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L' INTRODUCTION Results of a study of selected aspects of the life history and behavior of the northern squawfish (Ptychocheilus oregonensis) are sum- marized in this report. This project was initiated as part of a contract entered into in April 1980, between the Idaho Cooperative Fishery Research Unit, University of Idaho, and the U.S. Fish and Wildlife Service, Colorado River Fisheries Project. Study objectives were chosen to supplement areas on northern squawfish life history and behavior where information was found to be lacking in a thorough review of the literature. Problems considered are analogous to those of Colorado squawfish which might be useful in explaining past and present distribution patterns and in preventing further declines in the range and abundance of that species. Four topic areas are considered in this report: spawning biology, habitat selec- tion, early life history, and laboratory behavioral comparisons. STUDY AREA . The St. Joe River, in the northern panhandle region of Idaho (Figure 1), was selected for this study because it contains a large reproducing population of northern squawfish in a river environment. It has the additional advantages of easy accessibility, water clear enough to allow direct observations of fish, a size small enough to work in without great difficulty, and a large body of information on its squawfish population. A forested watershed of approximately 1,900 square miles on the west slope of the Bitterroot Mountains is drained by the St. Joe River before it empties into the south end of Lake Coeur d'Alene at an elevation of 2,128 feet. Mean annual air temperature in the region is 8.5° C and annual precipitation averages 77.3 cm (Falter 1969). Mean annual flow at Calder is 2,364 cfs <USGS 1980). Flow regimes are characterized by a large spring peak in April and May and a smaller peak in November and December. Low flow for the 1980 water year occurred in November at 154 cfs and a high of 12,400 cfs was reached in April (USGS 1980). Large winter flood events are also common. A major tributary, the St. Maries River, enters the St. Joe near river mile 16 at a mean annual flow of 352 cfs. Closure of the Post Falls Dam at the outlet of Lake Coeur d'Alene in 1906 raised the lake level and backed water into the lower 34 miles of the St. Joe River. This section, referred to as slackwater, consists of smooth-surfaced water flowing over a coarse silt and sand bottom (Falter 1969) at an average depth of 9.0 meters and an average width of 80 meters (Davis 1961) in a series of meanders through a broad, flat • valley. Above the slackwater, in the fastwater, the gradient increases and riffles alternate with long pools. The substrate consists of well washed rubble and the maximum depth is 10 meters. The river valley in the lower reach of the fastwater remains fairly broad; meanders and 47