Population assessment of Western Gray Whales in 2006

Authors

Justin G. Cooke, CEMS, Alexanderstr. 10, 79261 Gutach, GermanyFollow
David W. Weller, Southwest Fisheries Science Center
Amanda L. Bradford, Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington
Alexander M. Burdin, Kamchatka Branch of Pacific Institute of Geography, Russian Academy of Sciences, Far East Division, Petropavlovsk, Kamchatka 683000, Russia
Robert L. Brownell Jr., Southwest Fisheries Science CenterFollow

Date of this Version

6-2006

Comments

Paper SC/58/BRG30 presented to the International Whaling Commission Scientific Committee, June 2006, St Kitts and Nevis, WI.

Abstract

A population assessment of the western gray whale (Eschrichtius robustus) was conducted using the Photo-identification data collected off Sakhalin Island under the joint Russia-U.S. program from 1994 to 2005. This is an update of the assessment by Reeves et al (2005) which used data up to 2003, fitted to the same, individually-based population model. New median estimates of key population parameters (with 90% Bayesian confidence intervals) are 0.986 (0.975 - 0.995) for the adult survival rate; 0.72 (0.60 - 0.83) for the survival rate from calf to yearling; 3.0% per annum (2.1% – 4.2%) for the average annual rate of population increase over 1994-2005; 0.43 (0.37 - 0.50) for the female sex ratio and 122 whales (113 - 131) for the 1+ (non-calf) population size in 2006. The updated assessment is more optimistic than the Reeves et al assessment. This is mainly due to reduced calving intervals observed in recent years, implying a higher reproductive rate. The modal calving interval has shortened from 3 years to 2 years in the most recent seasons, which is consistent with reduced disturbance from industrial activity during 2002-04. Forward projections of the population model to 2030, assuming no additional mortality or disturbance to reproduction, indicate a high probability (>99%) of population increase. Three whales (all female) were killed in fishing nets on the coast of Japan in 2005 during the northward migration. Projections of the female population incorporating extra mortality at the 2005 level indicate a high probability (~75%) of population decline and a substantial risk (>25%) of extirpation by 2030. It is important to avoid any further human-caused deaths in this depleted population.