The blue whale (Balaenoptera musculus) is the largest animal ever known to have lived on Earth . Blue whales are found in all oceans worldwide and are separated into populations from the North Atlantic, North Pacific and Southern Hemisphere . Each population is composed of several stocks that typically migrate between higher-latitude summer feeding grounds and lower-latitude wintering areas. The largest numbers of blue whales in U.S. waters are within the eastern North Pacific stock. Other U.S. stocks occur in waters off the coast of Hawaii and the Northeast .
Pre-whaling blue whale populations had about 350,000 individuals . In 1868, the invention of the exploding harpoon gun made the hunting of blue whales possible and in 1900, whalers began to focus on blue whales and continued until the mid 1960s [1, 3]. During this time, it is estimated that whalers killed up to 99% of blue whale populations . Currently, there are about 5-10,000 blue whales in the Southern Hemisphere and about 3-4,000 in the Northern Hemisphere . Current threats include collisions with vessels, entanglement in fishing gear, reduced zooplankton production due to habitat degradation, and disturbance from low-frequency noise . The offshore driftnet gillnet fishery is the only fishery likely to take blue whales, but few mortalities or serious injuries have been observed .
The Eastern North Pacific Stock feeds in waters off the coast of California from June to November and then migrates south to Mexico (sometimes going as far south as Costa Rica) in winter/spring . Recently, blue whales seen off the coast of Alaska were photo-matched to photos from the Southern California area indicating that California animals now migrate as far north as Alaska . This is probably a re-establishment of a traditional migratory route . The number of whales reported off the coast of California increased from 704 in 1979/80 to 2,584 whales in 1996 . Blue whale surveys conducted in 2000-2002 reported fewer whales than previous years, but it is uncertain whether this represents a true decrease in numbers . The best estimate of current blue whale abundance for this stock is 1,744 . It is not certain if the overall increasing trend indicates a growth in the size of the stock, or just increased use of California waters , but in general, the stock is thought to have increased . Because this is the largest stock in U.S. waters, it dominates the trend of the species in U.S. waters.
Blue whales feeding along the Aleutian Islands are probably part of a central western North Pacific stock that is thought to migrate to offshore waters north of Hawaii in winter . Sightings of blue whales in Hawaiian waters are infrequent, although acoustic recordings indicate that blue whales occur there. There are no estimates of population size for this stock . No blue whales were sighted during aerial surveys of Hawaiian waters conducted from 1993 to 1998 or during shipboard surveys conducted in the summer/fall of 2002 . In 2004, three blue whales were seen in the western Aleutians, the first US sightings of blue whales from this western N. Pacific population in several decades 
The blue whale is an occasional visitor along the Atlantic coast of the Northeast . Sightings of blue whales off Cape Cod, Massachusetts, in summer and fall may represent the southern limit of the feeding range of the western North Atlantic stock that feeds primarily off the Canadian coast . Blue whales have been sighted as far south as Florida, however, and the actual southern limit of this stock’s range is unknown . Because blue whales are not frequently seen in U.S. Atlantic waters, there are insufficient data to determine the stock's population trend . In 1997, the total number of photo-identified individuals for eastern Canada and New England was 352 .
 NMFS. 1998. Recovery plan for the blue whale (Balaenoptera musculus). Prepared by Reeves R.R., P.J. Clapham, R.L. Brownell, Jr., and G.K. Silber for the National Marine Fisheries Service, Silver Spring, MD. 42 pp.
 NOAA Fisheries. 2004. Stock Assessment Report. Blue Whale (Balaenoptera musculus): Eastern North Pacific Stock. Revised 3/15/05. National Oceanic and Atmospheric Administration, Washington, D.C.
 American Cetacean Society. 2005 American Cetacean Society Fact Sheet: Blue Whale (Balaenoptera musculus). Website http://www.acsonline.org/factpack/bluewhl.htm (accessed on 11/30/05).
 NOAA. Fisheries. 2004. NOAA Scientists Sight Blue Whales in Alaska. Press Release 7/27/2004.
 NOAA Fisheries. 2004. Stock Assessment Report. Blue Whale (Balaenoptera musculus): Western North Pacific Stock. Revised 3/15/05. National Oceanic and Atmospheric Administration, Washington, D.C.
 Rankin, Barlow and Stafford. (in press) Marine Mammal Science.
 NOAA Fisheries. 2002. Stock Assessment Report. Blue Whale (Balaenoptera musculus): Western North Atlantic Stock. Revised Jan. 2002. National Oceanic and Atmospheric Administration, Washington, D.C