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I I I I I I I I I I I I I I I I I I I BIOLOGICAL OPINION Status of the Species apparently recent invaders (since 1900) to.the northern portions of the LCR Valley. Crayfish were introduced for use as a fish bait about 1934 and may be a limiting factor determining rail occurrence today. Population Estimates In 1987, the Yuma clapper rail population size in the LCR Valley was estimated to be between 400 and 750 individuals in the United States, with 450-970 in Mexico (Eddleman 1989). In 1994, the last year a full river-wide survey was taken, the population on the Colorado River in the United States was estimated to be 1,145 individuals. This figure is based on call-counts taken between late April and mid-June, the period of peak responses to taped calls (Todd 1986, Bennett and Ohmart 1978, Montgomery 1987). The percentage of breeding birds responding to taped vocalizations has been estimated at 70-80% for paired birds and 90-95 % for unpaired rails (Smith 1975, Bennett and Ohmart 1978). These estimates are based on knowing the locations of all vocalizing birds from repeated visits to study marshes, then determining the percentage of the birds that responded during call-count surveys, but see Eddleman (1989). Table 8 shows the total number of responses to taped calls from 1990 to 1995. Based on the annual call count survey taken on the LCR, the Yuma clapper rail population appears to be stable. Overall associated habitat trends have been downward, however. Rangewide Present Status Population numbers of the Yuma clapper rail appear to be stable. However, the habitat throughout the species' range is not secure. Due to daily and seasonal fluctuations in river flow, breeding habitat for the Yuma clapper rail cannot establish permanently along the main channel. Many of the backwaters have also undergone successional changes or were adversely impacted by siltation or channel degradation due to the high flows of 1983 and 1984. These backwaters no longer provide habitat for the rail. However, the Yuma clapper rail appears to expand into desired habita t when it becomes available. This is evidenced by the colonization of the California Department of Fish and Game Finne-Ramer habitat management unit in Southern California. This unit was modified to provide marsh habitat specifically for Yuma clapper rail. A substantial resident population exists there. 70