Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
The Science and Practice of Salmonid Spawning Habitat Remediation
David A. Sear, Paul DeVries, and Stuart M. Greig
Salmon and trout are evocative symbols of natural river ecosystems. Despite their symbolic (and economic) importance for humans, especially in the case of anadromous salmon and trout, we have inflicted great losses in their numbers and distribution. Within Europe, the Atlantic salmon Salmo salar is currently extinct in four countries—Germany, Belgium, the Netherlands, and Switzerland— and populations are close to extinction in another six—Spain, France, Portugal, Denmark, Finland, and the Baltic states. Only Scotland, Norway, Iceland, and Ireland have comparatively healthy populations, although figures suggest that even there salmon numbers are significantly depleted when compared to historical densities (WWF 2001; Youngson et al. 2002; Montgomery 2003). Within North America, current figures indicate that 84% of Atlantic salmon populations are now extinct, with the remaining populations in a critical condition (WWF 2001). In Canada, the picture is less severe, although only 8% of populations have recently been classified as healthy. Figures for Pacific salmon Oncorhynchus spp. indicate that populations have also declined, and 17 Pacific salmon runs are now extinct, with a further 214 runs at risk of extinction or of special concern (Nehlsen et al. 1991; Huntington et al. 1996; Shea and Mangel 2001). Alaska remains the primary natural haven in North America where one can observe Pacific salmon populations in a more or less pristine state, although even here returning salmon numbers are affected by fisheries harvest. Unfortunately, declining salmon numbers are not a recent phenomenon and historical accounts reveal a tortuous path of decline that traces human influence over riverine landscapes (Montgomery 2003). For some, the future for many salmon and trout populations can appear bleak (Lackey et al. 2006).
In the case of anadromous salmonids, the decline in stocks has been attributed to four main factors: overharvesting of fish, impacts of aquaculture, impacts to migratory movements, and reductions in the potential productivity of salmon habitats (Lichatowich 1999; Montgomery 2003). To varying extents, the same factors influence nonanadromous species and stocks as well. Each topic is complex and has been the subject of extensive writing. Each factor may by itself contribute seriously to population decline, and the cumulative effects of all four have in many cases been catastrophic.