| Annual shore censuses of southbound migrating gray whales were conducted from points near Monterey, CA, for 13 years beginning in 1967/68, by the U.S. National Marine Mammal Laboratory. To utilize these daylight counts in an estimation of total population size, both analyses of existing data and field verification experiments were conducted. It was determined that approximately 20 percent of the population passes by out of sight from land, and that there were significant observer biases in estimating the distance to and number of whales within passing groups. Through the quantification of migratory timing, the effects of visibility conditions on recorded counts were determined, and the "toils" of the annual migrations estimated. A statistical model was defined to estimate total abundance, taking these results into consideration. The best estimate of the 1980 population level was 15,647. A net increase of 2.5 percent per year occurred during the 13 years studied, concurrent with a 1.2 percent removal rate by the Soviet fishery, indicating a net productivity of 3.7 percent per year.; The implications of the documented production rate were considered in a review and reevaluation of the vital parameter estimates for the stock. All available data apply to approximately the 1967 level of 11,500 whales. At that time pregnancy rate was 0.407, age at maturity 8 years, adult survival 0.945 and juvenile survival 0.899 on an annual basis. Also reviewed were information on gray whale catch history, aboriginal whaling and pre-exploitation (by western civilization) population level.; A simulation of population history 1800-1980 was run for 375 combinations of carrying capacity population size, various forms of dynamic response of vital parameters to population density, and various levels of pre-historic aboriginal removal rate. Trajectories were selected for further scrutiny if they passed through the 1967 and 1980 confidence intervals from the shore station censuses. Only 23 trajectories, or 6.1 percent, passed through both windows. The parameters which produced a trajectory most closely approaching expected behavior during the 180 years were: a carrying capacity of 24,000 reduced to under 12,000 in 1800 by an aboriginal removal rate of 600 whales. |
