Research Document - 2013/118

Mortality patterns in St. Lawrence Estuary beluga (Delphinapterus leucas), inferred from the carcass recovery data, 1983-2012

By Véronique Lesage, Lena N. Measures, Arnaud Mosnier, Stéphane Lair, Robert Michaud and Pierre Béland

Abstract

A total of 469 beluga was found dead in the Estuary or Gulf of St. Lawrence between 1983 and 2012. The number of cases reported varied among years, with a median of 15 beluga annually. Beluga deaths were mainly reported in the St. Lawrence Estuary (SLE), between April and November, and peaked in May to August. Over the first 24 years of the study period, newborn deaths varied from 0 to 3 beluga per year, and followed a 3–4 year cycle. In 2008, this cycle changed to biennial peaks, and was accompanied by annual report rates 3 to 5 times higher than the maxima observed previously. Mortality patterns among adults followed no clear temporal trends over the study period. There was an equal probability of finding an adult male or an adult female regardless of season or year of study. Age distribution of collected carcasses followed a U-shaped distribution typical of long-lived mammals, with the highest mortalities in the first year of life, and among the older (35+ growth layer groups [GLGs]) age classes. The oldest individual had 72 GLGs. Age-at-death distributions were similar among adult males and females. A comparison of cumulative frequency distributions of age-at-death between the periods 1983–1999 and 2000–2012 indicates beluga died at a younger age in the latter period, and that deaths of adult females but not males were responsible for this pattern. Finally, the relationship between age and standard length of SLE beluga confirmed sexual dimorphism with males reaching an asymptotic standard length longer than females (416 cm vs 365 cm). SLE beluga are intermediate in size compared to other beluga populations.

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