Insects, the Favorite Food of Trout, are Abundant in Stream Reaches Cooled by Streamside Forests...

Water temperature, habitat structure, food availability and sediment flux are four important factors influencing the survival of trout (Salmonids) and other fish that are directly affected, to a large extent, by streamside forests.

In most small streams and rivers, the seasonal pattern of water temperature, the first of these factors, is determined largely by the extent that direct solar radiation and air temperature can modify the temperature of the water. In a given region, groundwater stays fairly constant in temperature throughout the year (± 1 degree C of mean annual air temperature for the region) and provides most of the baseflow for stream systems. Loss of shade from streamside forests can greatly warm streams, increasing a trout's demand for dissolved oxygen and, at the same time, reducing the amount of dissolved oxygen available in the water.

Photo right: A Quality water resources, as evidenced by a natural, healthy headwater stream.

Headwater streams of first to third order comprise about 85% of the total length of running waters, and because of the ratio of stream bottom to shoreline, are most readily influenced by exposure to solar energy. Agricultural drainage systems which intercept cool groundwater and drain it to streams in unshaded ditches contribute significantly to the increase in stream temperature.

Manipulation of the streamside forest canopy can be used to moderate and stabilize stream temperature to optimize the survivorship, growth, and reproductive needs of fish and aquatic macro-invertebrates and even benthic algae.

Habitat structure, the second factor affecting Salmonid survival, is enhanced by the addition to the stream channel of large woody debris which forms pools and important rearing areas.This debris also provides cover from predators and protection from high flows.

To understand the third factor, food availability, the natural stream must be viewed as a continuum from headwaters to mouth with a significant amount of the energy for aquatic life coming from organic material such as leaves, twigs, flowers, animals and insects originating from the streamside forest. These kinds of materials dominate the food base of small headwater streams flowing through forests. The food supports a diverse invertebrate community which, in turn, provides the principal food source for Salmonids in healthy ecosystems. Large amounts of leaf litter and other coarse organic matter enter small forested headwater streams and are rapidly consumed by aquatic invertebrates. These animals function as shredders because they reduce large pieces of organic debris to smaller pieces which move downstream and can be used by other animals who feed by filtering or gathering these fine particles of food.