Ecosystem regime shifts have not affected growth and survivorship of eastern Beaufort Sea belugas

Abstract

Large-scale ocean-atmosphere physical dynamics can have profound impacts on the structure and organization of marine ecosystems. These changes have been termed "regime shifts", and five different episodes have been detected in the North Pacific Ocean, with concurrent changes also occurring in the Bering and Beaufort Seas. Belugas from the Eastern Beaufort Sea (EBS) use the Bering Sea during winter and the Beaufort Sea during summer, yet the potential effects of regime shifts on belugas have not been assessed. We investigated whether body size and survivorship of EBS belugas harvested in the Mackenzie River delta region between 1993 and 2003 have been affected by previous purported regime shifts in the North Pacific. Residuals from the relationship between body length and age were calculated and compared among belugas born between 1932 and 1989. Residual body size was not significantly related to birth year for any regime, nor to the age group individuals belonged to during any regime. The percentage deviation in number of belugas born in any given year that survived to be included in the hunt (survivorship) did not show any significant trend within or between regimes. Accounting for lags of 1-5 years did not reveal any evidence of delayed effects. Furthermore, neither population index was significantly related to changes in major climatic variables that precede regime shifts. Our results suggest that EBS beluga body size and survivorship have not been affected by the major regime shifts of the North Pacific and the adjacent Bering and Beaufort Seas. EBS belugas may have been able to modify their diet without compromising their growth and survivorship. Diet and reproductive analyses over large and small time scales can help understand the mechanisms enabling belugas to avoid significant growth and reproductive effects of past regime shifts.