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4.6 Aquatic Habitat and Biota

Fish and invertebrates depend on a variety of stream physical characteristics including temperature, dissolved oxygen, turbidity, light, nutrients, sediment particle size distribution, and refuge opportunities. Chapter AQUA-5 contains a complete discussion on the range of aquatic habitats and species in the South. Most studies on the impacts of silvicultural activities on aquatic biota and habitat have been conducted in the Pacific Northwest and northwestern California, areas dominated by steep slopes, frequent landslides, erodible soils, and high precipitation levels. Under these conditions, forest practices can have a substantial impact on stream channel conditions if BMPs are not fully implemented and maintained over time.

Sullivan and others (1987) document several case studies in northern California that took place between 1950 and 1970 when several extreme storms after extensive logging resulted in substantial alterations to stream channel morphology. Streambeds were raised by as much as 4 meters, stream widths were doubled, stream channels were shifted, average particle size was increased, pools were filled in, riffles became less pronounced, summer flows were reduced, riparian vegetation was degraded, and stream banks were eroded. It was difficult to separate the contribution of harvesting impacts from the general storm effects, but fish populations declined over this period. In the 1982 National Fisheries Survey (Judy and others 1984), forestry activities were estimated to produce adverse effects on fish in about 7.5 percent of assessed river and stream miles, compared to 29.5 percent for agricultural land and 6.7 percent for urban areas.

Tebo (1955) studied the effects of early logging practices in steep mountainous watersheds on siltation and the impacts on bottom organisms in western North Carolina. There were no limitations on logging method and the logging operations were not supervised by the USDA Forest Service. Tebo (1955) compared the number and volume of bottom-dwelling organisms upstream of the harvested area to a site located below the mouth of the stream draining the logged watershed that received an accumulation of silt. The author found a statistically significantly larger population and higher volume of bottom-dwelling organisms at the control site upstream. After the removal of accumulated sediments and reduction in numbers of organisms due to flooding, the section of stream impacted by sedimentation still produced a slightly but statistically insignificantly lower number of organisms than the control section.

Vowell (in press) examined the effects of intensive forest management activities on aquatic habitat in northern Florida using a stream condition index (SCI) based on benthic macroinvertebrate sampling measures. Biological indicators such as this are believed to be more accurate measures of water quality than chemical indicators since the presence, or absence, and abundance of aquatic organisms, benthic macroinvertebrates in particular, better reflect the overall ecological health of waterbodies because they integrate pollutant stressors over time. Vowell also evaluated aquatic habitat using an average habitat assessment value based on a composite of physical stream attributes including substrate type and availability, water velocity, artificial channelization, habitat smothering, stream bank stability, riparian buffer width, and riparian buffer quality.

Vowell (in press) found no significant differences between pre- and post-treatment SCI values at any of the four sites, indicating no effect due to silvicultural activities. Average habitat assessment values were also within the "optimal" range both before and after treatments. The only notable differences found after treatment were changes in the score for water velocity and riparian zone width. The measured increase in water velocity was attributed to minor temporal variability rather than the treatment. Riparian zone widths after harvesting, while considered "marginal" from a scoring point of view, were still within the required width for primary streams. No change was recorded for habitat smothering or stream bank stability, two components of the habitat assessment considered especially sensitive to impacts from silvicultural activities and critical to maintaining macroinvertebrate population integrity.

Interestingly, some studies have actually documented increases in fish populations and fish size after logging (see Hall and Lantz 1969, Murphy and Hall 1981, Murphy and others 1981, Hawkins and others 1983). These increases are generally attributed to alterations in the foodweb (National Council for Air and Stream Improvement 1994). For example, increased light penetration or nutrient concentrations may lead to increases in primary productivity that may increase herbivore populations. Slight increases in stream temperature can actually favor fish growth and increase survival of young fish, particularly in northern latitudes or high-elevation streams (Holtby 1988)