Use of habitat heterogeneity in salmonid spawning habitat rehabilitation design

A shortage of salmonid spawning habitat on dammed and regulated rivers has led to the popularity of spawning habitat rehabilitation projects. Habitat heterogeneity is thought to be an important feature of aquatic ecosystems, but specific metrics for design and assessment are lacking. In August 2002, ~ 2,786 metric tons of spawning gravels and 7 large boulders were placed in a 155 meter reach on the lower Mokelumne River, California, USA. Habitat heterogeneity was incorporated into the design as part of a spawning habitat integrated rehabilitation approach (SHIRA) developed by the authors. A mix of conceptual and numerical models (2D hydrodynamic with habitat suitability and sediment entrainment submodels ) were used to test the effectiveness of design scenarios. Although optimal spawning habitat as defined by habitat suitability models is generally found in riffles, proximity of habitat to structural cover (pools, large woody debris, boulder clusters and overhanging vegetation) and hydrodynamic shear zones provide equally important refuge from predation and resting zones for energy conservation. The increased heterogeneity appeared highly effective in terms of redd utilization with 70 redds located in close proximity to 93% of the available structural cover, and 42 redds located in close proximity to 90% of the available shear zone refugia. Partial results emphasizing habitat heterogeneity availability and utilization metrics are presented to illustrate their potential in rehabilitation design and assessment. Habitat Simulation and Impact Assessments 2 framework was used to plan, design, construct and monitor the project; but details of the approach, provided in Wheaton et al. (2004a; 2004b), are not discussed here. The structural features used to provide habitat heterogeneity are themselves microhabitat scale (10 to 10 m) features but produce heterogeneity over the macrohabitat or morphological-unit scale (10 to 10 m).