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Population Trends of the Marbled Murrelet Projected From Demographic Analyses

A demographic model of the Marbled Murrelet is devel- oped to explore likely population trends and factors influencing them. The model was structured to use field data on juvenile ratios, collected near the end of the breeding season and corrected for date of census, to estimate fecundity. Survivorship was estimated for the murrelet based on comparative analyses of allometric relation- ships from 10 species of alcids. Juvenile ratios were generally low, and were higher for counts made from shore or in kelp beds (typically 10 percent) than conducted offshore (<5 percent). An- nual survivorship was strongly related to body size in alcids. Survival for the Marbled Murrelet was predicted to be 0.845 and range to 0.90. Lambda, the expected annual growth rate of the population, was estimated for likely combinations of fecundity and survival, and indicated that under all combinations murrelet popu- lations are expected to be declining. Based on the best data, rates of decline are predicted to be 4-6 percent per year, but the rate of decline could conceivably be twice as large. Studies in Alaska and British Columbia suggest population declines at 3-5 percent per year, supporting model predictions. Results are discussed in rela- tion to the factors affecting murrelet population growth, and the use of juvenile ratios for monitoring murrelet populations.