Stream pH as an Abiotic Gradient Influencing Distributions of Trout in Pennsylvania Streams

Abstract

Elevation and stream slope are abiotic gradients that limit upstream distributions of brook trout Salvelinus fontinalis and brown trout Salmo trutta in streams. We sought to determine whether another abiotic gradient, base-flow pH, may also affect distributions of these two species in eastern North America streams. We used historical data from the Pennsylvania Fish and Boat Commission's fisheries management database to explore the effects of reach elevation, slope, and base-flow pH on distributional limits to brook trout and brown trout in Pennsylvania streams in the Appalachian Plateaus and Ridge and Valley physiographic provinces. Discriminant function analysis (DFA) was used to calculate a canonical axis that separated allopatric brook trout populations from allopatric brown trout populations and allowed us to assess which of the three independent variables were important gradients along which communities graded from allopatric brook trout to allopatric brown trout. Canonical structure coefficients from DFA indicated that in both physiographic provinces, stream base-flow pH and slope were important factors in distributional limits; elevation was also an important factor in the Ridge and Valley Province but not the Appalachian Plateaus Province. Graphs of each variable against the proportion of brook trout in a community also identified apparent zones of allopatry for both species on the basis of pH and stream slope. We hypothesize that pH-mediated interspecific competition that favors brook trout in competition with brown trout at lower pH is the most plausible mechanism for segregation of these two species along pH gradients. Our discovery that trout distributions in Pennsylvania are related to stream base-flow pH has important implications for brook trout conservation in acidified regions. Carefully designed laboratory and field studies will be required to test our hypothesis and elucidate the mechanisms responsible for the partitioning of brook trout and brown trout along pH gradients.

Specific Niche Characteristics Facilitate the Invasion of an Alien Fish Invader in Boreal Streams

“…Thus, only longterm monitoring will tell whether brook trout is indeed continuing its range expansion to invade the whole study The most important variable explaining the distribution and niche characteristics of brook trout was water pH. In their native range, brook trout are known to prefer cold and variable headwater environments with low pH [16][17][18]30]. Brook trout has established in similar species-specific habitats also in our northern Finnish study system, and similar observations have been made in Swedish streams [31].…”

Relating fish biomass to habitat and chemistry in headwater streams of the northeastern United States

“…This study is consistent with results from other work evaluating pH in conjunction with other metrics in headwater streams from this region (Baldigo and Lawrence 2001;Nislow and Lowe 2003;Kocovsky and Carline 2005). The results presented here along with the recent literature suggest that despite slight increases in surface water pH across the region (Driscoll et al 2001), acid deposition continues to influence fish abundance and production in many northeastern streams.…”

“…Nislow and Lowe (2003) found that summer pH of 6.0 or less could be used as a rough threshold below which Brook Trout populations were substantially reduced relative to other streams with comparable riparian forest conditions. Kocovsky and Carline (2005) did not identify a specific threshold beyond which Brook Trout relative abundance or biomass declined. In evaluating the presence or absence of Brook Trout, however, they documented the lowest pH conditions in which trout were still present: streams with summer pH as low as 4.6 in the Appalachian Plateau province and 5.6 in the Ridge and Valley Province of Pennsylvania (Kocovsky and Carline 2005).…”

“…Nislow and Lowe (2003) evaluated the influence of low-flow pH and logging history on Brook Trout abundance in New England streams; however, their study did not address specific in-stream habitat factors that may have more directly influenced fish biomass. Similarly, in a recent analysis of landscape features influencing Brook Trout abundance in Pennsylvania, stream pH in summer was also found to be a key factor, but the relative influence of specific reach-scale habitat features were not evaluated (Kocovsky and Carline 2005). The current study assesses local habitat and relative stream buffering capacity both independently and together in evaluating factors associated with the variability in fish biomass in northeastern headwater streams.…”

“…Thus, the summer/fall low-flow pH represents the conditions of the system when additional acid inputs are low and as such our data provide a proxy measure of potential buffering capacity and associated effects of episodic acidification. Kocovsky and Carline (2005) also used baseflow pH as their metric to evaluate the effect of acidity on trout distribution in Pennsylvania, as did Nislow and Lowe (2003). Nislow and Lowe (2003) found that summer pH of 6.0 or less could be used as a rough threshold below which Brook Trout populations were substantially reduced relative to other streams with comparable riparian forest conditions.…”