Linking Stream Carrying Capacity for Salmonids to Habitat Features

—Stream carrying capacity for anadromous salmonids that rear to the smolting stage in freshwater can be predicted from a sequence of cause-response functions that describe fish preferences for macro-habitat features. The channel unit (e.g., pool, glide, riffle) is a useful stratum for quantifying rearing capacity for salmonids, and is a hydrologically meaningful unit for predicting the response of stream morphology to watershed processes. Thus, channel units are the natural link between habitat-forming processes and habitat requirements of salmonids. Maximum densities of juvenile salmonids that can be supported in a channel unit are related to availability of preferred habitat features including velocity, depth, cover, and substrate. Within channel unit types, maximum densities of salmonid parr will shift predictably as availability of cover from wood and boulders increases. Within stream reaches, additional variation in maximum rearing densities can be accounted for by light penetration and nutrient load. As salmonids grow, their habitat preferences change and the preferred habitat associated with their increasing size becomes less and less available. Further, territory size of salmonids increases exponentially with fish length, such that the demand for territory to support surviving members of a cohort increases at least through their first year of life. Changing habitat preferences and space demands, juxtaposed against shrinking habitat availability with the onset of summer low flows often results in a bottleneck to rearing capacity for age >1 salmonids in wadable streams. Habitat measurements in Oregon streams indicate that depths preferred by steelhead (anadromous rainbow trout) Oncorhynchus mykiss become scarce as parr exceed 15 cm in length, which coincides with the approximate threshold length for steelhead smolts. We present a generalized framework, called the Unit Characteristic Method, for accumulating effects of these habitat factors at the channel unit and reach-level scales to estimate carrying capacity for rearing salmonids in a basin. Our subsequent chapter in this book presents a demonstration of how this method can be applied to predicting salmonid production in streams. 226 Cramer and Ackerman Introduction How many salmon or trout should a given stream be capable of sustaining, and how will human actions affect those production capabilities? This is an urgent and often debated question, especially when resource managers constrain harvest, choose a hatchery strategy, regulate land or water use, or propose habitat restoration. All strategies to manage human activities so as to sustain desirable fish populations share a need to understand the primary drivers of fish population trends. The traditional approach to determining carrying capacity for anadromous salmonids has been through stock–recruitment analysis (Ricker 1954; Beverton and Holt 1957). That approach arose from an era that focused on determining maximum sustainable yield (MSY) for harvest. However, traditional approaches to quantifying stock–recruitment relationships have proven to be imprecise, because they are often based on an inadequate range of population sizes (Walters 1997) and they incorporate variation in survival through both the freshwater and marine phases of life. In the present era of depleted salmonid stocks across much of North America, with a mandate under the Endangered Species Act (ESA) to design recovery plans for ESA-listed populations, we need habitat-based approaches for estimating salmonid stream production capacities to inform harvest and habitat decisions. Stock–recruitment analysis requires a long time-series of data that includes a wide range of run sizes, but such data are lacking for the great majority of salmonid-producing basins. Even when data are available, the approach usually leaves a large share of recruitment variation unexplained (Figure 1), and leads to wide confidence intervals on estimated parameters of the curve (Cramer 2000). Some of the most robust data sets available appear as a cloud of data points when scattered against one another, without a clear pattern to indicate the form of the stock–recruitment curve that should best fit them. Further, that approach is not helpful for identifying the specific habitat factors that are limiting the population, or for estimating the benefits from potential stream alterations in a small portion of the watershed. If stream features change, those changes will influence the stream’s capacity to produce salmonids. Field studies of salmonids and their habitats have rapidly expanded over the last decade, and provide opportunities to develop more accurate and utilitarian approaches for parameterizing the stock-recruit function of salmonid populations. Promising methods have emerged and are being refined to estimate carrying capacity and productivity directly from measures of stream habitat (e.g., Bartholow et al. 1997; Cramer and Ackerman 2009, this volume; Blair et al. 2009, this volume). An ideal approach to predicting carrying capacity and survival rates of salmonids based on habitat features in a stream would offer the advantages of easily available data, and the potential to predict fish benefits from proposed habitat restoration or protection strategies. A great challenge in determining the effects of land and water management on fish has been the inadequacy of efforts to quantify cause-effect relationships between watershed changes and changes in fish populations (Imhof et al. 1996). The key to quantifying this linkage is to first determine the specific stream features that substantially influence salmonid populations, and then use watershed process models to predict how those features will change due to watershed management actions. In this chapter, we synthesize empirical evidence to link stream carrying capacity for salmonids to habitat features, and we explore possible cause-response relationships that determine the life stage of salmonids for which suitable habitat is most limiting. We show that channel unit types provide reliable 227 Linking Stream Carrying Capacity for Salmonids to Habitat Features strata for predicting maximum rearing densities, and that these units can be used to link habitat-forming processes to stream carrying capacity for salmonids. In addition to the physical features of habitat, we also account for the influence of food supply on the capacity of streams to produce salmonids. We recognize that competition, predation, and water quality also influence carrying capacity, but these complex features are beyond the scope of this paper. High summer temperatures, for example, commonly restrict or reduce salmonid use from certain areas of basins where the habitat is otherwise suitable. The framework described here for determining habitat production potential was developed from studies in salmonid-producing streams, and therefore will produce best results when applied to stream reaches having the typical range of conditions for streams that consistently support salmonids. Importance of Stream Area At the broadest scale, the size of a basin constrains its capacity to produce salmonids. Correlations have been demonstrated between several measures of basin size and the run sizes of anadromous salmonids it produces (Figure 2). These correlations clearly indicate that salmonid production is a function of stream area or volume, but more detail 0 2,000 4,000 6,000 8,000 10,000 0 3,000 6,000 9,000 12,000 15,000 Spawners R ec ru it s R = 0.72 (A) North Umpqua Winter Steelhead (1974-2002) 0 200 400 600 800 1,000 1,200 0 100 200 300 400 500 600 700 Spawners R ec ru it s R = 0.64 (B) Oregon Coast Coho (1950-1977) 0 20 40 60 80 100 120 140