Marbled Murrelet (Brachyramphus Marmoratus) Population Trends and the California Current WP-24-04
EXECUTIVE SUMMARY
Population trend is a key metric used in wildlife management and conservation. Trend can be estimated by regression, but there are various models that can be fit. It is important to explore different regressions to ensure the most appropriate model is used to support management actions or ecological understanding. The marbled murrelet (Brachyramphus marmoratus) is a federally protected seabird species that occurs in the Pacific Northwest of North America. To better understand population trends of this declining species, multiple trend models were fit to 2001-2020 at-sea survey data for marbled murrelet in California, Oregon, and Washington. The statistical properties and plausibility of each model were compared. Linear, exponential, and Monte Carlo exponential models all indicated that a flat trend (λ close to zero) existed. A quadratic model had a plausible fit which may reflect the pattern of fluctuating ocean conditions. A segmented model had a good fit but had issues of biological plausibility. The best model overall was a sinusoidal model with a linear term, which also may correlate with ocean conditions. We show that conditions in the California Current are likely related to murrelet productivity. The linear and exponential models have utility for regulatory purposes, whereas the other models may suggest mechanisms regulating the population. We concluded that trend can be summarized in multiple ways, but a thorough analysis, perhaps using multiple distributions, is necessary to avoid incorrect conclusions.
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AUTHORED BY
Craig Loehle, Principal Scientist; Jake Verschuyl, Director of Forestry Research, Western U.S. & B.C.
Keywords
Brachyramphus marmoratus, conservation, demography, endangered species, marbled murrelet, California Current, Oregon, population trend, Washington, California
For more information, contact gro.isacn@ofni.