body sodium concentrations in trout between the three sites. Following the storm event, in trout from the site that experienced the lowest pH, whole-body sodium levels were reduced significantly relative to a) the pre-storm condition and b) trout from the other sites. Results demonstrate that stream acidification can negatively affect native southern brook trout physiology in the GRSM under actual field conditions. Trout lose the ability to regulate critical blood ions, as exemplified by a loss of whole-body sodium, when stream pH was less than 5.0 for 20 hours. Loss of sodium is an important indication of physiological stress in fish exposed to acid waters. This observation supports the hypothesis that episodic acidification of streams could be limiting native brook trout from occupying headwater streams in the GRSM