Brook trout are often cited as “indicator species” of stream health. They require clean, cold water, so finding them in abundance means you have an especially healthy stream. A healthy stream is usually a forested stream: forests shade the watercourse, keeping it cool, and add structure that slows water flow and creates resting, feeding, and egg-laying areas for trout, while at the same time providing the stream with nutrients.
The wood and leaves that trees shed into streams also determine, in part, whether or not the stream environment is suited to the feeding and living requirements of various stream insects – also called macroinvertebrates – that trout like to eat. These insects are generally categorized, based on how they eat, in one of four ways: as predators, grazers, collectors, or shredders. In forested streams, the type of forest surrounding the stream – and the debris it sheds – often determine which of these four insect types dominates a particular area of the watershed.
Winsor Lowe, formerly of the Institute of Ecosystem Studies, and Keith Nislow, of the USDA Forest Service, examined headwater streams in New Hampshire to find out how logging affects these four types of stream insects, and, in turn, how brook trout are affected. Their results appeared recently in the journal Freshwater Biology.
By examining riparian forests that were logged both recently and long ago, Lowe and Nislow were able to come up with some basic relationships between forest cover, stream insects, and brook trout density, though they note that their results may not hold true for all locations. They found that recently logged forests with less canopy cover fostered more stream insects – and more of the type preferred by trout (the grazers) – than did forests with no recent logging history and more canopy cover (which tend to favor shredders). The streams in recently logged forests also had more trout.
These results may not be intuitive at first glance; it’s important to note that they’re just part of a much larger story. Recently logged forests can release more sediment into streams, which provides nutrients that allow algae – the main food of grazers – to grow. A thinner canopy lets in more light, which also helps the algae flourish. Grazers provide a more reliable food source for trout than do shredders, since they’re attached to rocks and not dependent on stream flow (which may be negligible in summer) for movement. And higher light levels may help trout forage for these grazers.
The flip side of the story is largely a matter of scale. Logging around headwater streams in New England is often of low intensity due to the nature of the terrain. In none of the streams studied was the extent of logging such that enough sediment was released to choke pools or gravelly egg-laying spots, nor did it let in enough light to warm the water to a level uncomfortable for trout.
However, the researchers caution, these same logging activities, if taking place in many tributaries of a main river, may cumulatively reduce the quality of downstream trout habitat. Headwater logging may also inadvertently reduce populations of other types of stream insects and small fish, especially those that are favored by the newly abundant trout. Everyone appreciates a little variety, after all.