Estimated trends in abundance of eastern Pacific gray whales from shore counts (1967/68 to 1995/96)

Abstract

Estimates of abundance of eastern Pacific gray whales (Eschrichtius robustus) are obtained from counts made during their southbound migration past a shore-based station near Monterey, California. Assuming an exponential rate of increase, the population is estimated to have increased at 2.5% per annum (SE = 0.3%) between 1967/68 and 1995/96. However, there is some indication that the population growth is slowing, so that an asymptotic growth curve may be more appropriate. The estimated asymptote from a logistic model is 26,046 (SE = 6,281) and the inflection point is approximately in 1971 (SE = 6.5). The onset of the migration, when 10% of the whales have passed the station, has occurred increasingly later through this sample period, by approximately one day every two years. Median dates show a similar trend of roughly one day every three years. However, there is no significant change in the date at which 90% of whales have passed the station.