Population Assessment of Western Gray Whales in 2007

A population assessment of the western gray whale (Eschrichtius robustus) was conducted using photoidentification data collected off Sakhalin Island under the joint Russia-U.S. programme from 1994 to 2006. This is an update of the assessments by Reeves et al (2005) and Cooke et al. (2006) which used data up to 2003 and 2005 respectively, fitted to the same, individually-based population model. New median estimates of key population parameters (with 90% Bayesian confidence intervals) are 0.982 (0.972 0.991) for the noncalf annual survival rate; 0.76 (0.66 0.85) for the survival rate from calf to yearling; 2.9% per annum (1.9% – 4.0%) for the average annual rate of population increase over 1994-2006; 0.45 (0.37 0.52) for the female sex ratio and 121 whales (112 130) for the 1+ (non-calf) population size in 2007. The updated assessment is more optimistic than the Reeves et al (2005) mainly because of the reduced calving intervals observed in recent years, implying a higher reproductive rate. The modal calving interval has shortened from 3 years up to 2002 to 2 years post-2002. This is consistent with reduced disturbance from industrial activity during 200204. Forward projections of the population model to 2050, assuming no additional mortality or disturbance to reproduction, indicate a high probability (>99%) of population increase. Four whales (all female) have been killed in fishing nets on the Pacific coast of Japan during the past 24 months. Projections of the female population incorporating this level of extra mortality indicate a high (>25%) probability of population decline and a substantial (>10%) risk of extirpation by 2050. It is important to avoid any further human-caused deaths in this depleted population.