Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Hyporheic Influences on Salmon Embryo Survival and Performance
Iain A. Malcolm, Stuart M. Greig, Alan F. Youngson, and Chris Soulsby
Salmonids bury their eggs in open gravel structures known as redds to depths of between 0.05 and 0.5 m (DeVries 1997) beneath the streambed. Redds are temporarily modified volumes of mixed sediment (Peterson and Quinn 1996) in which ova of one or more females are deposited and incubate (Taggart et al. 2001). The period of time between spawning, when adult fish bury their eggs, and emergence, when juvenile fish (fry) leave the streambed and enter the channel, accounts for a large, but highly variable proportion of total lifetime mortality (Peterson and Quinn 1996). Understanding the causes of this mortality is prerequisite to designing meaningful fisheries management and conservation tools (Malcolm et al. 2005) and becomes increasingly important as salmon populations decline to the point where adult fish (spawner) numbers are insufficient to fully stock available juvenile habitat. Beyond the effects of outright mortality, the sublethal effects of the hyporheic environment on embryo performance (Alderdice et al. 1958; Silver et al. 1963; Shumway et al. 1964; Youngson et al. 2005) may affect the recruitment of young fish to stream populations in more subtle ways.
Embryo survival and performance in the redd environment are dependent on the interaction between the biological requirements of developing embryos and the quality of streambed (hyporheic) water parameters. Critically, although not exclusively (e.g., Massa et al. 2000), survival is dependent on the provision of dissolved oxygen (DO) and the removal of metabolites (Silver et al. 1963; Malcolm et al. 2003a; Youngson et al. 2005; Figure 1). The processes controlling hyporheic water quality are complex and dynamic, including physical, chemical, and biological interactions associated with the redd, its hydrological and geomorphological setting, and its state of disturbance (Figure 1). Previous studies of embryo survival have tended to focus on particular aspects of this complicated picture. However, to increase understanding, an interdisciplinary perspective that considers the complex processes identified in Figure 1 is required.