The brook trout is the most abundant sport fish found in Maryland’s freshwater streams. But it might not have much of a future in many parts of the state.

The Maryland Biological Stream Survey estimated that about 318,000 brook trout live in state streams today. But that may be only about a tenth of the number found a few centuries ago.

Trout require cold, clean, undisturbed streams. Much of their habitat has been lost since Colonial days.

Today, according to the survey, streams with good habitat conditions average 599 brook trout per mile. If that number were multiplied by the nearly 5,000 miles of streams in the Piedmont and mountain portions of the state — which historically would have been suitable for brook trout — Maryland streams would have once contained nearly 3 million brook trout, according to an estimate by Paul Kazyak, a Department of Natural Resources biologist.

Instead, brook trout are only found in portions of seven of 17 river basins in the state.

What’s happened to brook trout habitat? In a word, development.

The survey never found brook trout in watersheds having more than 2 percent impervious surfaces — things like roads, parking lots and roof tops.

In fact, brook trout were only rarely seen in watersheds with more than 0.5 percent impervious surfaces. For reference, a two-lane road running through a square mile is equivalent to 0.5 percent impervious surface.

The bottom line: Only a little development can make a watershed out-of-bounds as far as brook trout are concerned.

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.

Also, rapid runoff from pavement tends to increase stream erosion, increasing sediment buildup in the spaces between rocks — areas important for brook trout spawning.

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 can tolerate warmer streams — up to 26 degrees Celsius — and more development. 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. “There are some others we haven’t found at about that same threshold,” he said. “But brook trout are a good example because most people like them.”

Several species of amphibians have also disappeared with low amounts of development. Several are never found in watersheds with more than 3 percent imperviousness. They include the mountain dusky salamander, seal salamander, Jefferson salamander and the northern slimy salamander.