An Emerging Pattern of Declining Growth Rates in Belugas of the Beaufort Sea: 1989–2008
Keywords: Beaufort Sea beluga, Mackenzie Delta, Paulatuk, harvest, length, GLG, size-at-age, growth, blubber thickness
AbstractStandardized, hunter-based sampling of harvested beluga whales was initiated in the three main harvesting areas of the Mackenzie River delta (Shallow Bay, Kendall Island, Kugmallit Bay) in 1980 and near Paulatuk, Northwest Territories, in 1989. Standard length and sex of landed whales have been recorded for 90 to 110 belugas per year since 1980, and ages have been determined since 1988. Hunters select larger and older belugas; males outnumbered females 2.0 to 1 between 1980 and 1989, 3.0 to 1 between 1990 and 1999, and 3.6 to 1 between 2000 and 2009. Age classes younger than 10 growth layer groups (GLGs) were essentially absent from harvests both in the Delta and near Paulatuk. Asymptotic lengths, calculated using GLG counts from teeth of 839 males and 225 females sampled over 16 seasons (1989, 1993 – 2008), were 435.3 cm (SE 2.0) for males and 380.1 cm (SE 1.8) for females. Males were 14.5% longer than females. Belugas landed in the Delta and Paulatuk were mainly mature adults (by hunter selection). Mature belugas landed by Paulatuk hunters were younger and shorter than those taken in the Delta because of age and sex segregation in the hunting areas. There was no difference in the size-at-age relationships for belugas landed at Paulatuk vs the Delta. The linear trend in size-at-age indicated a decline of 0.08% (SE 0.038%) per year, or 1.75% over the 19 year time series. The 314 male belugas landed in the Delta between 2000 and 2007 showed statistically significant differences in blubber thickness among years, but only a slight downward trend over this short period. Mean blubber was thickest in 2002 and 2003 and thinnest in 2005. The thinness of belugas in 2005, and subtle changes in growth of the belugas over the time series, may reflect ecosystem changes that have reduced the availability or quality and quantity of their prey in recent years. Further research, including isotope and fatty acid profiling, would be helpful in substantiating the declining trend in growth and elucidating the causative factors.