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

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.

Multi-state mark-recapture model to estimate survival of a dispersed-nesting seabird, the Kittlitz's Murrelet

“…The predicted annual survival probability for marbled murrelets from allometric and comparative methods was 0.835–0.845 (Beissinger , Beissinger and Peery ). These predictions were confirmed with field studies in California ( Ŝ = 0.868–0.896; Peery et al ) and British Columbia ( Ŝ = 0.829–0.929; Cam et al ) using similar mark–recapture methods as used in our study. For further comparison, at Triangle Island off the coast of British Columbia, apparent annual survival of rhinoceros auklets ( Cerorhinca monocerata ) was similar ( Ŝ = 0.86), whereas that of tufted puffins ( Fratercula cirrhata ) was markedly higher ( Ŝ = 0.91–0.96); in contrast, annual survival of Cassin's auklets ( Ptychoramphus aleuticus ) was lower ( Ŝ = 0.75–0.84), especially in years with extreme climate events ( Ŝ = 0.44 for females; Morrison et al ).…”

Timing and causes of mortality in the endangered Vancouver Island marmot (Marmota vancouverensis)

“…Estimating demographic rates for endangered species can be especially problematic because such species tend to be, by definition, rare. Recovery efforts on behalf of endangered species can also generate substantial public and political controversy, particularly when they involve consideration of socioeconomic or ethical issues in addition to biological ones (Cam et al 2003). The northern spotted owl (Strix occidentalis caurina (Xantus de Vesey, 1860)) and island gray fox (Urocyon littoralis (Baird, 1857)) are two well-known examples of endangered species that have generated controversy, in these cases, either through perceived economic costs to local resource-based communities (Franklin et al 2000) or by proposed control of naturally occurring predators (Courchamp et al 2003).…”

Full Issue

“…Significant population declines have been suspected in portions of Alaska since the early 1990s, including Glacier Bay and Icy Strait in southeast Alaska (Piatt et al 2007), but no population trend data are available for the Port Snettisham area. If reproduction in southeast Alaska is depressed compared with historical levels, as is suspected for murrelets nesting in California and British Columbia (Cam et al 2003), investigations into the causes of low reproductive success at Port Snettisham, as well as at other sites in this core breeding region, are needed to further elucidate the mechanisms that are limiting murrelet recruitment. …”

“…Sustained low recruitment, primarily from low reproductive rates, has been considered the key factor limiting population sizes of murrelets in the southern portion of their breeding range (Cam et al 2003. Suspected causes of low recruitment are mostly related to anthropogenic factors, such as increased rates of nest predation associated with habitat modification (Marzluff et al 2000, Malt andLank 2009), and reduced food availability and diet quality, possibly related to fishing practices and climate change (Becker and Beissinger 2006, Becker et al 2007, Ronconi and Burger 2008.…”