Summary

Description

We assessed the reproductive success of the California clapper rail Rallus longirostris obsoletus, an endangered species restricted to San Francisco Bay, and the relative importance of predation, flooding and contaminants as factors affecting that success. Our study was conducted in six tidal marshes in both the north and south reaches of San Francisco Bay. This assessment, conducted over a period of 4 years 1991, 1992, 1998, 1999, determined that fecundity of clapper rails was much reduced over the natural potential. Only 69 percent of clapper rail eggs whose viability could be assessed were viable. Rail egg hatchability in north and south bay marshes was 65 percent and 70 percent respectively. Only 45 percent of the nests successfully hatched at least one egg. Despite mean clutch sizes of 6.66 and 6.94 in the north and south bays respectively, clapper rails produced only 1.9 and 2.45 young per nesting attempt. Flooding was a minor factor in reducing fecundity, accounting for only 2.3 percent of eggs lost. Predation was a major factor in reducing nest success and accounted for onethird of the eggs lost. Failed clapper rail eggs were collected for embryo examination and chemical analysis of trace elements and organochlorines. Contamination appeared to exert an adverse influence over clapper rail reproductive success as evidenced by observations of deformities, embryo hemorrhaging, embryo malpositions, a depressed rate of hatchability, the exceedance of avian embryotoxic threshold concentrations for mercury, barium and chromium in some eggs, and the correlation of deformities with elevated concentrations of trace elements in failtohatch rail eggs. While all marshes had impaired hatchability, the marshes with the lowest rate of hatchability were those adjacent to potential contaminant sources. Mercury was the only significant contaminant common to all marshes. Mercury concentrations exceeded the LOAEC of 0.5 ugg on a fresh wet weight basis in at least some failed eggs in every marsh sampled. Mercury concentrations in failed clapper rail eggs in the south and north bays were 0.54 ugg and 0.36 ugg, respectively, and ranged from 0.17 to 2.52 ugg in the south and from 0.11 to 0.87 ugg in the north bay.