Monitoring Recovery Status and Age Structure of Cook Inlet, Alaska Belugas by Skin Color Determination

A study to describe a technique developed to monitor the conservation status of the Cook Inlet stock.

Belugas are gray at birth and gradually lighten with maturity. Thus, the fraction of animals in the population that are white (W_t) is an index of the fraction of mature animals in the population (M_t). M_t is expected to increase over time from the exploited level (hunters targeted white animals) to a rebound value (harvest stopped in 1999) if population abundance recovers. Population growth was modeled using a variable Leslie matrix and M_t estimated for each year from 1976-2050. Calf survival and age at sexual maturity were density dependent. Calf survival and fecundity were stochastic. Averages from 5000 simulations of the model estimated M₁₉₈₀ at 0.647 (CV 205%), M₂₀₀₀ at 0.559 (CV 07%), M₂₀₂₀ at 0.667 (CV 2.7%). The most likely year of recovery of M_t from its exploited to rebound level ranged from 7-15%, when differing life history parameters were used in the model. To determine W₂₀₀₀, aerial videos of whales collected in June 2000 were analyzed using image analysis software programmed to categorize animals as gray or white. A controlled experiment, designed to provide color calibration, was conducted using aerial video of life-size beluga models. Percent of control images correctly identified for color were: dark gray = 100%, medium gray = 74.0%, light gray = 84.3%, and bright white = 100%. Applying the color calibration to real whale images resulted in W₂₀₀₀ = 0.8109 (95% CI from bootstrap: 0.7613-0.8522). The value of W₂₀₀₀ may be positively biased due to low detection of mall dark calves. Despite the bias, the increase predicted for M_t, should be detectable if future measurements of W_t have similar variability to W₂₀₀₀.