Viability of a Small, Geographically-Isolated Population of Beluga Whales: Effects of Hunting, Predation, and Mortality Events in Cook Inlet, Alaska

A genetically distinct population of beluga whales (Delphinapterus leucas) in Cook Inlet, Alaska, displays strong site fidelity to the inlet year-round. A population viability analysis model was developed to evaluate this population’s risk of extinction and decline over 50 and 100 years. Model assumptions and parameter sensitivity were tested by:

• varying survival and fecundity rates
• frequency of catastrophic events
• predation level
• group mortality events and carrying capacity

While the different model scenarios showed considerable variation in extinction risk within 50 years (0–18 percent), and 100 years (0–38 percent), and probability of decline (1–71 percent), only the assumption of an intrinsic rate of growth greater than 2 percent reduced the risk of further decline to 1–2 percent.

Almost all model scenarios that included unusual mortality events (Models G to L) had probabilities of extinction within 50 years (2–18 percent, with the exception of Model G equaling 0 percent), unlike scenarios without. Both predation and group mortality events were shown to create thresholds below which the population could not recover. Models including threshold effects had probabilities of extinction as much as 25 percent higher than similar models without. In Model B, with no threshold effects, and with no subsistence hunt after 2014, the population declines in 53 percent of the cases, with a probability of recovery in 100 years of 14 percent. The model scenarios that best fit the existing Cook Inlet beluga whale data (Models B, F, M to O) included:

• a per capita mechanism increasing mortality (Models B, F, O)
• mortality from killer whale, (Orcinus orca) predation (Models F, N, O)
• a reduction in Cook Inlet carrying capacity (Models M to O)

Model scenarios B, F, K, M to O were used to estimate the range of the probability of extinction. These had a probability of decline between 42 percent and 71 percent and a probability of extinction between 0 percent and 14 percent in 100 years.

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Roderick C. Hobbs, Paul R. Wade, and Kim E. W. Shelden. Published in Marine Fisheries Review 77(2), 2015.