Best Management Practices, Cumulative Effects, and Long-Term Trends in Fish Abundance in Pacific Northwest River Systems

Abstract

Although it is widely believed that forest management has degraded streams and rivers, quantitative relationships between long-term trends in fish abundance and forestry operations have not been successfully defined. In this article we review the difficulties in describing cumulative effects of forest management on fishes of the Pacific Northwest. Despite uncertainties in interpreting long-term trends from catch and escapement statistics as well as widespread programs of hatchery production, many local fish populations are declining. We suggest that trends in the abundance of individual populations are often of limited use in identifying the cumulative effects of forest management within a river system. Shifts in the composition and organization of fish communities may provide more comprehensive evidence of the extent of environmental alteration. Reduced stream habitat complexity has been one of the most pervasive cumulative effects of past forest practices and probably has contributed to significant changes in fish communities, particularly when accompanied by other land use activities that have led to straightened, confined channels. In simplified streams a few fish species have characteristically been favored while others have declined or disappeared completely. Likewise, fish culture practices have resulted in overall losses of genetic diversity among species. In order to protect channel complexity and biodiversity, best management practices (BMPs) should include measures to preserve physical and biological linkages between streams, riparian zones, and upland areas. Connections must include transfer processes that deliver woody debris, coarse sediment, and organic matter to streams, as these materials are largely responsible for creating and maintaining channel complexity and trophic diversity. Past forest practice regulations have required attainment of individual water quality standards, such as temperature or dissolved oxygen, and have been aimed at protecting certain life history stages of single species (e.g., salmon eggs in spawning gravels). This approach is inadequate to achieve the goal of restoring and maintaining natural levels of complexity at the level of a stream ecosystem. New BMPs are beginning to address this issue by prescribing riparian management zones with a greater range of vegetative species and structural diversity, thus providing for future sources of large woody debris, floodplain connections, and other linkages important to ecosystem function. Benefits of new BMPs in terms of improved habitat complexity and increased diversity of fishes on the scale of a river basin will require coordinated planning and extensive application, and will take years—perhaps decades—to become apparent.