Estimates of northbound Gray Whales linked to spatial and temporal distribution of Arctic ice

Background/Question/Methods Most of the Eastern North Pacific population of gray whales migrates annually from feeding grounds in the Arctic to breeding and calving grounds off Baja California, MX. Estimates of calf production for this population are derived from shore based surveys that have been conducted for 16 consecutive years (1994-2009) during the return, northbound migration from the Piedras Blancas Light Station located near San Simeon, CA. Results of aerial surveys to determine whether migrating whales may pass far enough offshore to evade detection and day/night sampling with thermal imaging systems to compare migrating rates in daylight with those during periods of darkness indicate that visual survey data require no correction for either of these factors. In addition to the survey effort, bi-weekly images depicting the distribution of seasonal ice in the Arctic have been compared with known feeding grounds to investigate possible impacts of ice moderated access to prey and calf production in this population. We used a Bayesian model to explore the relationship between calf production and the temporal and spatial distribution of ice during the feeding season prior to ovulation and during the season of pregnancy. Results/Conclusions Estimates of total northbound calves were highly variable between years and the data show no indication of a trend in calf production over the 16 year time series. Although our index of ice moderated access to prey during the feeding season prior to ovulation explained little of the observed variability in calf estimates, the index values early in the feeding season of pregnancy explained almost 70% of the observed variability. We suggest that seasonal ice blocks access to feeding grounds for pregnant females, the first members of this population to return north, and this impacts the probability that existing pregnancies will be carried to term. Because it appears that ice was not a factor in estimates of northbound calves in 1980 and 1981, the link found in this study indicates that the observed overall northward shift in feeding grounds for gray whales may have outpaced the reduction ice cover resulting from the steady warming of the Arctic.