We reviewed mathematical models that predict standing crop of stream fish (number or biomass per unit area or length of stream) from measurable habitat variables and classified them by the types of independent habitat variables found significant, by mathematical structure, and by model quality. Habitat variables were of three types and were measured on different scales in relation to stream channels: variables of drainage basins were measured on the coarsest scale from topographic maps; channel-morphometry and flow variables were measured in the field along transects perpendicular to flow; and habitat-structure, biological, physical, and chemical variables were measured on the finest scale in the field. We grouped the 99 reviewed models by the types of independent variables found significant during model development: (A) primarily drainage basin (5 models), (8) primarily channel morphometry and flow (16 models), (C) primarily habitat structure, biological, physical, and chemical (25 models), (D) a combination of several types of variables (39 models), and (E) tests of weighted usable area as a habitat model (14 models. Most models were linear or multiple linear regressions, or correlations, but a few were curvilinear functions (exponential or power). Some used multivariate techniques (principal components or factor analysis), and some combined independent variables into one or more indices. We judged model quality based on simple criteria of precision and generality: coefficient of determination, sample size, and degrees of freedom. Most models were based on data sets of fewer than 20 observations and, thus, also had fewer than 20 degrees of freedom. Most models with coefficients of determination of greater than 0.75 had fewer than 20 degrees of freedom, which led us to conclude that relatively precise models often lacked generality. We found that sound statistical procedures were often overlooked or were minimized during development of many models. Frequent problems were too small a sample size, possible bias caused by error in measuring habitat variables, using poor methods for choosing the best model, not testing models, using models based on observational data to predict standing crop, and making unrealistic assumptions about capture probabilities when estimating standing crop. The major biological assumptionthat the fish population was limited by habitat rather than fishing mortality, interspecific competition, or predationusually was not addressed. We found five main ways stream-fish-habitat models are used in fishery management. To be useful for analyzing land management alternatives, models must include variables affected by management and be specific for a homogeneous area of land.

The symposium "Pacific Salmon and Their Ecosystems: Status and Future Options',' and this book resulted from initial efforts in 1992 by Robert J. Naiman and Deanna J. Stouder to examine the problem of declining Pacific salmon (Oncorhynchus spp.). Our primary goal was to determine informational gaps. As we explored different scientific sources, state, provincial, and federal agencies, as well as non-profit and fishing organizations, we found that the information existed but was not being communicated across institutional and organizational boundaries. At this juncture, we decided to create a steering committee and plan a symposium to bring together researchers, managers, and resource users. The steering committee consisted of members from state and federal agencies, non-profit organizations, and private industry (see Acknowledgments for names and affiliations). In February 1993, we met at the University of Washington in Seattle to begin planning the symposium. The steering committee spent the next four months developing the conceptual framework for the symposium and the subsequent book. Our objectives were to accomplish the following: (1) assess changes in anadromous Pacific Northwest salmonid populations, (2) examine factors responsible for those changes, and (3) identify options available to society to restore Pacific salmon in the Northwest. The symposium on Pacific Salmon was held in Seattle, Washington, January 10-12, 1994. Four hundred and thirty-five people listened to oral presentations and examined more than forty posters over two and a half days. We made a deliberate attempt to draw in speakers and attendees from outside the Pacific Northwest.

In the context of freshwater fisheries changing their strategies from the regulation of harvest and the enhancement of populations, to the creation and protection of habitats and the management of ecosystems, moves toward establishing an aquatic habitat classification system. Eight papers, from the February 1988 Symposium on the Classification and Inventory of Great Lakes Aquatic Habitats (the last in a series of Great Lakes Symposia), propose various classification approaches, most using a limited number of physical, chemical, and/or biological variables to produce some form of index. They also include overviews and summaries of the classification process.