Timing of the gray whale southbound migration

Abstract

The southbound migration of the eastern North Pacific stock of gray whales (Eschrichtius robustus) has been documented by the National Marine Fisheries Service most seasons since 1967 at or near Granite Canyon, in central California, and by the American Cetacean Society’s Los Angeles Chapter every season since 1985 at Point Vicente, southern California. This has provided a rare opportunity to examine cetacean migratory timing data over a relatively long time series. In 1998/99, anecdotal reports indicated a major change had occurred in the timing of the migration, which prompted this study to compare the observed timing relative to expected dates. Although no observers were at Granite Canyon in 1998/99, data collected from this site indicated that prior to 1980, annual median sighting dates ranged from 4-13 January (overall median = 8 January; CI = 1.3), but since then there has been a one-week (6.8 day; CI = 2.0) delay, with median dates now ranging from 12-18 January (overall median = 15 January; CI = 1.7). This delay in timing is better represented as a shift in dates than as a trend, and it occurred shortly after a major oceanographic regime shift in the North Pacific Ocean. The shift in whale sighting dates occurred equally in the onset of the migrations (when the first 10% of the whales passed a site), the median (50%) and end (when 90% of the whales passed). At Granite Canyon, there were no significant trends in these dates prior to 1980 or in dates following the shift. In mid-February (median = 15 February, CI = 1.9, at Point Vicente), few gray whales are still going south and some are already migrating north. Most of the migration (the period between the 10% and 90% sighting dates) occurs across a period of 34 days (CI = 2.0), but the entire southbound migration may take > 70 days to pass a location in any given year. It takes a whale approximately 54 days to migrate from the north central Bering Sea to the lagoons in Baja California (8,000km), but some whales may travel as far as 10,000km. Based on available observations and calculations using a travel rate of 147km/day, current median (peak) sighting dates of the southbound migration should be: 1 December in the north central Bering Sea (here considered the theoretical starting point for the migration); 12 December at Unimak Pass, Alaska; 18 December for Kodiak Island, Alaska; 5 January for Washington State; 7 January for Oregon; 15 January for central California; 18 January in southern California; and 24 January at the northern lagoons in Baja California (considered here to be the terminus of the migration). Although no observations were made at Granite Canyon in 1998/99, sightings made at Yaquina Head, Oregon (median sighting date = 7 January) and at Point Vicente (median = 20 January) indicate that the timing of that migration was consistent with previous years.