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|NOAA News Releases 2002
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New preliminary estimates show that the population of Eastern North Pacific gray whales is no longer increasing and may have declined, but scientists indicate that this change is not unusual and appears to be within a normal variation range. The gray whales migrate annually along the Pacific Coast between Alaska and Mexico.
Gray whales have been systematically counted along the Pacific Coast most years since 1967. In the 30-year population recovery (which allowed eastern North Pacific gray whales to be taken off the endangered species list) there have been several dips in population estimates. Since 1984, most gray whale estimates have been between 20,000 and 24,000, except for one other low value of 17,674 (in 1992/93), and a high value of 26,635 (in 1997/98).
Scientists at the National Marine Mammal Laboratory in Seattle consider these numbers to be in the natural course of events.
"Wild populations fluctuate, and abundance estimates can vary due to different observers, weather conditions, and so forth. We watch carefully for trends over time. This is a population count fluctuation which I do not expect to become a trend," said NOAA's National Marine Fisheries Service scientist Jeff Breiwick.
Breiwick is cautious to point out that the population estimates are exactly that estimates. Scientists do not count every single gray whale. Instead whale counters follow a rigid protocol of search effort, rotating through three-hour watches, and estimates are compiled from the samples provided. To produce an estimate of abundance, the shore-based counts are adjusted to correct for periods when no one is observing (such as during the night or with poor visibility), for whales that are simply missed within the viewing area even in good visibility, for different sighting rates among observers, of pod size, distance offshore, and differences in day and night travel rates of whales.
"Population estimates are a broad brush," said Breiwick, "but they still paint a picture." Wayne Perryman, also a biologist with NOAA Fisheries, reports that this year's whales look healthier than in 1998, and the winter calf count has rebounded. Stranding rates have been lower than usual during these last two years, indicating that the high mortality rate seen in 1999 and 2000 was probably an isolated event, not the beginning of a stranding trend.
Scientists reported unusually high numbers of strandings of gray whales in 1999 and 2000, which may have indicated a large die-off in the population. There were 200-300 whales found on the beaches in each of these years, compared to average counts of less than 40 per year. Many of the live whales were very thin. Calf counts were low. There is no clear explanation for the high number of strandings in 1999 and 2000, Breiwick said.
Although a published report showed the gray whale population increased by 2.5% per year between 1967 and 1996, a preliminary analysis of the gray whale trend indicates the population is no longer increasing and is likely at the carrying capacity of its environment.
"The analysis indicates the population appears to have been relatively stable since about 1990, and gray whale carrying capacity is estimated to be between 18,000 and 24,000," said NOAA Fisheries scientist Paul Wade. "Given the high number of strandings in 1999 and 2000, the population likely declined somewhat over that time, but we expect populations at carrying capacity to fluctuate as environmental conditions change, and the population experiences good years and bad years."
Whale counters working for the NOAA Fisheries tallied 2,800 whales migrating south past Granite Canyon, near Carmel, California, during 532 hours of systematic searching between December 12, 2001 and March 5, 2002. This resulted in a preliminary abundance estimate of about 17,500 whales. This estimate is still preliminary until peer-reviewed by the International Whaling Commission's scientific committee.
A year earlier, during the same time frame at the same site, whale counters tallied 2,754 whales during 593 hours, resulting in an estimate of 18,800 whales. The differences in these two estimates is primarily due to differences in the bell-shape curve that is fit to the daily sighting rates. The 2000/01 migration was irregular and protracted; the 2001/02 migration was the typical rise in numbers until mid-January followed by the typical fall.
Gray whales feed on a wide variety of organisms, both in the water and buried on the sea floor. They use their coarse baleen to filter out food items, creating plumes of mud when they have been feeding near soft sediment. Apparently they eat very little while migrating and throughout the winter, relying on stored fat. Their migration is one of the longest known of any mammal, covering 8,500 to 11,000 miles per round trip.
Gray whale mothers carry their calves for more than a year before birth, which usually takes place during the southbound migration or at the lagoons of Baja, Mexico, in January and February. Mothers nurse their calves from seven to nine months. A healthy, productive mother will bear a calf every other year.
NOAA's National Marine Fisheries Service is dedicated to protecting and preserving our nation's living marine resources through scientific research, management, enforcement, and the conservation of marine mammals and other protected marine species and their habitat.
To learn more about NOAA Fisheries in Alaska, please visit the Web site, http://www.fakr.noaa.gov.
For more information on gray
whales visit http://www.fakr.noaa.gov/protectedresources/whales/default.htm#gray.