Research Document - 2013/127

An age-structured Bayesian population model for St. Lawrence Estuary beluga (Delphinapterus leucas)

By Arnaud Mosnier, Thomas Doniol-Valcroze, Jean-François Gosselin, Véronique Lesage, Lena Measures, Mike Hammill


The St. Lawrence Estuary (SLE) beluga population was depleted by intensive hunting and declined from 5000-10000 individuals at the end of the 1800’s to a few hundreds in 1979 when it was afforded protection by the Fisheries Act. The low abundance estimate obtained from the photographic survey conducted in 2009 and the high mortalities observed in 2008, 2010 and 2012 raised concerns as to the status of SLE beluga. An age-structured hierarchical Bayesian model was used to describe the population dynamics of SLE beluga and trends in population abundance. The model included information on population abundance and proportion of young (0-1 year-old calves) obtained from 8 photographic aerial surveys flown between 1988 and 2009, and mortalities of newborns and older individuals documented by a carcass monitoring program maintained from 1983 to 2012. Results suggest that the population was stable or slightly increasing from the end of the 1960s until the early 2000s when it numbered approximately 1000 beluga. The population then started to decline to 889 individuals (95%CI 672-1167) in 2012. To explain this decline, the model suggests important changes in population dynamic parameters and age structure, moving from a stable period (1984-1998) characterized by a 3-year calving cycle, a population composed of 42% of immature individuals and 7.5% newborns, to an unstable situation (1999-2012) showing a 2-year calving cycle, and a declining proportion of newborns and immatures (respectively 6% and 33% in 2012), associated with a high newborn mortality. Independent abundance indices and observations of year-to-year variation in calf production and age-structure show similar trends to those predicted by the model, thus increasing our confidence in its conclusions. The lack of recovery, high overall adult mortality (6%) and highly variable newborn survival could indicate a population limited by resources in its environment, and thus particularly sensitive to climate variability and cascading effects in the ecosystem.

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