We analyzed the reproductive biology and demographics of the Lost River sucker Deltistes luxatus and shortnose sucker Chasmistes brevirostris, two endangered species endemic to the upper Klamath Basin of Oregon and California, from 1984-1997. Lost River suckers had distinct river and lake shoreline spawning stocks, and individuals of both species commonly spawned in consecutive years. In the Williamson River and lower Sprague River, spawning migration by both species occurred mainly during a 5-week period that started within the first three weeks of April and peaked between mid April and early May, although a separate, earlier (mid March) run of Lost River suckers may also spawn in the upper Sprague River. Migration of both species was several times higher at dawn (0500-0730 h) and evening (1800-2200 h) than other times of the day. Peak migrations almost always corresponded to peaks in water temperature, usually at 10-15°C. Lost River suckers were captured at springs along the east shore of the lake from late February through mid May, with peak spawning usually in mid March to mid April. Shortnose suckers were generally captured at the springs from late March through late May, but the time of peak spawning was not determined. Size and age at maturity was determined by recruitment from a strong year class (1991). Male Lost River suckers began recruitment into the adult population at age 4+ (375-475 mm). Substantial recruitment of females did not begin until age 7+ (510-560 mm). Male and female shortnose suckers began recruitment at age 4+, with the majority offish recruited by age 5+. Males recruited at 270-370 mm; females recruited at 325-425 mm. Fecundity estimates were quite variable ranging from 44,000-236,000 eggs per female Lost River sucker and 18,000-72,000 eggs per female shortnose sucker. In 1984 and 1985, the spawning populations of both species were dominated by large, old individuals, with little indication of recent adult recruitment. In the next 13 years, only one strong year class (1991) recruited into the spawning populations of both species. This year class temporarily boosted population numbers, but annual fish kills from 1995 to 1997 eliminated most adults of both species. Associated with poor water quality caused by the proliferation and decay of blue-green algae Aphanizomenonflos-aquae, these fish kills raise concern that alterations to the lake ecosystem over the past several decades have Perkins et al. Lost River and shortnose suckers 5 increased the magnitude and frequency of poor water quality. As a result, mortality rates of all life stages may have increased, thereby disrupting the species' life history pattern and potentially decreasing long-term population viability. Introduction The Lost River sucker Deltistes luxatus and shortnose sucker Chasmistes brevirostris are large, long-lived suckers endemic to the upper Klamath Basin of Oregon and California. Both species are typically lake dwelling but migrate to tributaries or shoreline springs to spawn (Moyle 1976, Scoppettone and Vinyard 1991). Once extremely abundant (Cope 1884, Gilbert 1898), both species have experienced severe population declines and were federally listed as endangered in 1988 (USFWS 1988). Much of the original habitat of these suckers has been destroyed or altered by conversion of lake areas to agriculture, dams, instream flow diversions, and water quality problems associated with timber harvest, loss of riparian vegetation, livestock grazing, and agricultural practices (USFWS 1988). Knowledge of the life history of Lost River and shortnose suckers is fundamental to recovery of these species. The objective of this report was to present the results of studies conducted from 1987-1998 on the reproductive biology and demographics of Lost River and shortnose suckers, and to compare these results with earlier unpublished data. Study Sites Studies were conducted on Upper Klamath Lake and the lower Williamson-Sprague river system (Figure 1). These waters form the upper portion of the Klamath River Basin in south-central Oregon and represent most remaining native habitat of Lost River and shortnose suckers. Upper Klamath Lake is a remnant of pluvial Lake Modoc that included eight major basins and encompassed 2,839 km2 (Dicken 1980). Today, Upper Klamath Lake serves as a storage reservoir that provides water for agricultural irrigation, waterfowl refuges, instream flow requirements of anadromous fish, and hydroelectric power generation. At full capacity, the lake covers approximately 360 km2 and has an average depth of 2.4 m. Most deeper water (3-12 m) is restricted to narrow trenches along the western shore. Lake elevation is controlled at the outlet by Link River