A survey of the fish assemblages between fiver kilometer 283 and 2 of the mainstem Willamette River, Oregon, was conducted in 1983 to evaluate the effects of improved water quality on longitudinal changes in fish assemblages and the usefulness of two indices of fish assemblage quality (index of well being and index of biotic integrity). Physical and chemical habitat quality and fish assemblage quality showed gradual, similar, and expected declines from the upper to the lower river, with only small changes near large point sources of pollution. More fish species, more species intolerant of poor habitat quality, and fewer species tolerant of poor habitat occurred in 1983 than in 1945. Stream order was not a predictor of fish assemblage patterns. A modification of the index of biotic integrity appeared to reflect changes in fish assemblage patterns and habitat quality better than the index of well being. A logical river classification is needed to study and manage 1otic ecosystems efficiently and to organize what we know of them. This classification should provide an improved perspective for thinking about rivers and serve as a guide for understanding relationships among sections of a river, among rivers, and between rivers and their watersheds. Since the 1950s, stream order (Strahler 1957) has been used as a framework for organizing information about 1otic processes and distribution patterns oflotic organisms (Kuehne 1962; Lotrich 1973; Vannote et al. 1980). It has been especially useful for explaining the patterns of fish distribution and diversity in small streams of the eastern and central United States (Kuehne 1962; Harrel et al. 1967; Whiteside and McNatt 1972; Lotrich 1973; Fausch et al. 1984). A formal model of fish assemblage-stream order relationships suggests that the assemblages should change most abruptly at or near places where stream order changes (Lotrich 1973). A corollary of that model is that only subtle changes occur within a single order. The prevailing model more closely resembles that of Fausch et al. (1984), who suggested that assemblages change gradually with order. A third model suggests that fish assemblages change abruptly or gradually because of abrupt or gradual changes in physicochemical habitat (Matthews 1986). Two indices of fish assemblage quality have been proposed. The index of well being (IWB) incorporates two diversity and two abundance estimates with approximately equal weight (Gammon 1976, 1980). The composite value reflects fish assemblage quality more realistically than a single estimate of species diversity or abundance. The index of biotic integrity (IBI) aggregates six speciescomposition metrics, three trophic-composition metrics, and three fish-condition metrics (Karr 1981). Scoring criteria for each metric are based on data from high-quality fish assemblages. Both the IWB and the IBI were developed and tested on fish assemblages in the Mississippi River drainage. Their applicability to the depauperate (in terms of species and families) ichthyofauna of the Columbia River...

The index of biotic integrity (IBI) integrates 12 measures of stream fish assemblages for assessing water resource quality. Initially developed and tested in the Midwest, the IBI recently was adapted for use in western Oregon, northeastern Colorado, New England, the Appalachians of West Virginia and Virginia, and northern California. The concept also was extended to Louisiana estuaries. In regions of low species richness, the IBI proved difficult to apply and often required extensive modification. Adapting the 1BI to those regions required that metrics be replaced, deleted, or added to accommodate regional differences in fish distribution and assemblage structure and function. Frequently replaced metrics include: proportion of individuals as green sunfish (Lepomis cyanellus), proportion of individuals as insectivorous cyprinids, proportion of individuals as hybrids, and number and identity of sunfish and darter species. The proportion of individuals as top carnivore metric was often deleted. Metrics added include total fish biomass and the number and identity of minnow species. These modifications generally followed the original IBI concept and its theoretical underpinnings. Problems remain in establishing tolerance rankings and scoring criteria, and adjusting scoring criteria for gradient differences in streams of similar size. The IBI holds promise for direct biological monitoring because of its strong ecological foundation and flexibility. Vermont, Tennessee Valley Authority, Ohio, Kentucky, and Illinois have incorporated the IBI into their monitoring or standards programs. The IBI thus serves as a quantitative, biological goal for water resource management.

1. We measured temporal variability of the fish assemblage in the Wabash River using a 25-year electrofishing dataset through a combination of a time-lag analysis and multivariate analysis. 2. The fish assemblage had substantial year-to-year variation, but the overall abundance pattern did not change. Based on time-lag analyses and ordinations, there was only weak predictable directional change at individual sites. At the scale of the entire reach, however, the overall combined data had a statistically significant directionality with the time-lag analysis. 3. These results support the hypothesis that the middle Wabash River stream fish assemblage has undergone change over a time period of 25 years, although at the scale of individual sites this change was not detectable.

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As the intensity and speed of environmental change increase at both local and global scales it is imperative that we gain a better understanding of the ecological implications of community shifts. While there has been substantial progress toward understanding the drivers and subsequent responses of community change (e.g. lake trophic state), the ecological impacts of food web changes are far less understood. We analyzed Wabash River fish assemblage data collected from 1974-2008, to evaluate temporal variation in body-size structure and functional group composition. Two parameters derived from annual community size-spectra were our major response variables: (1) the regression slope is an index of ecological efficiency and predator-prey biomass ratios, and (2) spectral elevation (regression midpoint height) is a proxy for food web capacity. We detected a large assemblage shift, over at least a seven year period, defined by dramatic changes in abundance (measured as catch-per-unit-effort) of the dominant functional feeding groups among two time periods; from an assemblage dominated by planktivore-omnivores to benthic invertivores. There was a concurrent increase in ecological efficiency (slopes increased over time) following the shift associated with an increase in large-bodied low trophic level fish. Food web capacity remained relatively stable with no clear temporal trends. Thus, increased ecological efficiency occurred simultaneous to a compensatory response that shifted biomass among functional feeding groups.