A Disturbance-Based Ecosystem Approach to Maintaining and Restoring Freshwater Habitats of Evolutionarily Significant Units of Anadromous Salmonids in the Pacific Northwest

Abstract

To preserve and recover evolutionarily significant units (ESUs) of anadromous salmo- nids Oncorhvnchus spp. in the Pacific Northwest. long-term and short-term ecological processes that create and maintain freshwater habitats must be restored and protected. Aquatic ecosystems through- out the region are dynamic in space and time. and lack of consideration of their dynamic aspects has limited the effectiveness of habitat restoration programs. Riverine-riparian ecosystems used by anadromous salmonids were naturally subjected to periodic catastrophic disturbances, after which they moved through a series of recovery states over periods of decades to centuries. Consequently the landscape was a mosaic of varying habitat conditions, some that were suitable for anadrornous salmonids and some that were not. Life history adaptations of salmon. such as straying of adults, movement of juveniles, and high fecundity rates, allowed populations of anadromous salmonids to persist in this dynamic environment. Perspectives gained from natural cycles of disturbance and recovery of the aquatic environment must be incorporated into recovery plans for freshwater habitats. In general, we do not advocate returning to the natural disturbance regime, which may include large-scale catastrophic processes such as stand-replacing wildfires. This may be an impossibility given patterns of human development in the region. We believe that it is more prudent to modify human- imposed disturbance regimes to create and maintain the necessary range of habitat conditions in space (10 3 km) and time (10 1 -10 2 years) within and among watersheds across the distributional range of an ESU. An additional component of any recovery plan, which is imperative in the short-term, is the establishment of watershed reserves that contain the best existing habitats and include the most ecologically intact watersheds.