Screening causal assessment of brook trout occurrence and road runoff

Although less sensitive than coho salmon, brook trout fry are sensitive to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene diamine-quinone (6PPDQ; 24-hr median lethal concentration that causes death in 50% of the population [LC50] = 0.2 µg/L 6PPDQ). Concentrations have been measured in United States streams exceeding that LC50. In a dataset of more than 5,000 sampling stations in Pennsylvania, USA, brook trout occurrence is negatively correlated with percentage of impervious cover, road density, percentage of area in agriculture, and population density, and positively correlated with mixed and deciduous forest (absolute Spearman's rho > 35), whereas the tolerant brown trout is weakly positively correlated with road runoff variables and negatively correlated with precipitation and some types of vegetative cover (absolute rho < 26). The probability of observing brook trout decreases with increasing road density, percentage of impervious surfaces, and total traffic volume. Brown trout were indifferent to these variables. Other stressors and toxicants also occur in the study area and direct effects in the field have not been conclusively attributed to 6PPDQ. Observing mortality in the wild is difficult because fry are small and remain in the gravel until their yolk sacs are absorbed. There is evidence that road-related contaminants such as 6PPDQ may have adverse effects on brook trout populations, and the issue deserves further study using targeted water monitoring and caging experiments.