Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Hierarchical Physical Controls on Salmonid Spawning Location and Timing
Tim Beechie, Hamish Moir, and George Pess
In order for salmonid populations to persist in specific spawning reaches, adult fish must arrive healthy at their spawning grounds, adults must be able to dig a redd, and the eggs must not be scoured or suffocated during incubation. Failure at any of these stages means that adults will not successfully reproduce and the population will not survive in that location. Over hundreds to thousands of years, salmonids have adapted to specific spawning site characteristics and locations, leading to spatial and temporal segregation of spawning areas or timing among species and races (Taylor 1991). For example, smaller-bodied salmonids generally spawn in smaller gravels and have shallower redds, whereas larger fish can dig deeper redds in larger gravels (Crisp and Carling 1989; Crisp 2000; Quinn 2005; Morbey and Hendry 2008, this volume). Such adaptations impose a first order spatial pattern on spawning salmonids because each species can only occupy locations where suitable habitats are available, and the distribution of gravel sizes and scour depths in river networks is controlled by geomorphic processes (e.g., Montgomery et al. 1999). Superimposed on these broad patterns are smaller scale patterns driven by either by local physical factors or by biological factors, leading to localized spawning within generally suitable reaches (Duker 1981; Essington et al. 1998; Torgerson et al. 1999; Baxter and Hauer 2000; Geist 2000). Temporal patterns are also apparent, as some species have overlapping spawning site preferences but segregate in time (e.g., spring spawning steelhead Oncorhynchus mykiss use the same reaches as fall spawning coho salmon O. kisutch in western North America).