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

The Cook Inlet stock of beluga whales (Delphinapterus leucas) is the most isolated of the five evident populations around Alaska. Declines in abundance of this stock, apparently from over harvest, have been detected since 1993. This study describes a technique used to monitor status of these whales. Belugas are dark gray at birth and turn white at sexual maturity, around 4-7 years of age. Thus, the fraction of white animals in year y, M(y) is an estimate of mature animals in the population. To determine M(2000), videos of whales taken during aerial surveys in June 2000 were analyzed using a computer program trained to categorize each animal as gray or white. A controlled experiment, designed to determine sampling error and provide color calibration, was conducted using aerial video of life size beluga models. Population growth was modeled using a variable Leslie matrix and values of M estimated for each year. Density dependence was incorporated into values used for calf survival, and values for both calf survival and fecundity were stochastic (drawn from beta distributions). The value of M(2000) in the model was fitted to the observed value of M(2000) from the aerial videos to estimate probable ranges for life history parameters. M is expected to increase over time from the exploited level (hunters targeted white animals) to a rebound value (the harvest stopped in 1999) if the population recovers. Thus, future observed values of M can be used to monitor the recovery of this population.