Research Document - 2012/168
Population modeling and harvest advice under the precautionary approach for eastern Hudson Bay beluga (Delphinapterus leucas)
By T. Doniol-Valcroze, J.-F. Gosselin, and M.O. Hammill
Abstract
Subsistence harvest of beluga whales by Nunavik communities is directed towards a mixture of several stocks, including the depleted Eastern Hudson Bay stock (EHB). The 2012 reported harvest consisted of 13 beluga taken in eastern Hudson Bay, 12 in Ungava Bay, 208 in Hudson Strait in the spring and 56 in the fall, 61 near Sanikiluaq (Nunavut), and 11 in the Long Island/James Bay area.
We incorporated recently updated information on stock structure and the results of the 2011 aerial surveys into a population model. Genetic variation at mtDNA loci was used to assess the contribution of each summering stock to the harvest and how these contributions vary spatially and seasonally. The model was fitted to survey estimates using Bayesian methods. The estimated stock size in 1985 was 3,799 animals with a 95% CI of 2,389–6,332. The lowest abundance point was estimated for the year 2001 at 3,016 individuals (95% CI 2,141–4,322), with a 2012 abundance of 3,229 individuals (95% CI 1,896–5,406). At current harvest levels, the stock abundance seems to have increased slightly over the last few years The model estimated struck-and-loss at 41% (95% CI 12–78%) and growth rate at 2.74% (95% CI -0.67 to +6.13%).
According to the model, removing 62 EHB animals per year for 10 years would have a 50% probability of causing a decline in the stock relative to its 2012 estimate. Limiting the harvest of EHB animals to 28 animals would reduce the probability of decline to 25%. Conversely, a harvest of 106 EHB beluga would have a 75% probability of leading to stock decline. In the absence of harvest, the probability of decline is 9%. A spring/summer harvest in Hudson Strait, with no harvest in the eastern Hudson Bay arc would have the lowest impact on the EHB stock, followed by a fall harvest in Hudson Strait only, again with no harvest allowed in the arc. If harvesting does occur in eastern Hudson Bay, then numbers taken in Hudson Strait must be reduced to obtain the same probability of increase, but the size of this reduction will depend on whether hunting occurs in the spring/summer or in the fall.
The model was used to estimate the probability of reaching a recovery target of 70% of the inferred maximum stock size under a precautionary approach framework. Projections over the next 25 and 50 years show that, at current harvest levels (~50 EHB beluga per year), there is a 33% probability of reaching a recovery target of 5,600 individuals after 25 years. After 50 years, this probability increases to 48%. In the absence of harvest, the probabilities of reaching the target are 58% after 25 years and 78% after 50 years. However, uncertainty about the historical stock size, current carrying capacity and density-dependent mechanisms place important limitations on our ability to make long-term predictions regarding the recovery of the EHB stock.
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