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The Effects of a Dam on Breeding Habitat and Egg Survival of the Foothill Yellow-legged Frog (Rana boylii) in Northwestern California

Although the debate continues on the relative roles of natural fluctuations and anthropogenic influences in recently observed amphibian population declines (Blaustein 1994; McCoy 1994; Pechman et al. 1991; Pechman and Wilbur 1994; Travis 1994), many species are clearly demonstrating local extinctions and largescale declines (Blaustein and Wake 1990; Blaustein et al. 1994b; Fellers and Drost 1993; Hayes and Jennings 1986; Jennings 1988). Causes of these declines can be placed in two broad categories: direct habitat degradation (Blaustein et al. 1994b), and more nebulous large-scale environmental changes and contamination (e.g., airborne pollutants, acid precipitation, ultraviolet radiation) (Blaustein et al. 1994a; Dunson and Wyman 1992; Fellers and Drost 1993; Hagstrom 1977; Harte and Hoffman 1989). Dams and water diversion projects have been proposed as causes of declines of many native frogs (Hayes and Jennings 1986; Jennings 1988;Moyle 1973), but the effects of these projects have not been quantitatively evaluated. Water diversions also have been implicated in region-wide declines of anadromous fish in the Pacific Northwest (Moyle and Williams 1990; Nehlsen et al. 1991) and are documented to adversely affect riverine macroinvertebrates (Petts 1984). Impacts to fish include loss and degradation of habitat because of reservoir filling and downstream changes in river morphology along with detrimental off-season andfluctuatingwaterreleases(BurtandMundie 1986; Petts 1984). A recent petition to list the arroyo southwestern toad (Bufo microscaphus califomicus) as endangered indicated that impoundments associated with dam construction have caused a 40% loss in the original habitat of this toad. In addition, it was suggested that habitat quality downstream of dams has been severely compromised by changes hi water temperatures, riparian vegetation, and habitat stability (Federal Register 1993). Our study provides information on both direct and indirect effects of a dam on the foothill yellow-legged frog (Rana boylii) in a northwestern California river system. Rana boylii is generally found in association with stream riffles that have rocky substrates and partly shaded banks (Hayes and Jennings 1988; Moyle 1973; Zweifel 1955). This frog ranges from northern Baja California, Mexico (Loomis 1965) to central Oregon, west of the Sierra Nevada and Cascade crests. Rana boylii is currently listed as a Species of Special Concern in California (Laudenslayer et al. 1991) and is also a candidate for Federal listing (Federal Register 1994). The most notable declines are in southern California and the west slope drainages of the Sierra Nevada and southern Cascade Mountains (Jennings and Hayes 1994; Leonard et al. 1993). Our objectives were: (1) to describe the changes in Rana boylii breeding habitat in a dammed river prior to and some years following dam construction, and (2) to document the effects of the timing of water releases from the dam on the reproductive ecology of this population. Our study took place on the main stem of the Trinity River, in Trinity County, California, USA, and covered the 63 km of river from Lewiston Dam (near Lewiston, California) downstream to the confluence with the North Fork Trinity, near Helena, California (elevation 420-550 m). Construction of two dams, an upper (Trinity) and a lower (Lewiston), was completed in 1963. At Helena, this river drains an area of 2968 km. Before dam construction the river had an average yearly (water year, Oct-Sept) discharge of 1.2 million acre-feet at Lewiston. During the four years of our study, the yearly discharge ranged from 270,800 to 367,600 acre-feet at Lewiston. The basin surrounding our study reach is 52% public land (USDA Forest Service and USDI Bureau of Land Management) and 48% privately owned. Land use activities within this watershed include logging and associated road building, recreation, and widely scattered home construction. Portions of river channel and adjacent areas were mined in the early to mid-1900's. Since 1984, the USDI Fish and Wildlife Service, in cooperation with the USDI Bureau of Reclamation (the dam administrator), has been conducting a flow evaluation study on the main stem of the Trinity River. Water has been released at varying levels above base flow (8.5 m/sec [cms]), each spring in order to facilitate migration of wild and hatchery-raised anadromous fish and to permit evaluation of fish habitat availability and changes as a function of discharge. The timing of these high flow releases has been influenced by many factors, though efforts have been made to mimic historical flow release patterns (Trinity River Restoration Program 1994).