Pacific Brant Age Ratios

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Pacific brant are an Arctic-breeding sea goose that stage and feed on seagrasses during the non-breeding season in coastal areas of Alaska.  Brant are an important subsistence and sport harvest species and the focus of several population surveys by state and federal agencies.  Each fall the entire population stages during migration in Izembek Lagoon, Alaska, presenting a unique opportunity to survey the proportion of young and derive and index of annual productivity.

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Pacific Brant family in Alaska

Brant family on the breeding grounds prior to migrating to Izembek Lagoon, Alaska.  
(Credit: Jeff Wasley, U.S. Geological Survey. Public domain.)

Since 1963, the USGS and U.S. Fish and Wildlife Service have conducted surveys to record the number of juvenile (i.e., first-year birds) and adult (i.e., birds ≥ 2 years old) Pacific brant (Branta bernicla nigricans) and estimate the age ratio (number of juvenile birds compared to total birds aged) for the overall population.  This fall age ratio estimate is the only measure of annual productivity for the entire population and a potential index of recruitment (i.e., number of young produced per female) given that most first-year mortality in Pacific brant occurs between hatch and migration to Izembek. USGS recently published model-based age ratio estimates through 2015 suggesting a slow decline in brant productivity (Ward et al. 2018).  However, age ratios have improved in recent years and although they vary widely, general trends appear stable.

To provide up-to-date information for managers, USGS will publish age ratio data  and model-based age ratio estimates annually on this site - data are currently available through 2019. 

Data

Data Download [87KB] | Metadata

Citation: Amundson, C. L., D. H. Ward, and P. J. Fitzmorris. 2020. Data and model-based estimates from Black brant (Branta bernicla nigricans) fall age ratio surveys at Izembek Lagoon, Alaska. U. S. Geological Survey data release, https://doi.org/10.5066/P9QIJIU2.

Model-based estimates of Pacific brant age ratios in Izembek Lagoon, AK

Model-based estimates of Pacific brant age ratios (juveniles:adults) in Izembek Lagoon, Alaska 1963-2019.
http://rpubs.com/camundson/557436
(Credit: Courtney Amundson, USGS. Public domain.)