For reference, a two-lane road running through a square mile is equivalent to 0.5 percent impervious surface. It's unclear exactly what factors cause the brook trout to be lost, said Scott Stranko, a DNR biologist working on the survey. But, he said, two likely suspects are temperature and sediment. Because impervious surfaces collect heat on hot summer days, the rain hitting them is raised to warmer than normal temperatures. When it runs off roads and parking lots, it can raise the temperature of small streams many degrees.
Brook trout require cold streams.
In Maryland, the survey never found one in a stream warmer than 23 degrees Celsius. As streams degrade, it gives a slight competitive advantage to the larger brown trout, a nonnative species that has been stocked in the streams. Brown trout were found in watersheds with up to 5 percent impervious surfaces. As development takes place and imperious surfaces expand, brown trout will continue to gain a competitive advantage over the native brook trout. Brook trout are not the only fish species to disappear with low levels of development, Stranko said.