Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Enhancing Salmonid Populations via Spawning Habitat Restorative Actions
Dudley W. Reiser
Freshwater controls on salmonid populations can extend across multiple geographic scales and borders and are manifest within all life stages, including adult upstream migration and holding, spawning and incubation, fry and juvenile rearing, smoltification in the case of anadromous salmonids, and out-migration (Quinn 2005). This book, as its title reveals, is focused on various elements of the spawning and incubation phase of the salmonid life cycle and includes chapters that describe the biological and physiological processes involved with spawning, the physical and hydraulic conditions that combine to produce suitable spawning habitats, and implications relative to population dynamics and density dependency when spawning habitats become fragmented or are inherently sparse. This chapter is centered around the realization that the quantity of spawning habitat in streams can be naturally limited or can become so from anthropogenic influences, thereby potentially affecting population intrinsic growth rates via compensatory or, in some cases, depensatory effects. The chapter focuses on addressing two important questions related to this: (1) what actions can be and have been done when situations of limited salmonid spawning habitat occur in streams, and (2) will such actions be successful in terms of increasing population viability? A case study is used to illustrate one type of spawning habitat enhancement project and to describe various metrics used to evaluate its success. Technical and logistical considerations that should be evaluated when considering spawning habitat restorative actions make up the remainder of the chapter. The chapter incorporates by direct reference and otherwise by inference various concepts and principles proffered in other chapters that relate to the subject at hand.
Constraints or limits in spawning habitat within streams are most commonly linked to issues of gravel quality and/or quantity, both of which have been cited as prominent factors that can limit salmonid production (Hobbs 1937; Benson 1953). In the context of quality, which is generally gauged by the percentage of fine sediments within the spawning gravel matrix, deleterious effects may include reduced egg-to-fry survival owing to increased sediment concentrations in spawning gravels that either reduce egg survival directly within the egg pocket (Moring and Lantz 1975; Tagart 1976, 1984; Reiser and White 1988) or prevent fry emergence via plugging of interstitial egress pores or formation of a sand seal (McCuddin 1977; Beschta and Jackson 1979; Tappel and Bjornn 1983; Young et al. 1991). Studies have repeatedly documented the deleterious effects of high sediment concentrations in salmonid spawning gravels on the quality of spawning habitat (Carling and Mc- Cahon 1987; Chapman 1988; Sear 1993; Waters 1995; Sear et al. 2008, this volume).