Gray whales born north of Mexico: Indicator of recovery or consequence of regime shift?

Every winter, most gray whales (Eschrichtius robustus) of the eastern North Pacific stock migrate from feeding areas in the Arctic to warm, shallow lagoons in Mexico, covering a distance of 15 000-20 000 km roundtrip. It is hypothesized that this migration to warmer climes is undertaken to reduce the whales' thermoregulatory energy requirement during winter when food resources are low. Calves are particularly vulnerable as they have yet to acquire a thick layer of blubber. Prior to the mid-1970s, newborn calves were seen primarily in Mexico's lagoons. However, since 1980, shore-based observers have reported increased numbers of calf sightings north of Mexico. Calves were greatly under-represented in the shore-based records as rarely did more than one independent observer at a time recognize the presence of a calf and a strong nearshore preference was not evident from the aerial data. Although cows with calves were difficult to detect, significant increases in average annual calf counts occurred at two counting stations in California, USA; counts increased in the late 1970s at a station near San Diego (southern California) and in the mid-1980s at a station near Carmel (central California). This trend is probably more than an increased emphasis on reporting calf sightings over the years for two reasons: (1) The first reports of calves stranding north of Mexico during the southbound migration occurred after 1976; and (2) calves were absent during many of the earlier censuses, and when they were seen, most appeared near the end of each migration. In subsequent years, calf sightings spread through the respective seasons, first at the southern stations (sometime after 1969) and then farther north (sometime after 1980). Increased calf counts at the northern stations were strongly correlated with warmer sea surface temperature anomalies. The interannual increase in calf sightings may be related to the increased abundance of the population, to changes in ocean climate, or to both factors. A one-week shift in the timing of the southbound migration since 1980 placed the mean passage date for pregnant females near Carmel at 8 or 9 January, coinciding with earlier estimates of median calving date (10-13 January). Assuming the median parturition date has not changed, this would mean that nearly half of the calving now occurs north of Carmel.