Population trends and vital rates for Marbled Murrelet (Brachyramphus marmoratus) in the Pacific Northwest
Northwestern Naturalist 103(1):20-29
The Marbled Murrelet (Brachyramphus marmoratus) is a small seabird found on the western coast of North America, listed as federally threatened in the US. It has been in decline for a prolonged period over most of its range. To better understand this population trend, we analyzed relative abundance from at-sea survey transects of Marbled Murrelets throughout the California, Oregon, and Washington region, 2001–2017. We assessed the consistency of trends at different spatial extents, evidence of among-region movements, and consistency between long-term trends and published estimates of demographic parameters. Yearly changes in population survey estimates in Oregon and northern California versus Washington were negatively correlated, suggesting significant bird movement between the 2 regions. Observed declines in the north and gains in the south may be due to Marbled Murrelet movements rather than differing regional demographics. Annual fluctuations were, in some cases, greater than what observed at-sea recruitment rates would allow for the species, especially at the state and zone scales, and were more variable by year in smaller regions. Trends for smaller spatial extents did not align with the 3-state stable trend estimate, suggesting that larger survey regions may be more reliable. The lifespan/recruitment rate scenarios that would allow persistence (λ=1) were also evaluated. Recruitment levels of 0.06/y require a longevity of 16.7 y for population stability, whereas a 0.1/y recruitment only requires a 10-y lifespan. Better resolution of Marbled Murrelet movement and vital rates is needed to resolve the contradiction between demographic analyses that project population declines and the observed stability of Marbled Murrelets quantified through the 3-state at-sea survey transects.
Brachyramphus marmoratus, California, conservation, demography, endangered species, Marbled Murrelet, Northwest Forest Plan, Oregon, population trend, Washington