Demographic Assessment of a Marbled Murrelet Population from Capture‐Recapture Data

Abstract

Abstract: Population size is a fundamental variable of interest in most conservation programs, as are the demographic processes underlying variation in numbers. The Marbled Murrelet ( Brachyramphus marmoratus ) is a seabird nesting in coastal old‐growth forest in North America and is at the heart of a controversy concerning forest management. Despite the urgent need for a careful assessment of the demographic state of populations, the species is so secretive that its life history is poorly known and reliable estimates of the required vital rates are rare or missing. Here we present the first survival estimates obtained through capture‐recapture data from a population in British Columbia, Canada. This population is in a site that has been heavily logged in the last 50 years and continues to be logged. Survival estimates ( 0.8289 and 0.9289 based on different samples corresponding to two capture techniques ) fell within the range specified for small alcids, indicating that neither mortality nor emigration out of the study area is particularly high in the population. We used capture‐recapture data to estimate the realized population growth rate ( λ ) from 1991 to 2000. λ^ was < 1 ( 0.9851 ), but the confidence interval was large and included 1 ( stationary population ). This estimate was obtained using part of the data ( the sample with the higher survival rate ) to meet the assumptions of the analytical approach. This lambda value might not reflect the growth rate of the overall population but may be specific to the segment of the population that successfully bred. The hypothetical value of λ corresponding to a survival probability of 0.8289 was 0.8593, which suggests that the population may be declining if the local survival rate based on the combined samples is closer to the true value. Because of our low recapture rates and poor precision of estimates, there is still uncertainty regarding the health of this population. Development of field techniques and achievement of higher detection probability should reduce uncertainty in future demographic studies of Marbled Murrelets and allow refined assessment of the conservation status of populations.