Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Adaptation of Salmonids to Spawning Habitats
Yolanda E. Morbey and Andrew P. Hendry
Salmonids (i.e., salmon, trout, and char in the subfamily Salmoninae) show a remarkable diversity in life history, morphology, and behavior (Ricker 1972; Groot and Margolis 1991; Fleming 1998; Hendry and Stearns 2004; Quinn 2005). Some of this diversity reflects adaptation to spawning habitats because these habitats vary dramatically in factors that affect reproductive success. This adaptation may take the form of overall suitability to the conditions experienced by a particular species, adaptive differences among populations within species, or even adaptive variation among individuals within populations. This present chapter reviews the evidence for these adaptations.
The life history and reproductive behavior of salmonids sets the stage for a consideration of the role of habitat. In general, the salmonid breeding system is characterized by competition among females for access to nest sites and among males for access to females (Fleming and Reynolds 2004). In most species and populations, spawning females select and prepare a nest site by digging a depression in the gravel. Oviposition then takes place, during which sperm from the attendant males fertilizes the eggs. Females then bury the fertilized eggs by covering their nest with gravel. Spawning usually takes place over several days to weeks for individuals, or several weeks to months for populations. In some species, primarily among the Pacific salmon Oncorhynchus spp., all individuals inevitably die after spawning (e.g., Finch 1990). In other species, some individuals live to spawn again in subsequent years (Fleming and Reynolds 2004). The fertilized eggs hatch after 1 to 8 months of incubation in the gravel, with the length of this period depending primarily on water temperature and species (Beacham and Murray 1990; Berg and Moen 1999). The newly hatched alevins (larvae with an attached yolk sac) then remain in the gravel for another 1 to 5 months, again depending on temperature and species. The development of eggs and alevins (collectively, embryos) relies almost exclusively on endogenous energy sources provided by the yolk (Berg et al. 2001). After emerging from the gravel, fry begin to obtain exogenous food.