Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Assessing Physical Quality of Spawning Habitat
G. Mathias Kondolf, John G. Williams, Timothy C. Horner, and David Milan
Human activity often degrades natural spawning habitat, so there is a frequent need to assess the quality of spawning gravels and determine whether gravel quality limits spawning success. Degradation of spawning gravels is recognized as a primary contributing factor in the widespread decline of salmon and trout populations throughout North America and Europe. The bed material may be too coarse for spawning fish to move, a problem common where dams eliminate the supply of smaller, mobile gravels (e.g., Parfitt and Buer 1980; Buer et al. 1981). Excessive levels of interstitial fine sediment may clog spawning gravels, an effect that has been documented downstream of several types of land use that increase sediment yields, such as timber harvest, road construction, and agriculture (Cederholm and Salo 1979; Everest et al. 1987; Meehan 1991; Theurer et al. 1998; Sear et al. 2008, this volume).
If salmonids spawn successfully in a gravel (i.e., if they dig a pit, deposit, and bury eggs; the eggs incubate and hatch; and the alevins develop and emerge), then we might assume that the hyporheic habitat in the gravel is suitable for spawning. However, a deeper analysis of the problem should also consider the quality of the subsurface or hyporheic habitat and the fitness and viability of emerging alevins or fry and include biological factors in the evaluation of spawning habitat.